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GustavL Fletcher 

Mmdm MEW PHYStomPHY ty 

From the collection of the 

z n m 

o Prelinger 

i a 

v XJibrary 

San Francisco, California 











No part of the material covered by this 
copyright may be reproduced in any form 
without written permission of the publisher. 




This text is based upon New Physiography by Arey, Bryant, 
Glendenin, and Morrey. It retains the good features of the older 
text, but the order of topics has been entirely changed, most of 
the illustrations are new, and the subject matter has been com- 
pletely rewritten. The author's long experience with high school 
pupils has made him aware of their difficulties with the printed 
text, and mindful of that, he has tried to use language within 
their comprehension. 

The introductory chapter gives a general idea of what has hap- 
pened on the earth and what is going on now, so that the pupil 
is made aware of the aim of the entire subject. This is followed 
by a study of the materials of which the earth is made, rocks, and 
of the forces acting upon those materials. Having learned that 
much, the pupil is in a position to understand how these forces 
have modified the earth's surface and made it what it is today. 

The land is studied first, because pupils are more likely to know 
something about land, to begin with. It is for that reason they 
find land studies easier than the rest of the subject. 

This is followed by a short history of the earth, designed to 
teach the student how the earth came to its present condition. 
The chapter is optional, but the author feels that many of the 
better students will be eager to read it. 

The study of the land will probably occupy the first half of a 
year. The pupils who have successfully completed the first half 
will then study the earth's relations in space, seasons, latitude, 
longitude, time, the atmosphere and associated phenomena like 
weather and climate. And the year's work is brought to an end 
by the study of the sea with special emphasis on harbors. 

The text is printed in type of two sizes: larger type for the 
essential material and smaller for the optional. Each chapter has 
a completion summary which the pupil is required to copy and 
complete. This avoids the objection that many teachers have to 
the ordinary summary: that some pupils read only the summary. 
The completion summary acts as a self -test, for if the pupil is 
able to fill in the blanks, he knows that he has learned his lesson 
and this knowledge carries with it a sense of mastery and hence 
a feeling of satisfaction. 



At the end of each chapter are questions on every important 
point in the text and the teacher may well use these questions as a 
chief part of his assignment. There is also a set of optional questions 
which will challenge the best students to extend themselves. 

Summaries in tabular form are used wherever the complexity 
of the subject seemed to require them, as, for example: the cycle 
of stream erosion, page 91; classification of rocks, page 24; 
ground water, page 150; harbors, page 525. 

The author believes that illustrations should be used chiefly to 
help the pupil understand the text, and he has therefore been at 
great pains to use illustrations wherever they would help: for ex- 
ample, Fig. 263, page 393, on the cause of cyclones, Fig. 285, page 
424, on the rainbow, and Figs. 189 to 196 in " Stories in Stones." 

Several new topics have been introduced into the book: flood 
control, soil erosion, and dust storms are shown to be related to 
each other and to the problems of irrigation and water power. 
"Stories in Stones" is a short history of the earth never before 
attempted in a book of this kind. The chapter on "The Economic 
Importance of Rocks and Minerals" is also new. 

Besides the newer phases of the subject, many of the older 
topics are treated in new ways. 

There is a clear analysis of rocks and the relation of each class 
to the others. 

The chapter on rivers has a logical development. 

Earthquakes, volcanoes, and mountains are all related to each 
other and to the margins of the continents, where crustal 
movements are taking place. 

Geysers are explained as similar in action to coffee percolators. 

An attempt is made to explain the cause of the cyclonic whirl. 

The monsoon is shown to be nothing but a land breeze on a 
continental scale. 

Lightning is discussed in a practical way. 

Climates are classified scientifically, following Finch and Tre- 
wartha's up-to-date treatment. 

Relative humidity, dew point, frost, fog, cloud, rain, and snow 
are all treated as related topics. 

The author wishes to express his thanks to Mr. Abraham Fialkoff 
of the New Utrecht High School, Brooklyn, N. Y., for his con- 
structive criticism of the manuscript; and to Mr. Chas. Halgren, 
whose painstaking work with the illustrations is largely responsible 
for their success. 

January, 1938 GUSTAV L. FLETCHER 









7. RIVERS 73 


9. GLACIERS 107 



12. PLAINS 167 

13. PLATEAUS 187 


15. MOUNTAINS 203 


17. VOLCANOES 238 




21. THE MOON 322 





26. WINDS 384 











INDEX 561 





1. What is physiography? Education may be defined as 
the process which fits a person for the life he has to lead. 
In many respects we cannot predict what that life will be 
and therefore we cannot prescribe the proper kind of edu- 
cation. But we all live on the surface of the earth and it 
would seem that knowledge about that surface ought to 
make up an important part of everyone's education. What 
is the surface like at the present time? What kind of change 
is it likely to undergo today, tomorrow, next month, next 
year, in the near future, and in the distant future? For 
things do not remain the same for very long. In the words 
of the old hymn, " Change and decay in all around I see." 

To be more specific, what kind of an earth is this on 
which we live? 

What is it made of? 

Can we make use of any of the materials of which it is 

What changes will they undergo? 

What is the relation of continents and oceans? 

Is that relation likely to change? 

Will the mountains always remain as they are? 

What are earthquakes? 

What causes the seasons? 

Can we predict the weather? 

Why does it rain? 

How is climate determined? 

These are a few of the questions which physiography tries 
to answer; and if we summarize them we might say: Physiog- 



raphy is the study of the face of the earth, the home of man, 
and the changes in physiognomy which that face undergoes. 

2. The earth in space. The earth is a member of the 
solar system, consisting of the sun and nine other heavenly 
bodies called planets (Fig. 1). 

FIG. 1. The Solar System 

The earth, as well as the other planets, moves about the 
sun in a path which is almost circular, and in a plane, called 
the ecliptic, which almost coincides with the sun's equa- 

3. The origin of the earth. The sun and all the planets 
are composed of the same elements, as shown by spectro- 
scopic analysis. This evidence would lead us to the con- 
clusion that these bodies have had a common origin, and 
all the hypotheses about the origin of the earth start that 
way. These hypotheses will be taken up, in detail, in a 
later chapter; but it is necessary, here, to indicate how we 
believe the earth attained its present condition, in order 
that we may understand some of the changes that are going 
on now. 

We believe that the present solar system started as a 
large mass of very hot gas, much like the present sun, and 
including all the other bodies of the present solar system. 

Some event took place which tore out masses of this hot 
material and started the rotation which still continues, 
since there is nothing to stop it. These masses cooled to 


form the planets, and, according to the Jeans and Jeffreys 
version, as well as the nebular hypothesis, the mass which 
is now the earth was at one time liquid. 

The liquid mass cooled, on the surface, to form a solid 
crust, but it remained very hot inside. However, we have 
evidence to prove that the earth is a very rigid solid inside, 
despite its great heat; and this is in perfect accord with 
scientific principles; for at the great pressure of the under- 
lying rock, the melting point of the rock is raised. In other 
words, it takes a much higher temperature to melt rock 
which is under great pressure. But if that pressure were 
released, for example, by the cracking of the crust al cover- 
ing, then the interior material would become liquid as we 
see it sometimes in volcanic eruptions. 

4. The origin of the continents. Let us imagine the earth 
as it must have been, if our hypothesis is correct, a ball of 
very hot gas, consisting of all kinds of material, most of 
which now is either liquid or solid. As it cooled more and 
more, most of the gas became liquid, which collected to- 
gether in the form of a ball surrounded by the remaining gases. 

As the liquid, consisting of a mixture of all sorts of molten 
substances, cooled sufficiently, some of these substances 
began to separate out as 
solids. The very dense sol- 
ids, like iron, sank in the 
liquid, while others floated 
about like great icebergs, 
some sticking out of the 
liquid higher than others, FIG. 2. Ice, Wood, and Cork 

like cork and wood and ice Floating in Water 

a + ' : + /T?,' o\ The cork is higher than the ice be- 

floating in water (Fig. 2). cauge itg gpedfic * ravity fa legg 

Now we find that the 

earth's crust is composed chiefly of two kinds of rock, which 
we call by the general names of granite and basalt. The 
granite, with a specific gravity of 2.7, and the basalt with a 
specific gravity of 3.0 both floated in the liquid earth mass, 


whose specific gravity was about 5.7; but the granite stuck 
out farther, like cork in water, while the basalt floated like 
wood (Fig. 3). 

FIG. 3. Masses of Granite and Basalt Floating in 
Liquefied Rock or Magma 

When the entire surface had cooled to a solid, it must 
have been irregular and have consisted of masses of granite 
and basalt, with the former standing higher than the latter. 
Now on further cooling, there came a time when the water, 
all of which must have been in the atmosphere as steam, 
began to fall as rain, boiling hot rain, which at first evapo- 
rated from the hot rocky surface of the earth. But ulti- 
mately, some of the hot water remained on the earth, running 
down from the high places into the lowest ones; from the 
granite masses on to the basalt (Fig. 4). 

FIG. 4. Formation of Oceans and Continents 

This figure also shows how the pressure of sediments formed by erosion 
pushes the basalt down and the granite up, re-elevating the mountains. 

If this were true, the continents should be made chiefly 
of granites while the oceans should rest on basalt, and this 
appears to be the case. Ever since that first rain fell, water 
has been evaporating from the ocean, falling as rain, and 
running down from the continents to the ocean again. 


5. The theory of isostasy or balance. The action of air 
and water on the granites, in time, wore off pieces, small 
and large, which the rain water moved downhill toward the 
ocean; slowly in most cases, rapidly in others where the 
slope was steep. The running waters, or rivers, pushing along 
the loose bits and pieces of rock, ground down the bedrock 
and wore it still more, just as a file wears down a piece of 
iron. In time, millions of years in most cases, the high places 
of the continents, the mountains, were worn down, since the 
wearing process is most rapid where the slope is steepest. 

The loose material carried by the rivers was ultimately 
dragged down to the seas, ground down to sand and silt, 
and this material was deposited at the mouths of the rivers, 
the coarser material nearer the shore, the finer material a 
little farther out, but none of it very far from shore, because, 
as soon as the river flows into the ocean, it slows down and 
soon stops moving altogether, so that it can no longer push 
particles of sand or even silt. 

The process we have been describing, called erosion, planes 
down the high places of the continents and carries the debris 
into the seas, dropping it close to the shore on the con- 
tinental shelf, as we call it (Fig. 5). 

FIG. 5. Loose rock, formed by erosion, is deposited near the shore. 

When these mountainous masses have been deposited on 
the continental shelf, they create a pressure on the under- 
lying basalt. (See Fig. 4.) It will be remembered that the rock 
underneath the crust is solid but very hot; and we have 
reason to know that it is plastic. Now just imagine a ball 
composed of plastic material with a thin cover, the crust, and 
two pieces of wood sticking through the crust into the plastic 


material. If one piece of wood is pushed in, the other will 
be pushed out until the pressures throughout are again equal 

(Fig. 6). We call this an isostatic 

And where would that adjust- 
ment take place, if our theory is 
correct? The granite masses have 
been worn off and thrown upon the 
basalt, which therefore must sink 
deeper, pushing up the underlying 
granite mass into mountains again. 
This should take place at or near 
the contact between the granite and 
the basaltic masses, that is, near the 
shore line, and that is just where 
our mountain ranges are formed; 
parallel to the shore lines (Fig. 7). 

This plausible explanation of the 
origin of the continents and of 
mountains is called the theory of 
isostasy and we are developing it 
early in the course, because it will 
help us to understand many of the forces that seem to be at 
play, changing the face of the earth. Isostasy means literally 
equal pressures or balance. The earth's crust is composed 
of granite and basalt blocks which are balanced. When the 
basalt block is made heavier, it sinks down and pushes up 
the granite block until balance is again restored. 

Completion Summary 

Physiography is an important subject because - .* 
The earth was at one time in the - - condition, and 
although even now, we have evidence that it is a solid. 

We believe that 

liquids began to separate from the 

* The student is required to copy this summary, filling in the blanks with the 
correct word, phrase, sentence, or paragraph. 


FIG. 7. North America, Showing the Mountain Ranges 
Parallel to the Shore Lines 

gaseous mass ; and as the liquid cooled, the - - separated 
from it, sinking to the center, if they were - , or floating, 
if- -. 

The lighter solids were chiefly of two kinds: - - and 

; of which the - stuck out farther than the - . 

Subsequent rains - - basalt, forming oceans, while the 

granite masses 

Erosion wore down the granitic mountain masses and de- 
posited the sediments - . 

The pressure brought about - - in the hot, plastic 
, and ultimately mountains again. 



1. State two reasons for your belief that physiography is going 
to be an interesting study. 

2. Give a brief outline of the origin of the earth. 

3. What is our present belief concerning the nature of the 
interior of the earth? 

4. How do we explain the fact that the continental masses are 
chiefly granite? 

5. Explain how the oceans were formed. 

6. How are mountains worn down? (Use the word erosion.) 

7. If the mountains have been worn down in the past, why is 
it that we still have mountains? Explain, using the theory of 



6. Composition of the crust of the earth. In order to 
understand what is going on, we must also know something 
of the nature and properties of the materials that make up 
the surface of the earth. In its original gaseous state, we 
believe there were only the chemical elements, about ninety- 
two in number. About half 

the total amount seems to 
have been oxygen, a gas, and 
about one quarter silicon, 
now a solid. The others are 
shown in Fig. 8. These figures 
are based upon analyses of 
minerals throughout the 
world, all of them derived 
from the crust and from lavas 
which may have come from 
the depths of the earth. 

As these hot gaseous ele- 
ments cooled, some of them combined with others, forming 
compounds like water, silica, better known as quartz, and 
thousands of others, in some places separated from each 
other, in most places mixed up with each other. 

7. Rocks and minerals. A simple element or compound 
found in the earth, but not formed by plant or animal, we 
call a mineral. Quartz is a mineral. It is composed of silicon 
and oxygen and always has the definite chemical composi- 
tion Si0 2 . In chemistry, such a substance is called a com- 
pound, because it always has the same composition, while in 
geology we call it a mineral. According to this definition we 


FIG. 8. Composition of the Earth 


must even consider water a mineral, because it is a definite 
compound, found in the earth. 

The earth's crust is made of rocks, and these, as we have 
seen, consist chiefly of mixtures of minerals. Granite, for 
example, is a rock composed of at least three minerals: 
quartz, feldspar, and mica. 

Some rocks consist of only one mineral. Limestone is called 
a rock, because it makes up whole areas of the surface of the 
earth, but it consists often of only one mineral. Quartz, the 
most common mineral, is never formed in very large masses; 
it does not cover very large areas of the earth never do 
we find even one whole mountain made of quartz. Therefore 
quartz is not called a rock. A rock is mineral matter found in 
the earth in large quantities. 

To sum up then, the earth is made of rocks each of which 
is composed of minerals, usually more than one. 

8. Kinds of rocks. If we go back to our theory of the 
formation of continents, we shall be able to understand how 
the different kinds of rocks arose. 

The earth was at one time liquid, and it is still hot enough 
inside to become liquid under certain conditions. We be- 
lieve the original crust of the earth was formed by the cooling 
of the liquid. Any rock derived from the molten condition 
we call igneous (Latin, ignis, fire), as, for example, the 
granitic continental masses and the basaltic rocks under- 
lying the oceans. 

The loose material carried down by rivers and deposited 
on the continental shelf as sediment becomes consolidated 
into rocks which we call sedimentary rocks. We include also 
under this head any sediments deposited from solution such 
as rock salt or gypsum; or from air, like dune sands; or 
from glacial ice, like tillite. 

Both these kinds of rock, igneous and sedimentary, when 
subjected to the action of water, heat, pressure, movement, 
and other forces, are changed into what we call metamorphic 
rocks (from Greek words meaning changed form) . 



The earth's crust, then, is composed of three kinds of 
rocks, igneous, sedimentary, and metamorphic. 

About three quarters of the continental area is covered 
by sedimentary rocks and in most places we see only these 
rocks on the surface; but wherever these sediments are worn 
off by long erosion, we find igneous or metamorphic rock 

FIG. 9. The Grand Canyon 

Underneath, near the river, the rock is metamorphic, while above we see 
sedimentary or stratified rock. 

The sedimentary rocks, then, form only a thin surface 
covering, seldom more than a mile thick, over the igneous 
or metamorphosed crust. This is well shown on the Grand 
Canyon where we have a mile of sediments underlain by 
metamorphic rocks (Fig. 9). 

9. Igneous rocks. Whenever material cools from the 
liquid condition to the solid, it usually forms crystals. If 
the cooling is slow the crystals are large; if rapid, they are 
small. Igneous rocks therefore contain minerals in crystal 

When the molten rock is poured out on the surface, it 
cools rapidly, and the crystals are small. If the igneous mass 



is forced into the rock, perhaps miles below the surface, it 
cools slowly and the crystals are large. When the surface 
covering is worn off, these rocks are laid bare. 

Igneous rocks are usually easy to distinguish because they 
are massive, as we say. That is, when looking at a wall or 
a cliff of igneous rock, no banding or stratification will 

show (Fig. 10). The entire 
rock seems to be uniform. 

However, upon examining 
a hand specimen, it is not so 
easy as that, since a piece of 
sedimentary rock shows no 
layers, either. But neither 
does the sedimentary rock 
show crystals. We can distin- 
guish a specimen of igneous 
rock, therefore, by the crys- 
tals, since sedimentary rocks 
do not, as a rule, contain 
crystals. Igneous rock is also 
much harder than most sedi- 
mentary rock. 

The hand specimen of ig- 
neous rock shows a uniform 
distribution of crystals, which 
may, however, be too small 
to see without a glass or mi- 

There are two classes of igneous rocks: light colored or 
acidic rocks like granites, and dark colored or basic rocks 
called gabbros or basalts. 

10. Coarse-grained igneous rocks. Ordinary granite usu- 
ally consists of at least three minerals: quartz, mica, and 

The quartz and feldspar are light colored and the mica 
may be, too; but it is often black. Yet on the whole the 

FIG. 10. Igneous Rock 
It is not stratified. 



general effect of a granite is that of light color. It usually 
has a granular appearance, and in fact, if the crystals are 
much larger than usual or much smaller, it is not popularly 
called granite. When the crystals are very large, sometimes 
up to many inches in diameter, the rock is called pegmatite. 
It is from pegmatites that we get large pieces of mica or 
feldspar as specimens. 

Most people use the name granite for any granular igneous 
rock, although strictly if there is no quartz, the rock should 
be called either syenite, dio- 
rite, or gabbro. Diabase is a 
rather fine grained, dark 
colored igneous rock, often 
called trap. The word trap is 
derived from the Swedish 
trappar, which means steps. 
The rock is apparently so 
called because of its columnar 
structure which gives it the 
appearance of steps (like the 
Giant's Causeway in Ireland, 
and the Palisades on the 
Hudson River near New 
York City). 

11. Other igneous rocks. 
When the grains of the min- 
eral are unequal in size, so that one kind of crystal stands 
out from a background of the rest, we call the rock a por- 
phyry (Fig. 11). We believe such rocks were formed in two 
different locations; at first, cooling of the molten rock or 
magma took place very far below the surface, forming the 
large crystals, but before the entire mass could crystallize, it 
was transported to a new location nearer the surface. Here 
the cooling took place more quickly, forming smaller crystals. 
When the crystals are so small that they cannot be seen with 
the naked eye, the rock is called felsite; and when no crys- 

FIG. 11. Porphyry, an Igneous Rock 

Containing Large Crystals Set in a 

Background of Small Crystals 



tals are seen even under the microscope, the rock is called 
a glass. Obsidian is a glass that has been poured out on the 
surface and hence chilled very rapidly, so that no crystals 

12. Sedimentary rocks. Most sedimentary rocks were 
formed by erosion of other rocks. The action of air and 

water causes exposed rocks to 
crumble, and the rain water 
gradually moves the pieces 
downhill. After movement 
has ceased, some substance 
in solution in water deposits 
a cementing material which 
holds the pieces together to 
form a rock. If the pieces 
are large we call the rock a 
conglomerate. Pieces about 
the size of sand grains 
form a sandstone. When the 
mass is like a fine smooth 
powder, the rock becomes a 

Conglomerates may contain pieces of any kind of rock. 
Usually the pieces are rounded because they were worn by 
movement from their original position on a hill to their 
final resting place, perhaps many hundred miles away. Very 
few minerals can stand being transported such distances; 
they wear down to powder before they get to their ultimate 
resting place. Quartz is one of the most common minerals 
and it is very hard; therefore conglomerates and sandstones 
are usually composed of quartz. However, the coquina of 
Florida is a limestone conglomerate (Fig. 12). 

Shale is composed of the smallest particles, which remain 
suspended even in quiet water, and that material is chiefly 
clay. The term shale is derived from the German word 
schale, a shell, used to denote the fact that shales often 

FIG. 12. Coquina, a Conglomerate 
Made of Shells 


break off in layers shaped slightly like a shell. Some shales 
do not show the shelly fracture and seem to be nothing 
but slightly consolidated mud. These are called mudstones. 
Sometimes a little sand will find its way into the mud and 
then it is called a sandy shale. 

Since the sedimentary rocks are deposited from water they 
form horizontal layers or strata, and this stratification is very 
noticeable at a distance 
(Fig. 9) where one can see 
a layer of conglomerate 
topped by a layer of sand- 
stone, for example. But it 
is a mistake to think that 
sedimentary rocks show 
layers and then expect to 

find such layers in a single FlG - 13 - Specimen of Limestone, 

f , Showing Fossils 

piece of conglomerate 

(Fig. 12). The strata as originally laid down are horizontal, 
but subsequent earth movements may tilt them up at an 
angle and also bend and twist them. 

Limestone is often formed from the shells of marine 
animals, and it is from these specimens or fossils that we 
learn something of the life of bygone periods of the earth's 
history (Fig. 13). When formed quite near the shore, mud 
may be mixed with the shells, and it gives the mass a gray 
color. We call this rock shaly limestone or if it is chiefly 
shale, calcareous shale. Chalk is a form of limestone made 
up of the shells of microscopic animals. 

*Limestone,* which chemically is calcium carbonate, CaCO 3 , is 
precipitated from marine waters containing calcium acid car- 
bonate, Ca(HCOs)2, in solution; when this solution is warmed, 
carbon dioxide, CO2, is driven off according to the equation: 
Ca(HCO 3 ) 2 - CO 2 + CaCO 3 4- H 2 0. This forms a limy mud, 
which is very fine grained; and most limestone has this origin. All 
kinds of limestone bubble when acid is applied. 

"Text in smaller type, marked by a *, is not essential and may be omitted. 


13. Other sedimentary rocks. The four types of sedi- 
mentary rocks already mentioned, conglomerate, sandstone, 
shale, and limestone make up the chief portion of the sedi- 
mentary rocks of the earth. But some of the rarer sediments 
are of considerable economic value. We shall say a word 
here about each of the following sediments: rock salt, gyp- 
sum, certain kinds of coal, and some iron ores. 

14. Rock salt. The largest salt deposit in the world, with 
an area 650 miles by 200 miles, is found in the rocks of 
Kansas, Oklahoma, Texas, and New Mexico. It is about 300 
feet thick. Much salt is also found in New York and 

In Europe there are large deposits of rock salt near 
Cracow, Poland, and the enormous deposit at Stassfurt, 
Germany, yields the salt, potassium chloride, valuable for 

In northern Chile, in the desert of Atacama, we find de- 
posits of Chile saltpeter, sodium nitrate, which is very useful 
for fertilizer and for the manufacture of explosives and dyes. 

Since the salt must have come from the evaporation of 
sea water, we believe the rock salt was formed while desert 
conditions prevailed. 

The great Salt Lake of Utah and the Dead Sea of Palestine 
are probably now undergoing evaporation which will, in 
time, produce salt beds. These lakes are both in semiarid 
regions so that they do not receive as much water as evapo- 
rates, and, as a result, the per cent of salt increases from 
year to year. 

+Gypsum, like rock salt, was deposited by evaporation of sea 
water. It is a soft rock and looks like some kinds of limestone, but 
it can easily be scratched by the nail and does not effervesce with 
acid. It occurs usually near the salt deposits in New York, Michi- 
gan, and Iowa. 

*15. Sedimentary iron ores. Two important ores of iron, hema- 
tite and limonite, are often sedimentary. Hematite is red and 
limonite yellow or brown. The sedimentary types are more like red 


or yellow earths. In fact the yellow, brown, or red color of many 
rocks and soils is due to iron compounds and in some places these 
compounds, dissolved out by water, have been precipitated in 
lakes or bays by the action of organic material. 

16. Sedimentary coal. Coal is usually found interbedded 
with shale and sandstone and sometimes limestone, showing 
that it is of sedimentary origin. Coal is really carbonized 
plant remains, trees, roots, and leaves, and we often find 
fossilized plant remains in the coal. 

*When trees fall in the forest, they usually decay or oxidize. 
The chief elements present in wood are carbon, hydrogen, and 
oxygen. On oxidation the carbon forms carbon dioxide while the 
hydrogen changes to water; nothing but a little ash remains. But 
when the tree is covered by water, so that oxygen cannot get at it, 
the chemical changes are quite different. Slowly some of the hydro- 
gen and oxygen combine to form water, and some of the carbon 
unites with the hydrogen to form marsh gas or fire damp, CH 4 , 
which is often found in coal mines. Thus gradually the hydrogen 
and oxygen are eliminated and the carbon is left. 

Partially changed roots, leaves, and other matter when 
consolidated by pressure become peat. As the change pro- 
gresses, brown coal or lignite is formed. 

17. Metamorphic rocks. When any kind of rock is sub- 
jected to markedly changed conditions within the earth, 
usually heat, pressure, water, and motion, it is changed in 
many ways and we say it has been metamorphosed. In 
biology we speak of plants and animals adapting themselves 
to their environment. In geology we find, likewise, that rocks 
frequently adapt themselves to their environment. For ex- 
ample, under great pressure a rock takes up less room - 
it becomes denser, ft it can; and sometimes if the chemical 
elements are present to form a new mineral, which is denser 
than any of those present, that change will take place. For 
example, graphite is more dense than coal; hence when coal 
is metamorphosed, graphite is often formed. Again lime- 
stone and marble are both calcium carbonate, but marble, 


formed by metamorphism, is more dense than limestone. 
If the limestone contains silica as an impurity, then a new 
mineral, calcium silicate, is formed, which is denser than 
calcium carbonate. 

Under heat and pressure, a sandstone, which is quite 
porous, is converted into a quartzite, in which the pores 
have disappeared; the edges of the grains of sand have been 
fused together. But it is under heat, pressure, and motion 
that we get the most profound metamorphism. We get more 

FIG. 14. Banded Metamorphic Rock 

dense minerals, as before, but now the crystals arrange them- 
selves in the direction of the movement, giving a banded 
appearance; those minerals are formed which have flat faces, 
like mica, and which are smooth, like talc and graphite. 
Shale becomes slate, which, under the microscope, reveals 
flakes of mica with their flat faces parallel to each other, 
producing the characteristic slaty cleavage or schistosity. 
Some minerals, like mica, and some rocks, like slate, break 
or split readily in one direction, leaving flat, shiny faces. 
This is called cleavage. 

Metamorphic rocks often have a banded appearance due 
to segregation of minerals of one kind in layers, which at 
first sight give the appearance of strata (Fig. 14). These 
bands will often show in a small hand specimen. They can 
easily be distinguished from sedimentary rocks. Their con- 


stituents are crystalline and hard, resembling igneous rocks. 
Igneous rocks, however, show no bands. Schistosity or fo- 
liation (Fig. 15) is characteristic of metamorphic rocks, but 

Photo by Keith, U. S. G. S. 

FIG. 15. Metamorphic Rock (Slate) Showing Foliation 

there are some, like quartzite and marble, which do not 
show it at all. In both these cases, schistosity, which is due 
to segregation of different minerals, could not develop, since 
each rock consists of only one mineral. Quartzite is made 
entirely of quartz, and marble entirely of limestone. 



18. Kinds of metamorphic rocks. The following table 
shows the sedimentary and igneous rocks from which the 
metamorphic rocks were formed. 







slate and schist 

coarse granite 
fine-grained granite 


schist and slate 

hornblende schist 
and serpentine 

A few words about each of these metamorphic rocks will 
be sufficient to make one acquainted with them. 

Gneiss (pronounced nice) 
frequently looks like granite, 
and consists of quartz, feld- 
spar, and mica with the min- 
erals arranged in bands 
(Fig. 16). 

Quartzite. A quartzite dif- 
fers from a sandstone by be- 
ing much less porous and 
much firmer. It looks rather 
glassy. The magnifying glass 
reveals the fusion of one grain 
of sand with the other, and, 
when broken, it will be found 
that the grains have frac- 
FIG. 16. Gneiss Showing Bands tured rather than separate 

from each other. 

Slate is partially metamorphosed shale. It cleaves into 
thin plates, whence its use for roofing and blackboards. 

Schists resemble gneisses except that the bands are thinner. 
They consist chiefly of quartz and mica and these are ar- 
ranged in bands, producing schistosity. 

Mica schist consists chiefly of mica. It often contains garnet 
as secondary mineral. When the amount of garnet is rather 
noticeable, we call the rock garnet schist. If it contains much 
talc, it is called talc schist. A rather common variety is horn- 


blende schist, dark green or black, with hornblende crystals 
in parallel layers, imparting a silky appearance to the rock. 

Marble is formed by metamorphism of limestone. It is 
crystalline, usually harder than limestone and therefore can 
be polished, but shows no banding or cleavage when pure. 
It is often white, but all varieties of colored marbles are 
found; the color is due to small quantities of impurities, 
which often are segregated in streaks. It is easily scratched by 
a knife and therefore is easily worked, which makes it an ideal 
medium for statuary, ornamental objects, and building stone. 

Coal. We have mentioned the origin of sedimentary coal 
on page 19. When this is subjected to heat and pressure, 
bituminous coals are formed which contain less hydrogen 
than the brown coals, and more free carbon. They are dull 
black in color and crumble readily. Bituminous coal burns 
with a smoky flame. When the beds of coal are intensely 
folded we get a very compact coal called anthracite. It is 
almost free of hydrogen, contains about 90% free carbon, 
has a high luster, and burns without smoke. 

Completion Summary 

A natural inorganic substance which is not a mixture is 
called a - . Such a - - must have the same - 
whenever or wherever we find it. 

The crust of the earth is made of - . Some - - con- 
sist of one mineral; most rocks are composed of - . 

There are three classes of rocks : - , - , and - . 

- rocks are formed from - material, sedimentary 
rocks from- ; metamorphic rocks may be - either. 

The surface rocks are usually - , while deeper down 
they are - . Granite consists of - . 
Dark colored igneous rocks are called - . 

- rocks are stratified, but the - - cannot be seen 
in a hand specimen. Small specimens which seem to show 
stratification are - . This arrangement of minerals in 
parallel layers is called . 


Three kinds of sedimentary rocks are . These sedi- 
mentary rocks differ from each other in - . 
Rock salt is a - - rock. It was formed - . 
Metamorphic rocks are formed from or . They 

- igneous rocks, because they often show crystals, but 
they differ from igneous rocks - . 
The true coals are - - rocks. 







from a 

Large crys- 


Small crys- 

Granite, diorite, syenite, 
gabbro, diabase 

Large and 
small crystals 



Felsite, basalt 


Obsidian, pumice, basalt 




Conglomerate, sandstone 


Shale, limestone, peat, some 
iron ores 


Salt, gypsum 


Igneous or 
tary rocks, 
changed by 
heat, pres- 
sure, water, 
and move- 

banded and 

Gneiss, schist 

Very fine 



Quartzite, anthracite 





1. What is a mineral? Why do we consider water a mineral? 

2. Name five minerals. 

3. Name three rocks. 

4. What is a rock? How does it differ from a mineral? 

5. Why is not quartz considered a rock? 

6. Name a rock which is also a mineral. 

7. Name a mineral which is not a rock. Explain. 

8. Name a rock which is not a single mineral. Explain. 

9. Name the classes of rocks, with one example of each class. 

10. How can one distinguish sedimentary rocks, in the field? 
Make a diagram to illustrate. 

11. How can one tell an igneous rock in a hand specimen? 

12. In what ways are igneous and metamorphic rocks alike? 

13. How can one distinguish banding from stratification? 

14. Describe a granite, both in composition and appearance. 

15. How does a porphyry differ from a pegmatite? 

16. Name the classes of sedimentary rocks, with one example 
of each class. 

17. Why is sand usually made of quartz grains? 

18. Why are sedimentary rocks stratified? 

19. What is a shaly limestone? 

20. Name a sedimentary coal. Why do we consider it sedi- 

21. How are metamorphic rocks formed? 

22. How do limestone and marble resemble each other? How 
do they differ? 

23. How do sandstone and quartzite differ? 

24. Explain how the banded appearance of some metamorphic 
rocks has been brought about. 

25. What is schistosity? How does it arise? 

26. Name three kinds of metamorphic rocks and tell how each 
was formed. 

27. In what way is gneiss like granite? In what is it different? 

28. How can one distinguish a schist from a gneiss? from a 
sedimentary rock? . 

29. How is bituminous coal formed from peat? 

30. How can slate be distinguished from shale? o'Cr'Yv 


if Optional Exercises 

31. Is air a mineral? Explain. 

32. We say the earth is made of rocks. Why can we not say the 
earth is made of minerals? 

33. Why is it that a great part of the cores of mountains is 
made of metamorphic rocks? 

34. How does a felsite differ from a porphyry? 

35. How can one distinguish felsite from a fine-grained sand- 

36. Why are the grains of sedimentary rock usually uniform 
in size? 

37. Why are sedimentary rocks often in horizontal layers? Why 
are they not always horizontal? 

38. Why do we consider salt a sedimentary rock? In what ways 
does salt differ from the ordinary sedimentary rocks? 

39. How does the decay of wood differ from the decomposition 
that forms coal? 

40. Explain why metamorphic rocks are usually denser than 

41. Explain why some metamorphic minerals, like graphite and 
talc, are smooth. 

42. Explain the metamorphism of wood into the various forms 
of coal. 

43. What is the most common surface rock in the United 
States? Which is the least common? 

44. Coarse-grained granites are now on the surface in New 
England. What does that prove about the elevation of the sur- 
face in those localities? 



*19. It is worthy of note that from 1900 to 1925 more mineral 
products were taken out of the earth than during all of previous 
history. Modern industry depends to a great extent on power 
and machinery. Power is chiefly dependent on coal and oil, 
while machinery is practically all iron and other metals. In other 
words, modern civilization is dependent on our mineral resources, 
and those nations that are fortunate enough to be rich in mineral 
resources, particularly coal and iron, are the wealthy nations. 
Wars are often caused by the desire of one nation to possess itself 
of natural resources. 

The " Great Powers in World Politics" by Simonds and Emeny, 
recognizes two groups of world powers. (1) Those richly endowed 
with natural resources. (2) Those that lack sufficient resources. 
The second group must either accept material inferiority or seek, 
by conquest, to expand its territory. The first group seeks to 
maintain the status quo. 

The per cent production of some important industrial minerals 
by the major countries is shown in the following table: 










Great Britain 

and possessions 







France and pos- 
























USSR (Russia) 







United States. . . 









"This entire chapter is optional. 


It will be noticed in the table that the United States ranks very 
high in mineral resources and if to these be added its agricultural 
resources, our prosperity is readily understood. 

*20. Coal. The world's production of coal from 1900 to 1935 
is shown in Fig. 17 and the percentage of this total now being 




FIG. 17. The World's Production of Coal 

produced by the major countries is shown on page 27. The annual 
value of coal produced in the United States is about one and one 
half billion dollars. Pennsylvania produces about one third, West 
Virginia one quarter, Illinois one tenth, and Kentucky one tenth. 
The estimated coal reserves of the world at the present rate of 
consumption will last about five thousand years. 

Anthracite is rather rare, being found only in Pennsylvania and 
in Great Britain. The reserves are rather small, and it is estimated 
that in about one hundred years there will be no more anthracite. 

*21. Petroleum was first used because kerosene, useful in 
lamps for illumination, could be extracted from it. The chief 
product obtained from petroleum today is gasolene for automobile 
and airplane fuel. Other products include lubricating oils, fuel oil, 
petroleum jelly, and paraffin. 

In this country the annual production is about one billion 
barrels, worth about one billion dollars. This is about sixty per 
cent of the total world production, and the estimated reserves in 
the United States will last less than twenty years. 


Other important world producers of oil are Russia, Persia, 
Venezuela, and Mexico. Germany has recently developed a process 
of distilling gasolene and other products from bituminous coal and 
shale, but this gasolene costs much more than that obtained from 

Texas produces 40% of the total output of the United States, 
California, 20%, and Oklahoma, 20%. 

*22. Metallic ores. Of the metallic minerals in the earth's 
crust, iron makes up about 5% and aluminum 7%, but these are 
not in the form of metals. The only metals that are found free or 
native are those that are inactive chemically, that do not combine 
readily with oxygen, water, and carbon dioxide of the earth's 
atmosphere, and sulphur compounds, found in the depths of the 
earth. Gold, silver, platinum, and a little copper are found native, 
while most of the others, including all the useful metals, iron, zinc, 
lead, aluminum, and most of the copper, are found as compounds. 
Furthermore, many of these compounds are difficult to work, that 
is, to extract the metal from them. For example, most of the 
aluminum of the earth's crust is found in the form of feldspar and 
clay from which we can extract the metal, but only at great cost. 
Only one compound of aluminum is workable ; the mineral bauxite. 
Likewise many minerals like pyrite and hornblende contain iron, 
but it would not be profitable to extract it from these minerals. 
On the other hand, it is comparatively easy to extract iron from 

We call bauxite an ore of aluminum, and hematite, an ore of 
iron, but clay, feldspar, and hornblende are not called ores. 

*23. Origin of ore bodies. If we turn back to page 5 where we 
discussed the nature of the interior of the earth, we shall recall our 
hypothesis that there is a core of metal at the center of the earth. 
It is very likely that this consists chiefly of iron, but it is also 
likely there are other metals in the metallic core. Most of our ore 
bodies are found in or near a zone of contact between igneous and 
sedimentary rock; and we believe, therefore, that the metallic 
compounds were brought from the depths in the molten igneous 
rock, from which they were diffused into the sedimentary rock by 
means of hot gases and water under great pressure. 

Near the surface, percolating ground waters carrying oxygen 
and carbon dioxide have usually changed the original ore body 



into oxides and carbonates, but deeper down most ores are sul- 
phides. A common way of mining ore bodies is to blast through the 
rock from the surface until the ore body is penetrated (Fig. 18). 
This opening is called a shaft, and from the shaft, tunnels may be 
driven in all directions into the ore body by blasting it out and 
carrying it to the surface in cars. One of the deepest mines in the 
world, at Calumet, Michigan, is over a mile deep. It becomes ex- 
pensive to mine at great depths 
and it would be impossible to 
mine at depths of ten miles or 
more. The deeper we go, the 
greater the pressure of the over- 
lying rock, and if we get down 
far enough, the pressure is be- 
yond the breaking point of the 
rock, so that if a shaft were 
blasted into the rock, it would 
collapse and be filled with rock 
again. There is no known way 
to solve this problem and we 
seem confined, for our ore re- 
sources, to surface deposits or 
those less than two miles deep. 
*24. Iron. The United States 
leads in producing more than 

one quarter of all the world's iron ore. Russia is second, Germany 
third, Great Britain fourth, and France fifth. 

Our production in 1935 was thirty million tons, from which iron 
worth three hundred sixty million dollars was made. The principal 
producing states are Minnesota, twenty million tons, Michigan, 
seven million, and Alabama, three million. There is as yet no sign 
of exhaustion. The principal ore of iron is hematite. It is dark red 
in color, but looks black when compact, as in specular iron ore (ore 
with metallic luster). Other ores are magnetite (lodestone) and 
limonite, a rusty looking mineral. 

*25. Copper. Copper is used chiefly in electrical transmission 
because it is a good conductor of electricity. It is also used to make 
brass, bronze, and other alloys which do not rust. 

The United States produces about 20% of the world's copper, 

FIG. 18. Cross Section of a 
Coal Mine 


Chile is second with about 17%, Canada about 13%, and Rhodesia 
about 12%. 

In 1935 we produced about seven hundred million pounds of 
copper, worth sixty million dollars. The chief producing states are 
Arizona, Montana, Utah, Nevada, and Michigan. Michigan pro- 
duces native copper in a very pure condition. It is known in the 
trade as lake copper. 

The chief ore of copper is chalcopyrite, a complex sulphide of 
copper and iron. According to estimate, the world's copper ore 
reserves will be exhausted in about seventy years. 

*26. Zinc. Zinc is used chiefly to make galvanized iron. The 
zinc prevents the iron from rusting. Much zinc is also used to make 
brass, an alloy of copper and zinc. Zinc oxide, an important paint 
base, is made from zinc. 

We produced in the United States in 1935 about one half mil- 
lion tons of zinc worth about forty million dollars. 

The principal zinc ore producing states are Oklahoma, Kansas, 
New Jersey, and Montana, and the reserves are sufficient to last 
only about twenty years. The chief ore is sphalerite, a sulphide of 
zinc, called by the miners rosin jack or black jack because of its 
luster and color. In the metal market zinc is called spelter. 

*27. Lead. Lead is used chiefly to make white lead, a carbonate 
of lead, which is the chief paint base. It has a variety of other uses 
among which are storage batteries, lead pipe, solder, and type metal. 

We produce about one fifth of the world's output chiefly in the 
states of Missouri, Idaho, Utah, and Oklahoma. The chief ore is 
gakna, a sulphide of lead which occurs in easily recognized heavy 
metallic cubes. 

*28. Aluminum. Aluminum finds its chief use today in the 
making of alloys combining light weight with strength and resist- 
ance to corrosion. These alloys are used very largely in aircraft, 
automobiles and other vehicles, and innumerable small articles. 
Cooking utensils of aluminum are light in weight, conduct heat 
well, and do not tarnish. 

As mentioned before, more than seven per cent of the earth's 
crust is aluminum, and it will at some future time supplant iron; 
but none of it is native, and most of the compounds are not ores. 
There is practically only one ore, bauxite, an oxide of aluminum. 
Practically all of our bauxite is in Arkansas. 


During 1935 we produced about one hundred million pounds of 
aluminum, valued at twenty million dollars. 

*29. Sulphur. Sulphur is used chiefly to make sulphuric acid, 
which is necessary in some part of practically every industrial 
process. Other uses are in making paper, explosives, dyes, and in 
vulcanizing rubber. Sulphur is usually found around volcanoes, 
and during an eruption the smell of sulphurous gases is very 

In Sicily and in Texas and Louisiana the sulphur is found 
native. In Sicily it occurs in volcanic rock from which it is easily 
melted out. In Texas it is mined by forcing hot water down to the 
deposit through a pipe. The melted sulphur mixed with water is 
forced up through another pipe by air pressure. The sulphur 
produced in the United States in 1935 was one and one half 
million tons, worth thirty million dollars. Much of our sulphuric 
acid is obtained from pyrites or fool's gold, a sulphide of iron, and 
as a by-product of the smelting of copper, lead, and zinc sulphide 
ores. In each of these smelting operations, the metal cannot be 
extracted without burning out the sulphur and, since it is illegal 
to permit the noxious fumes of sulphur to escape into the at- 
mosphere, the smelters are forced to convert it into sulphuric 

*30. Building stones. In comparatively young countries, like 
ours, wood is the chief building material; but in older countries, 
most of the wood has been used up and they have had to look 
about for more lasting materials. There is nothing so lasting as 
stone, especially stone taken from a surface outcrop, because, since 
it has survived exposure to the natural agents of change, air, and 
water for perhaps millions of years, it will probably last the few 
score years for which it is needed as a home. 

All kinds of rock are used as building stone, but as a general rule, 
the softer and more porous sedimentary rocks are not so durable 
as the igneous rocks. If the rock is very porous, as a sandstone is 
apt to be, it will get soaked with water and in cold weather when 
the water freezes, the rock will chip off, because the water expands 
when it changes to ice. 

Quartzite, a metamorphosed sandstone, makes a very durable 
building stone, because it is very compact. In 1935 we used about 
twenty million dollars' worth of stone for buildings, about fifty 


million for roads, etc. In other years we spent about two hundred 
million dollars for stone. 

In quarrying stone, dynamite is used and if there are any joints 
in the rock these are taken advantage of. Channeling machines 
are often used before blasting and, in soft stones like slate, wire 

The blocks of stone are subsequently cut up by sawing and 
chiseling and finished by chisels, lathes, rubbing tables, and polish- 
ing machines. 

*31. Granite. This term, in the trade, includes all igneous rocks 
as well as gneiss. Granites are the most durable building stones but 
they are much more difficult to quarry and to work. Most of the 
Egyptian monuments like the Sphinx, and the statues of Egyptian 
kings and queens are made of granite. At Baalbek, in Asia Minor, 
the Romans built a temple containing one hundred and fifty 
granite columns, each seventy feet high and seven feet in diameter. 
These columns are over two thousand years old, but their surfaces 
are still smooth and lustrous. 

There are many places where granites can be obtained since the 
chief mass of all continents is granite. But in most places it does 
not outcrop, that is, it does not appear on the surface, because it is 
covered by sedimentary rocks. In an old region, however, like the 
Appalachians, the sedimentary cover has been worn off in many 
places and reveals the igneous or metamorphic core of the moun- 
tains. Granites are quarried in the hilly belt from Maine to Ala- 
bama. In the west the chief producing state is California. Traprock 
(diabase) from New Jersey is used a great deal as crushed stone 
in building roads. On account of its columnar structure it is easily 
quarried (Fig. 19). 

*32. Limestone and marble. Both are forms of calcium car- 
bonate and, as such, they are soft, easily scratched by metal, and 
slowly dissolved by carbonic acid. For that reason characters en- 
graved on these stones are gradually erased by exposure to air. 
However, they are not too porous and hence do not chip; and 
since they are soft stones, they are easy to work up into statues 
and monuments. 

Some of them are streaked with other minerals, pyrite being 
particularly obnoxious because it rusts badly. Many of the lime- 
stones, like the famous Indiana variety, contain fossils which give 




U. 8. G. S. 
FIG. 19. Columnar Structure in Traprock 

Such rock is easily quarried. 

the stone a pleasing granular appearance, but at the same time 
they collect dust and require frequent cleaning. 

In the United States, Vermont is the chief source of marble. 
Indiana produces most of the limestone used in building. Much 
limestone is used in extracting iron from its ores. 

*33. Sandstones. These make rather weak building stones ; but 
quartzite, which is often called sandstone, is strong and resistant 
to weathering. 

There are many varieties of sandstone: bluestone, brownstone, 
flagstone, and freestone. Brownstone from the Connecticut Valley 
was at one time very popular in New York City, but time has not 
been very kind to the buildings in which it was used. Flagstone is 
a clayey sandstone used for sidewalks, because it is thinly bedded 
and splits into blocks of proper thickness. Freestone is used fre- 
quently because it is easily worked. 

The famous Berea grit from Ohio is a sandstone much used for 
fancy work because of the uniformity of its grain. Most whetstones 
are made of sandstone, because the mineral quartz of which it is 
composed is harder than steel. 

*34. Slate. Slate is a metamorphosed shale which has good 
cleavage, splitting easily into thin, smooth sheets, because of the 
arrangement of scales of mica parallel to each other. Slate is fairly 
durable. Its chief use has been for roofing, but of late years 


many artificial roofing materials have been developed which are 
depriving slate of its market. 

Most of our slate and sandstone comes from Pennsylvania. 

*35. Clay products. Bricks, earthenware, porcelain, tile, and 
some cements are made of clay. The product is decorated after 
the first baking and before the glaze is applied. 

The red color of bricks is due to iron compounds in the clay 
which, on burning, change to ferric oxide. A pure variety of clay 
which remains white on burning is called kaolin. The value of the 
clay products of the United States, in 1934, was one hundred and 
twenty million dollars; in 1927, four hundred million, while ce- 
ments add another two hundred and fifty million. 

Clay is very widely distributed over the earth, since it is a 
product of the weathering of feldspar, one of the constituents of 

*36. Precious stones. Gems are minerals which are valued for 
ornamental purposes because of their color, luster, brilliance, and 
durability. A piece of glass, for example, might be cut and polished 
so as to give it considerable brilliance, but it is not durable and 
therefore has not the value of a diamond or ruby, which are very 
hard and tough. By the same standard, a diamond with a bad 
flaw has very little value. 

Durability in a gem depends mainly upon its hardness, which is 
measured by scratching. If a mineral makes a visible scratch on a 
piece of glass, it is harder than the glass. The standard scale of 
hardness is : 

diamond 10 apatite 5 

ruby 9 fluorite 4 

topaz 8 marble 3 

quartz 7 gypsum 2 

feldspar 6 talc 1 

Diamonds are pure crystalline carbon, the hardest substance in 
the world. Dark colored, poorly shaped diamonds are called bortz 
and are used for polishing other diamonds. Carbonadoes are tough, 
dark diamonds, used for drilling rock. Diamonds seem to have been 
formed when an igneous mass under great pressure penetrated a 
shale or other sedimentary rock containing carbon. It is a meta- 
morphic mineral with a specific gravity of 3.5. At Kimberley, 


South Africa, the diamonds occur in volcanic necks or pipes which 
cut through carboniferous shales. 

The chief producing areas are in South Africa and in Brazil. In 
the United States a few diamonds are found in Arkansas in forma- 
tions resembling those in South Africa. 

Emerald is a variety of a mineral called beryl, a beryllium 
aluminum silicate. Its green color is due to chromium. It is harder 
than glass but seldom without flaws. If without flaws, emeralds 
are equal to diamonds in value. Brazil and India are the chief 

Ruby and sapphire are both the same mineral, corundum (alumi- 
num oxide). Ruby is red and sapphire is blue; both colors are due 
to impurities. They are, next to diamond, the hardest substances 
known. Most sapphires come from Siam, but a few are found in 
Montana. The best rubies come from Burma, but a few are found 
in North Carolina. 

Rubies are rarer than sapphires and hence more expensive; 
a large, fine specimen is worth more than a diamond. They are very 
heavy minerals, even denser than diamond (sp. gr. = 4). Very 
good rubies are now made artificially. They are in every respect 
the same as the natural specimens and can be distinguished only by 
microscopic test. 

Rubies must not be confused with garnets, which are not nearly 
so hard nor so heavy (nor so valuable). 

Topaz is aluminum fluosilicate. Its color is often yellow, but 
other colors are frequent. It is as hard as emerald, but not so hard 
as ruby. It is as dense as diamond (sp. gr. = 3.5) and hence it may 
be inferred that it is usually found in metamorphic rocks. There is 
a yellow variety of quartz called false topaz, but this material is 
light in weight and not so hard as topaz. 

The best topazes come from Ceylon and Brazil. In the United 
States we find some in Maine and Colorado. 

Tourmaline is a silicate of aluminum and boron, about the same 
hardness as quartz, but not nearly so heavy as the other gem 
minerals. It is widely distributed in metamorphic rocks, and it 
varies in color from brown to black. The red and green varieties 
are highly prized. The green tourmalines are easily distinguished 
from emeralds. The tourmalines are very dark green and not so 
hard as emeralds. The red variety is not so dark as ruby nor so 


hard. Brazil, Russia, and Ceylon furnish many good tourmalines. 
Green tourmalines are found in Maine. 

Turquoise is a copper aluminum phosphate of waxy luster and 
bluish green color. It is rather soft, not so hard as quartz, and of no 
great value. The best variety comes from Persia. In the United 
States we get turquoise from Arizona and New Mexico. 

Quartz is silica, a crystalline mineral resembling glass in general 
appearance. It has a hardness of 7. It is sometimes called rock 
crystal and, when cut, it is 
known as rhinestone. Quartz is 
common all over the world, but 
especially good specimens are 
found in Brazil, Switzerland, 
and in New York State. 

Amethyst is blue quartz, car- 
nelian is red, and there are other 
varieties, such as milky quartz, 
rose quartz, bloodstone, contain- 
ing red spots in a green back- 
ground, and chalcedony, which 
has a waxy luster. FIG. 20. Agate 

Agate (Fig. 20) is banded 

chalcedony, and onyx is agate cut in slices which are parallel to 
the bands. 

False topaz is yellow quartz, and chrysoprase is a green variety, 
often mistaken for jade. 

Opal is a variety of quartz, but it is not crystalline. It occurs in 
all colors and is usually iridescent. 

It is often found in silicified wood, which makes it clear that the 
silica in solution filled the crack or opening and slowly precipitated 
out in the colloidal condition. 

The prettiest specimens, resembling beautiful landscapes, are 
full of flaws and hence are easily broken. Hungary produces the 
best opals. In North America, Mexico and Oregon furnish some 
inferior specimens. 

Jade. Much of the material sold as jade is chrysoprase, a green 
variety of quartz. Jade may be either one of two minerals: jadeite 
or nephrite, both of them resembling the feldspars. Their hardness 
varies between 6 and 7, and the specific gravity is about 3.3. The 


Chinese make very artistic carvings in jade, which they value 
above all other gems. The mineral jade, of itself, has no great 
value. The value of a finished specimen of jade is due to the rich- 
ness of color, the toughness of the piece, and the artistic nature of 
the carving. There are many imitations of jade in softer minerals, 
but these have very little value, because they are easy to make, 
and do not last long. Jade is found in Burma and China. 

^Completion Summary 

The - - of a nation depends upon its - resources. 

The United States is first in the production of - . 

The two states, - - and - - produce more than - 
the coal of the United States. 

The chief product of - is gasolene. Nearly half the petro- 
leum comes from - . 

Pyrite is not an ore of iron, because - . The chief ore of 
iron, , is mined in - . 

The chief ore of copper is 
The chief ore of zinc is - 
The chief ore of lead is 

Sulphur is mined in - - and in - . It is used in the 
manufacture of - , - , and - . 

Granite is a good building stone, because . 

Marble is - - limestone. 

Porcelain and earthenware are made from . 

Precious stones must be durable, , and . 


1. Explain the relation between natural resources and the 
wealth of a nation. 

2. Why is there so little anthracite coal in the world? 

3. Which are the chief coal producing states? 

4. Name some products of the refining of petroleum. 

5. Name three important oil fields in the world. 

6. Name three important oil fields in the United States. 

7. What two metals make up more than 5% each of the earth's 

8. Which metals are found native? Why? 


9. What is an ore? How does it differ from a mineral? 

10. Explain the probable origin of ore bodies. 

11. Name two ores of iron. Where is iron found in the United 

12. Name one ore of copper. Where is it found? 

13. Name one ore of zinc. Where is it found? 

14. What is the chief ore of lead? Where is it found? 

15. What is the ore of aluminum? 

16. Name two important uses of sulphur. What is the source of 
our sulphur? 

17. Name three rocks used as building stone. 

18. Why do granite and quartzite make durable building 

19. Why does marble take a smoother polish than limestone? 

20. Why is most sandstone unsuited for long exposure to 

21. What is a whetstone? What is it made of? 

22. Of what use is slate in building? 

23. What are bricks made of? Name one other product of the 
same material. 

24. What properties should a precious stone have? 

25. What is the difference between diamonds, bortz, and car- 

26. What are the two chief sources of diamonds? 

27. In what way do ruby and sapphire differ? 

28. Name several gems which are varieties of quartz. 

29. In what way does agate differ from ordinary quartz? 

30. What is opal? 



37. The surface of the earth, as we find it, is covered 
by loose, unconsolidated material, such as gravel, sand, clay, 
soil, or earth. The original surface of the earth was every- 
where rocky and it has been brought to its present condi- 
tion by the action of "wind and weather." In physiography 
this process is called weathering. Superficial examination of 
rocks convinces us that they last forever but more careful 
examination shows that even rocks change. Rock just taken 
from the quarry looks quite different from the same rock 
which has been exposed to weather for a long time. A 
granite boulder has a dull surface but, if a piece is chipped 
off, the fresh surface is bright and sparkling. 

The chief agents responsible for weathering of rocks are 
(1) air, (2) water, (3) temperature changes, and (4) plants 

and animals. The methods by which 
they cause changes are of two kinds, 
mechanical and chemical. 

38. Frost action. Most liquids 
contract in freezing, but water is 
an exception; it expands, and the 
force of this expansion is irresistible. 
When water freezes in iron pipes, 
the iron is broken by the expansion 
of the water. Just so, water finds its 
way into pores, fissures, and joints 
of rocks and, in winter, as the water 
freezes, it breaks off pieces of rock. 
At the base of every cliff, in humid regions where freezing 
takes place, we can find accumulations of rough, broken 


FIG. 21. Iron Pipe Broken by 

the Expansion of Freezing 




pieces of rock. This is called talus. Igneous and metamorphic 
rocks are least affected by frost action because they are not 
highly porous. Sedimentary rocks, particularly sandstone, 
are most easily attacked be- 
cause they are very porous. 
Hence, sandstone used for 
building purposes in a moist 
region, where the winters are 
severe, usually scales off and 
becomes unsightly. 

39. Weathering due to 
water and air. Water alone 
has practically no effect on 
rocks. Rock salt and gypsum 
are soluble in water but such 
rocks are found only in arid 
regions. Air alone has no ef- 
fect on rocks, but air and 
water together are the chief 
agents of weathering. Oxy- 
gen and carbon dioxide are the gases of the air which, with 
water, cause important chemical changes in rocks of all 

Oxygen and water attack the iron found as a constituent 
of many dark colored minerals, changing it to ferric oxide 

(rust) . That accounts for the 
yellow, brown, or red colors 
of most rocks and soils. 

Limestone is completely 
dissolved by water carrying 
carbon dioxide in solution, al- 
though pure water has prac- 
tically no effect whatever. 

FIG. 23. Caverns in Limestone 

Dissolved limestone makes 

hard water. This is carried to the ocean, where animals ex- 
tract the limestone to build their shells. Limestone in humid 

FIG. 22. Talus at the Foot of a Cliff 


regions is most rapidly eroded, and the solution of great 
masses of the rock forms caves and other structures. 

Most limestones contain some sand or clay and when the 
limestone is dissolved, the sand or clay remains, unaffected 
by weathering, as a loose covering or earth. The fertile soils 
of some limestone regions, as in southern Kentucky, are due 
to this kind of weathering. 

Sandstones whose cementing material is either a calcium 
compound (limestone) or an iron compound (rust) are rap- 
idly disintegrated by water and carbon dioxide, which dis- 
solve out the calcium or iron compounds and leave a mass 
of sand. But where the cement is silica, the sandstone is 

Slate and shale crumble to clay under weathering, but 
other metamorphic rocks, the gneisses and schists, are as 
resistant as the igneous rocks. 

When granite is weathered, the quartz is unaffected and 
when the other minerals are removed, the quartz remains 
as sand. The mica is decomposed very slowly. 

*The feldspar, KAlSiaOg, is slowly attacked by water and carbon 
dioxide; the potassium, K, forms potassium carbonate, which is 
soluble in water, while the other elements, aluminum, silicon, and 
oxygen, Al, Si, and 0, combine to form clay. The products, then, 
of the decomposition of granite are potassium carbonate, which 
helps to fertilize the soil, residual clay, and sand. 

As we have seen, the continents were made of granite, 
and hence the weathering of granite gave us the chief ma- 
terials which now cover the rocks, clay and sand; and these 
mixed together are called earth. Loose material of any kind 
which covers the rock is called rock mantle. It may consist 
of boulders, gravel, sand, clay, silt, earth, or soil. 

40. Effect of temperature changes. Exfoliation. In arid 
regions, frost action plays no part in weathering, but in 
those regions the clear atmosphere permits great variation 
in temperature. During the day the heat of the sun easily 
penetrates the air and bakes the surface of the rock. Since 



most rocks are very poor conductors of heat, only a thin 
surface skin is baked and expanded. At night, radiation is 
rapid, because of the clear 
air, and the rocks cool rap- 
idly. The alternate expansion 
and contraction splits off 
pieces of the rock, like the 
skin of an onion. This proc- 
ess is called exfoliation. 

Exfoliation occurs also in 
humid regions where it can- 
not be accounted for in this FlG 24 Exfohation of a Boulder 
way. Some geologists explain 

the splitting off of shells as due to chemical changes, chiefly 
absorption of water (hydration) with a consequent increase 
of volume which causes pressure. 

Sedimentary rocks under the direct heat of the sun are 

sometimes found to buckle and 
break, like cement sidewalks in 

41. Weathering due to plants 
and animals. Plant roots find 
their way into openings in rocks 
and enlarge them. The roots of 
a tree are often found growing 
in a rock crevice. In this way as 
the roots increase in size they 
split the rock. Animals that 
burrow in the earth, like ants 
and worms, increase the extent 
of surfaces exposed to weather- 
ing and therefore hasten the 

Man assists weathering by 
clearing the forests and removing the vegetable covering 
of the soil. This exposes new surfaces to wind and water. 

FIG. 25. What split the rock? 


All these processes of plants and animals are mechanical, 
but plants also help in chemical weathering. Decaying plant 
material in the soil, called humus, is a source of the car- 
bonic acid and the other acids which attack the underlying 

The carbonaceous material of plants also causes a change 
in the color of the soil. The red color is due to oxidized 
iron (rust) produced by the chemical weathering of some 
rocks. There are two classes of iron compounds, ferrous and 
ferric. The latter are red, yellow, or brown in color. The 
ferrous compounds are almost colorless. Hence the reduc- 
tion of ferric to ferrous compounds by the carbonaceous 
material of the humus results in a change from red soils 
to colorless and finally to black. 

Completion Summary 

The action of natural agents on rocks is called , 

Weathering is of - - kinds : - . 
Talus is formed chiefly by - . 

- and of the air cause chemical in rocks. 
Some of the compounds formed are soluble in water and 
are renioved in that way. The other constituents of the 
rock then - , forming residual rock mantle. 

- is one rock which is completely soluble in water 
and carbon dioxide. 

The cementing material of sandstone is often . In 
weathering, this is - , and the material that remains 

- and - - are the chief products of the weathering 
of granite. Granite, therefore, - - soil. 

Exfoliation - - arid regions. 

Plants and animals - - weathering; the plants , 

and the animals - . 

Soils are often red because . 



1. Why do we have the impression that rocks last forever? 

2. What are the agents of weathering? 

3. Explain the effects of frost action. 

4. What is talus? 

5. What kind of rock is most easily attacked by frost 

6. In what rock is solution the most important factor in 

7. How is sandstone affected by weathering? 

8. How does weathering attack metamorphic rocks? 

9. Why is quartz always a product of the weathering of 

10. What are the chief constituents of rock mantle? 

11. How does rock mantle differ from earth? 

12. What are the chief constituents of earth? Wliy? 

13. What is exfoliation? Where is it an important factor in 

14. What part is played by plants in weathering? 

15. What part is played by animals in weathering? 

16. Why are soils often brown? 

* Optional Exercises 

17. What part is played by solution in the weathering of 
igneous rocks? 

18. Explain in detail the chemical effects of weathering on 
granite and show how this process produces the chief constituents 
of fertile soil. 

19. Why do we find deposits of pure clay or almost pure sand 
in some places, since both are produced together? 

20. Explain the changes in the color of soils. 



42. Residual mantle. The process of weathering breaks 
down the surface rocks mechanically and chemically into 
a loose material which mantles or covers the rocks. This 
loose material, including boulders, gravel, sand, clay, earth, 
soil, etc., is called rock mantle. In some places the mantle is 
still in the position where it was formed and some of the 
boulders will be found to have the same composition as the 
underlying bedrock. These are called residual boulders and 
the entire covering is called residual mantle. 

43. Movements of the rock mantle. In time the mantle 
is removed, transported, and deposited somewhere else, 
usually at a lower level. The entire process of wearing down 
the rock and carrying it away is called erosion. 

The agents responsible for the removal of rock mantle 
are (1) gravity, (2) wind, (3) streams, (4) the sea, and 
(5) glaciers. 

44. Movements due to gravity. When pieces of rock are 
loosened on a steep slope they fall and slide down until they 
reach a supporting mass of talus below. This mass comes to 
rest at its angle of repose, which will be steeper for coarser 
fragments. But any additional force, like the wind, an earth- 
quake, or the falling of a new boulder, may start the deli- 
cately balanced mass moving again. This is a landslide. A 
heavy rain soaking through a mass of loose material on 
a steep slope may initiate a landslide. It is dangerous for a 
mountain climber to ascend across the talus when it is on 
a steep slope, because he may loosen one boulder which will 
start an avalanche. 




Sometimes the entire mantle moves slowly down the slope, 
together with its covering of vegetation. This is called creep. 
It is assisted by frost action, since, in freezing, the expan- 
sion lifts some of the loose fragments and, when the thaw 
takes place, they creep down the slope. 

In some cases the mass of boulders is so great and the 
slope so steep that the entire talus creeps. In this case it is 
called a rock glacier. 

45. Wind erosion. By itself the wind has little if any 
effect on solid rock; but when it picks up and carries along 

FIG. 26. Wind Abrasion in an Arid Region 

dust, sand, and even gravel, it uses these tools, like a file 
or a sand blast, to wear away hard rocks. This is called 
wind abrasion. In arid regions the telegraph poles are some- 
times cut down by the abrasion of wind-blown sand. In 
humid regions this process is a negligible factor, except on 
the seashore, since the loose material is protected by a layer 
of vegetation; but in arid regions wind abrasion is the chief 
agent of erosion. It polishes hard rocks to a smooth surface ; 
it wears away the softer parts of rocks like granite, thus 
causing the rest of the mass to crumble into gravel and sand. 
In the intermediate stages of this process of abrasion, strange 
and weird forms are developed, especially in rocks as soft 
as limestone. 

46. Sand dunes. Wind action is very noticeable in dry 
regions, like deserts, where the air is frequently full of dust 



and sand. Even on the seashore, in humid regions, the sand 
is blown about when it is thoroughly dry. On the desert, 
these sand storms often overwhelm travelers. The enormous 
quantities of sand transported in this way are deposited in 
places where an obstruction, like a rock, a tree, or a house, 

" v ?'$?& \u) ''WIBSKS 

' .>' ' '-".-/ "wWfcftSjSL** 

FIG. 27. Sand Dunes 

breaks the force of the wind. Houses, forests, and even cities 
have been buried in sand in this way. The sandy deposit is 
called a dune. Dunes are roughly oval in shape. Many of 
them are 100 feet high and there are some in Africa as 
much as 400 feet high. 

Inspection of a dune reveals the direction from which the 
prevailing winds blew, since on the leeward side of the dune 

the sand assumes its angle 
of repose which is about 25, 
whereas on the windward 
side the slope is gentle. 
Along the shore of the sea 

or a large lake, dunes are developed by the winds that blow 
from the water to the land. These dunes are deposited at 
about the same distance from the shore all along, so that 
on the map they appear in a line paralleling the shore. See 
Fig. 29. 

Dunes are not stationary in shape or position; the sand 
that is deposited may be picked up by the very next wind 
and carried farther away. In this way the entire dune fre- 
quently moves about 20 feet a year or even as much as 

FIG. 28. Diagram of a Sand Dune 
Arrow shows wind direction. 



100 feet a year, as on the west coast of 
France. This migration of dunes may 
be prevented in humid regions by cov- 
ering the windward side, or sometimes 
the entire dune, by a protective layer 
of vegetation. In arid regions this can- 
not be done. 

47. Dust storms. Dust is more easily 
carried than sand. Much of the fine 
surface material from the Great Plains 
is carried far across the Mississippi. 
Dust from the Gobi desert is carried 
across the mountains into China. Vol- 
canic ash, or dust thrown high into the 
air, is carried far out to sea and some- 
times far around the earth. In the great 
eruption of Krakatoa in 1883, the dust 
was carried around the earth several 
times. Dust from the Sahara is found 
on the decks of ships in the Mediter- 
ranean and sometimes even in the 
countries of southern Europe. 

Dry farming in our semiarid Great 
Plains has removed much of the vege- 
tation which originally covered the soil 
and has left it almost bare. Every puff 
of wind now picks up the dry soil and 
carries it away. H. H. Bennett, of the 
Soil Conservation Service, says, "On 
the llth day of May 1934, the sun was 
blotted out over a vast area of North- 
western United States by a huge dust 
storm that originated in the drought 
stricken wheat and sorghum fields, west 
of the Mississippi." The effect was dis- 
astrous in the region covered by the 


FIG. 29. Sand Dunes 
near the Shore 


dust. Roads were blocked, vegetation ruined, and the life of 
the entire region was at a standstill. The subject of soil 
erosion resulting from this and other forces will be taken up 
in another paragraph. 

48. Loess. Deposits of fine wind-blown material are called 
loess. When this is deposited in a humid region it makes 
very fertile soil, because it contains grains of feldspar which 
has not been acted upon by water (coming as it does from 
an arid region). Feldspar is rich in the element potassium, 
an essential for plant growth. 

Loess stands up in steep cliffs, in spite of its loose texture. 
There is a certain amount of clay in the loess and this, no 
doubt, acts to hold the rest of the loose particles together. 

There are enormous deposits of loess in China, where 
many of the inhabitants live in holes dug in the soft mass. 
Here also some of the roads have worn deep canyons through 
the loess. The Yellow River probably owes its name to the 
loess which is carried down by its tributaries. There are also 
extensive loess deposits in central Europe and in the Mis- 
sissippi Valley. In southern Nebraska and western Iowa the 
loess reaches a thickness of 100 feet. At Council Bluffs, 
Iowa, we find a characteristic exposure of loess which has 
been eroded into steep-walled cliffs. 

49. Movement of the rock mantle due to wave action. 
Wave action affects the bottom only in very shallow water 
and here the loose material is dragged along toward the 
shore and sometimes hurled by the breakers on the shore 
itself. As the water returns, the undertow carries the fine 
material back although it may not be able to carry the 
coarser matter dumped on the shore by the breakers. Since 
most waves are driven against the shore at an angle, they 
cause a slow movement of rock mantle parallel to the coast. 

50. Transportation of rock mantle by glaciers. The ac- 
cumulated snow on a slope ultimately becomes consolidated 
into ice and as its weight increases it begins to slide. A 
moving mass of ice and snow is called a glacier. Sometimes 



it crashes down a mountain in an avalanche, but usually it 
moves very slowly, a few feet a day. 

Glaciers transport rock mantle in several ways. Some is 
swept along in front, some is imbedded in the ice and dragged 
along, and some loose boulders fall on the top of the ice 
from overhanging cliffs and are carried along. 

Some of the mantle transported by a glacier is dropped 
along its path, some is pushed to the sides, and the rest of 

^%1^%-^^-f-- "|M #3f'Jr C 

r^'W;>i^j;..:.g| j| ^HS|l%3 
^^^teft?'^ \^?^. .^"^^"i^-^-S^iS . 

^? ' -.^i -"" : ^ far a \ "'-'' }/J8hb ' "^X v'".r- ".".' -'fc. : r^"' _j^ 

- ^-' ? fc^ v**J^ ^^*/*^ 

"\ v^soL'^ZlK ,.' 

FIG. 30. Glacial Till 

it is deposited where the glacier melts, as an unassorted 
mixture of boulders, sand, and clay, called glacial till or 
drift, several feet thick, sometimes almost 100 feet thick 
(Fig. 30) . This material is then reworked by the other agents 
of erosion, principally water. Most of the soils of northern 
United States and Europe owe their origin to this process. 
(See Glaciers, Chapter IX, for details.) 

51. Transportation by streams. Streams lift up and carry 
fine material, like mud or sand, and push coarser matter 
along the bottom. With increased velocity, the carrying ca- 
pacity increases until the water is capable of moving huge 
boulders by rolling them over and over. When the slope is 
greater, the water is able to move larger boulders which are 
deposited at the foot of the slope. 


. In semiarid regions, when it rains, the water rushes down 
the hills, carrying a mass of rock mantle, but when a level 
spot is reached the water is suddenly arrested in its progress 
and the mass of miscellaneous debris is dropped. Since the 
water comes out of a narrow defile in the hills and spreads 
out on the more level ground, the deposit takes the shape 
of a fan. Such deposits are called alluvial fans (Fig. 31). 

In humid regions, where the water flows continuously, the 
load carried by the streams is sorted out as it is deposited: 

FIG. 31. Small Alluvial Fan 

here a mass of boulders, there gravel, somewhere else sand, 
and in other places mud. When the stream enters a large 
body of water, at its mouth its velocity is suddenly reduced 
and it must therefore drop the rest of its load, which usually 
consists of very fine matter like clay. 

When the stream is in flood, it overflows its banks and 
spreads over the surrounding lowlands. Here also its velocity 
is reduced and consequently deposition of mud occurs. 

The debris deposited by streams is called alluvium. 

52. Soil. We have seen that the bedrock of the earth is 
covered by a layer of loose material called rock mantle, 
which in some places is an unassorted mixture of coarse and 
fine matter, in others carefully graded and sorted, here coarse 


and there fine. Any of this rock mantle in which plants will 
grow is called soil. Some of the soil was formed by the 
weathering of the bedrock on which it rests. This is called 
residual soil. But in most cases soils have been transported 
and therefore they vary very much in texture and com- 

Texture depends on the size of particles, which varies from 
large to small as follows: 

1. Boulders are very large about six inches or more in 

2. Gravel has pieces smaller than boulders: down to about 
| inch. 

3. Sand particles are smaller than gravel but not so small 
that the wet mass will hold together. 

4. Very fine particles, which the wind can pick up, are dust. 
When wet, it sticks together and is called silt, if it con- 
tains fine quartz particles; but if it contains almost no 
quartz it is called clay. 

We find boulders of all kinds of rocks. Gravel and sand 
usually consist of quartz, probably because many rocks con- 
tain quartz, and when the rocks are mechanically weathered, 
the constituent minerals are either attacked chemically or 
are quickly worn because they are not hard. But quartz is 
not chemically changed by weathering nor is it easily worn 
because of its hardness. 

Clay is not chemically attacked either. Hence the mineral 
soils consist principally of mixtures of two minerals, clay 
and quartz. Besides these, all soils also contain organic 
matter, the product of the decay of leaves and roots, called 

Each constituent of soil serves some useful purpose in 
cultivation. The sand acts as the body or bulk of the soil, 
permitting the plant to penetrate and attach itself so that 
it can hold its leaves up in the sunlight ; but a soil composed 
entirely of sand has no cohesion and permits every wind to 



displace the roots of the plant and often to uproot it al- 

Furthermore, water and soluble mineral compounds are 
held by the surfaces of the particles of soil. The smaller the 
particles, the greater will be the total surface, and the greater 
the total quantity of water which can wet the surface. For 
example, one centimeter cube of rock has a total surface of 
6 square centimeters, but if it were broken down into coarse 
sand with a diameter of 0.1 centimeter, there would be one 
thousand particles with a total area of 60 square centimeters. 
The result of further subdivision is shown in the following 






1 cm. 3 of rock 

1 cm. 


6 cm. 2 

Coarse sand 

0.1 cm. 


60 cm. 2 


0.001 cm. 

1 billion 

6000 cm. 2 


0.000001 cm. 

1 billion billion 

6,000,000 cm. 2 

A consideration of the above table will show how much 
more effective clay is for retaining water as well as dissolved 
mineral matter. Clay is, therefore, essential for a good soil, 
since it furnishes feeding areas for the root hairs. In very 
fine soils, water may be drawn upward a distance of several 
feet by capillary action. 

Clay has plasticity and holds the sand together, but it is 
difficult to penetrate, and hence both clay and sand are 
essential for good soils. Too high a clay content causes the 
soil to hold too much water, which may drive out all the 
air. Soil which has no air cannot successfully be cultivated. 

Besides the mineral content, a fertile soil requires a cer- 
tain amount of organic matter derived from plants: leaves, 
roots, fruits, and other plant tissues. These are decomposed 
by microorganisms until the original plant structure is un- 


recognizable and nothing but a jelly-like or colloidal mass 
remains. This is called humus. It furnishes to the growing 
plants soluble nitrogen, potassium, and phosphorus com- 
pounds, in which the soil is otherwise deficient. The humic 
acids produced by decomposition attack the minerals of the 
soil chemically, causing them to yield up other elements 
which are needed by the plant. Humus keeps the soil in 
colloidal condition, during which it holds water in the proper 
proportion to air. The water held by the colloidal soil dis- 
solves mineral substances which would otherwise be washed 
out by rain. Such soil, therefore, has the proper consistency 
for cultivation. Soil very rich in humus is called muck. 

*Many soils have a red color due to oxidized iron compounds 
taken from minerals in the rocks. The red color indicates that 
the soil has little, if any, humus. The organic compounds of the 
humus reduce the oxidized ferric compounds, which are highly 
colored (red, yellow, or brown), to ferrous compounds which have 
practically no color. Besides, the ferrous compounds are more 
soluble and are carried away by water containing a little acid, 
like carbonic acid or any organic acid derived from the humus. 
For these reasons the red colors fade to grays and finally to black, 
which is the color of the carbon, some of which is formed from the 
partial decay of plants. 

Red soils are called laterites (from the Latin word later, 
a brick). They have very little fertility because they are 
deficient in humus. 

53. Classification of soils. For agricultural purposes, soils 
may be divided into three classes: mineral, calcareous, and 
organic. Calcareous soils consist chiefly of chalk, and or- 
ganic soils are chiefly peat with little mineral matter. Most of 
our soils are mineral soils, and we shall discuss only these. 

54. Mineral soils. There are three groups of mineral 
soils: sands, loams, and clays, but these grade into one 
another imperceptibly, so that we have sandy loams and 
clay loams of infinite variety. But all of them contain sand, 
silt, and clay, as can be seen from the following table show- 


ing the approximate composition of one sample of each of 
four kinds of soil. 


1. Ordinary sand 85% 10% 5% 

2. Sandy loam 60% 30% 10% 

3. Clay loam 50% 30% 20% 

4. Clay 10% 35% 55% 

Sandy soils are loose, easily penetrated by plant roots, 
and well supplied with air; but the water runs through 
rather too easily. They are easy to work and are used in 
gardens and nurseries. 

Loams are not so loose but permit easy penetration by 
roots and water. The water is held better by loams; hence 
they are used for all kinds of plants. Clays are difficult to 
cultivate, hold water too well (to the exclusion of air), and 
cannot easily be penetrated by roots. When they have suffi- 
cient lime and organic matter they lend themselves to the 
growth of wheat. 


55. The threat to agriculture. In its natural state the 
soil is adequately protected against the destructive effects 
of wind and running water by the vegetable cover grass 
and trees but when this is stripped off in order to culti- 
vate the soil, every puff of wind carries off some of the fine 
topsoil when it is dry, and every rain washes some of it 
away, especially on steep slopes. More than 80% of the 
agricultural land area of the United States has a slope 
greater than |% (a fall of half a foot per 100 feet), which is 
steep enough to be subject to soil erosion. 

Three billion tons of soil a year are being washed into the 
ocean and this is only a part of the total soil removed. Much 
more is transported and dropped over rich soils, which are 
thereby often damaged. Much of the sediment is deposited in 
reservoirs, irrigation ditches, and harbors, which are thereby 
silted up. The irrigation systems of the West may soon be- 
come useless because of this filling of the storage basins. 



Already 100 million acres of agricultural lands have been 
ruined, and the process is going on at the rate of one half 
million acres per year. Within 100 years, we shall have only 
150 million acres of farm lands left and it has been esti- 
mated that this cannot feed our population. 

A survey of the corn lands shows that in spite of improved 
knowledge and use of fertilizers, the average yield has 
dropped from 27 bushels per acre (1870-1880) to 26 bushels 
per acre (1920-1930) ; and we can readily understand that 
this is the result of the removal of 50 million tons per year 
of the three principal elements, potassium, nitrogen, and 
phosphorus, when only one half million tons are returned 
in the form of artificial fertilizers. 

The formation of soil is a slow process, requiring thousands 
of years. Its removal therefore constitutes for us an irrepa- 
rable loss. 

56. Causes of soil erosion. The normal geological proc- 
esses of erosion are very slow and they cannot be arrested 
by man, but when man removes the soil cover trees and 
grass --by clearing, burning, and excessive grazing, and 
loosens the soil by cultivation, resistance to the forces of 
erosion is very much lessened and these forces become ex- 
cessively destructive. 

In an experiment conducted by the Soil Conservation 
Service at Bethany, Missouri, it was found that the loss 
of soil and water was much greater from land growing corn 
than from grass-covered soil; and this was accentuated when 
the slope was increased. These results are shown in the fol- 
lowing table : 



WATER Loss IN % 














Apparently, the grass cover was very effective in pre- 
venting soil erosion and loss of water. It is noticeable that 
the soil loss was the same on both slopes under grass. 

The chief forces in soil erosion are the wind and running 
water, and these produce three types of erosion. 

1. Sheet wash 

2. Gullying 

3. Wind erosion 

U. S. Department of Agriculture 

FIG. 32. A Dust Storm 

57. Soil erosion due to wind action. Dust storms. Wind 
erosion has already been taken up in paragraph 45. It will 
be remembered that when cultivated land dries up, as it 
does under the hot sun, the wind can pick up and blow away 
not only fine dust, but even sand. The wind is the principal 
agent of soil erosion in arid and semiarid regions, like our 
Great Plains, while sheet wash and gullying are responsible 
for most of the destruction in humid regions. 

In the area from Texas to North Dakota, wind erosion 
has ruined about 4 million acres and has seriously affected 
about 60 million more. In 1934, a disastrous dust storm, 
caused by prolonged drought, removed several inches of the 


topsoil from much of this region and spread it out over 
the country to the east, some of it being carried into the 
Atlantic Ocean. 

A passenger on a train in Kansas describes one of these dust 
storms as follows: "I had read of dust storms, but they were 
vague in my consciousness. Now I see one, and it is a terrible, an 
awesome thing. They are clouds of dust-soil; soil blowing away 
from a ravaged and denuded land. 

I have had only one thought in my brain, as I sit on the train 
and look out at this desert through which we have been passing for 
two hours. It is a saddening, almost a heart-tearing thought. It 
is the thought that right here, under my very eyes, I am seeing 
this country blowing away. Here, in what was once the richest 
farm and stock land of the Middle West, I see the destruction of 
that soil which has fed us. 

I see death, for there is no life; for miles upon miles, I have seen 
no life, no human beings, no birds, no animals. Only dull-brown 
land, with cracks showing; ground that looks like clay. Hills 
furrowed with eroded gullies. You have seen pictures like that in 
the ruins of lost civilizations. 

Trees once in a while. But their branches, their naked limbs 
are gray with dust. They look like ghosts of trees, shackled and 
strangled by this serpent, flinging their naked arms skyward, as 
if crying for rescue from this encircling, choking thing." 

58. Sheet wash. When rain 
falls on the ground, some of it 
sinks through the surface and 
some runs off. This surface 
runoff is at first a sheet but 
soon it breaks up into tiny 
streams of muddy water. Ev- 
ery drop of water that fell on 
the soil has loosened some 
fine silt, picked it up, and is 
carrying it along. This proc- 
ess is called sheet wash. 

59. Gullying. When the runoff finds a depression it takes 


that path and if the slope is sufficient, rapid headward ero- 
sion sets in. A gully may be started by a natural depression 
or by an artificial one. Running a wagon or dragging a plow 
down a slope may be the beginning of a destructive gully. 
When the gully cuts down below the water table it drains 
a much larger area, and the increased velocity and larger 
volume of the water still further accentuate the soil erosion. 
The gully will develop branches which ultimately cover the 
entire area. 

60. Soil erosion _and flood control. Not only is the soil 
being lost from much of the cultivated upland, but the 
stream channels are being silted up. The water runs off the 
uplands much more rapidly for two reasons : 

1. Cultivated soil does not hold water so well as soil 
covered with grass and forest litter. 

2. Where the soil has been removed, the runoff is com- 

When this excessive runoff reaches the silted-up streams 
it overflows the banks and we have a flood. It seems ap- 
parent, therefore, that one important factor in any system 
of flood control must be a proper method of cultivation, 
since soil conservation will check rapid runoff. 

61. How to control erosion. The Soil Conservation Serv- 
ice of the United States studies soil erosion, finds remedies 
and preventives, and educates the public in the use of con- 
servation methods. They recommend the following for the 
control of soil erosion : 

1. Take out of cultivation all steep areas whose slope 
is greater than 15% and cover them with trees or grass. 

2. Protect the cultivated land by strip-cropping; that is, 
grow clean tilled crops, which lay bare the soil, like corn, 
tobacco, and cotton, between parallel bands of grass and 
other untilled crops, planted along contours. 

3. Terracing, to stop erosion of water. 

4. Plow down stubble, instead of burning it, to protect 
against wind. 



Completion Summary 

Rock mantle is the result of - . It is called residual 
soil when - , and transported soil - . 

FIG. 34. Strip-cropping 

Movements of loose mantle are brought about by 
and . 

- is chiefly effective in arid regions. 
Dunes are - - sand - - migrate 

-. Dust is 


carried - 

Loess is 
quent in the interior regions of the great continents, because 

-. Loess, dunes, and dust storms are fre- 

Alluvial deposits are made by - . 
Soil supports - . It consists chiefly of - . 
Soil erosion is brought about chiefly by cultivation, be- 
cause - . It can be prevented by - . 


1. What are the chief constituents of residual mantle? 

2. Name the factors in the transportation of rock mantle. 

3. Describe creep. 

4. In what kind of region is wind action an important factor 
in erosion? 

5. Why are sand dunes not often seen in humid regions? Why 
do we sometimes find sand dunes on the shore? 

6. In what kind of region are dust storms common? Why? 


7. What is loess? Why is it fertile? 

8. Explain how waves move rock mantle. 

9. What is glacial drift? 

10. What is an alluvial deposit? 

11. What is soil? How does it differ from earth? 

12. How does a residual soil differ from a transported soil? 

13. What is the function of sand in soils? 

14. What are the advantages and disadvantages of clay in soils? 

15. Why is clay an essential constituent of fertile soil? 

16. Make a tabular form showing the essential constituents of 
soil, together with the function of each constituent in cultivation. 

17. What advantage would there be in a soil which was very 
rich in sand? 

18. What advantage would there be in a soil very rich in clay? 

19. What does humus furnish to the growing plant? 

20. What is loam? 

21. What is meant by soil erosion? 

22. What are the chief agents of soil erosion? 

23. In what way does the cultivation of the soil for corn lay it 
open to erosion? 

24. Explain the relation between wind action, soil erosion, and 
dust storms. 

25. What is sheet wash? 

26. How does gullying hurt farm land? 

27. Explain how a gully starts. 

if Optional Exercises 

28. Explain how sand dunes are built up parallel to a shore. 

29. Why is the material in an alluvial fan rather heterogeneous, 
whereas that on the banks of a stream is assorted? 

30. Draw a diagram showing a hill on which is a stream flow- 
ing down to a plain. Show in the diagram where the following 
would be deposited: boulders, gravel, sand, silt, clay. 

31. Explain the change in the color of soils from red to black, 
introducing as much of the chemistry of the changes as you can. 

32. Explain how dry farming is in great part responsible for 
the dust storms of the Great Plains. 

33. How can gullying be prevented or remedied? 



62. Rainfall. Rain owes its origin to evaporation, chiefly 
from the surface of the sea, and to precipitation due to 
causes which will be discussed later. The amount of rainfall 
varies from two inches per year in Death Valley to five 
hundred inches per year in parts of India. In this respect 
we classify the regions of the United States as follows: 

Annual rainfall over 20 in. Humid region 

Annual rainfall, 10-20 in. Semiarid region 

Annual rainfall less than 10 in. Arid 

The rain which falls on the land may be disposed of in 
four ways : 

1. Part evaporates. 

2. Some collects in ponds. 

3. Some sinks into the ground. 

4. The rest runs over the surface to a river. Of that which 
sinks into the ground, a part finds its way, at lower levels, 
into the streams and adds to the volume of running water. 

63. How running water moves rock mantle. Water has 
two ways of eroding rock: solution and abrasion. It is an 
old saying that " water will wear away stone/' but pure 
water, by itself, has little, if any, effect on rocks. Carbonic 
acid, added to the water as it sinks through decaying vege- 
tation, enables it slowly to attack minerals containing cal- 
cium, magnesium, and iron, and to remove these substances 
in solution. Limestone is the most common mineral subject 
to this solvent action. Many sandstones, consisting of grains 
held together by a cement of calcium or iron carbonate, are 
disintegrated by solution. 



It has been estimated that about three billion tons of 
rock in solution are carried to the sea every year. These 
dissolved compounds include calcium and magnesium bi- 
carbonates and sulphates, sodium chloride (common salt), 
and silica. 

The process of weathering prepares the rocks for removal 
by breaking them into smaller pieces which the water can 
move mechanically. About ten billion tons of sediment are 
carried by streams and deposited in the sea every year. 

The size of particles which can be moved by running water 
depends on the velocity of the water. A few examples are 
shown below: 


0.2 mile per hr. Clay 

0.5 mile per hr. Sand 

1.0 mile per hr. Small pebbles 

2.0 miles per hr. Two-inch pebbles 

5.0 miles per hr. Small boulders 

When the velocity is doubled, the water can move particles 
sixty-four times as large! We need no longer be surprised 
when we see the huge boulders, some of them weighing 
many tons, which have been moved by running water. 

Of course the water does not actually lift the boulder off 
the bottom, but rolls it along. Lighter masses are picked up 
and thrown forward in little leaps, while the very fine ma- 
terial remains suspended during the entire course of the 
river, and settles only when the water comes to rest. In this 
connection, we must not forget the buoyant effect of water, 
which makes it appear that rocks lose weight when they 
are submerged (Archimedes's Principle). 

The amount of material carried by running water de- 
pends not only upon its velocity but also upon the kind of 
rock it flows over. Muddy streams usually flow through 
regions made of shale and other soft sedimentary rocks, 
whereas streams that are clear flow over metamorphic and 
igneous rocks. 


64. How a river erodes. The material carried by water 
furnishes the tools with which the river can wear away rock 
and cut a deeper and deeper channel. A moving body of 
water carrying its load is therefore like a sinuous file, and, 
in the process, the material hurled against the rock is itself 
worn ever finer and finer. 

A river rises on high ground and ultimately finds its way 
to the sea. At its source the water flows rapidly because of 
the steep slope or gradient; but this gradient flattens out as 
the river proceeds on its course and at its mouth the slope 
may be practically nothing. Where the gradient is steep, the 
stream cuts rapidly into its bed, deepening its channel and 
by that very process reducing its gradient. 

While the gradient is steep, the water pushes obstacles 
out of its path or flows over them, maintaining a generally 
straight course; but as the gradient is reduced it cannot 
push obstacles away or flow over them: instead, the water 
is forced to move aside and therefore it begins sidewise 
cutting. The stream has now developed from youth to ma- 
turity. It begins to widen its channel, cutting into its bed 
more slowly than during youth. 

There comes a time in the life history of a stream when 
the gradient has been so reduced that the stream is at the 
level of the body of water into which it empties. It has now 
reached base-level. It cannot cut any lower, for the water 
will not flow. This is old age. 

It must be remembered that, during most of its life history, 
a stream shows all stages of development in different parts. 
At its source, in the hills, it is youthful; at its mouth, where 
it enters a lake or sea, it is old; while in the rest of its course 
it is mature. 

One often hears the statement, "A river wears down the 
mountains," and may be unable to understand how the 
water in a river can wear down mountains it does not 
touch. The Colorado River, for example, will ultimately 
erode the entire mountain system through which it has cut 



its gorge. It must not be forgotten that every drop of water 
that flows into the Colorado has fallen on higher ground and 
run down into the river below, eroding as it flows. All these 
little tributaries are part of the Colorado River, and it is they 
that wear down the mountains; the parent stream merely 
removes the waste. 

A river formed on a surface of uniform hardness develops 

a pattern like that of a 
branching tree; hence the 
term, dendritic drainage (Fig. 

65. Deposition by running 
water. Whenever the velocity 
of the water is decreased, it 
can no longer carry the 
coarser material and this is 
dropped. The simplest case 
of this kind occurs in semi- 
arid regions where streams 
are intermittent because of 

FIG. 35. Map Showing Dendritic . rr p n ITTI > 

Drainage insufficient rainfall. When it 

rains the water runs rapidly 

down the slopes, since the surface is usually bare rock. 
As the water comes out on the more level ground at the 
foot of the hill, it spreads out, its velocity is suddenly 
checked, and it drops its entire load. The larger pieces are 
dropped right in its path as it emerges from the ravine and 
tend to choke up the outlet; but the next rush of water 
sweeps them away or overrides them. In this way the debris 
is piled up helter-skelter, large and small pieces in a con- 
fused heap, but with the coarser material nearer the ravine 
and the finer material on the valley floor, where for a short 
while the water spreads out like a lake. The shape of this 
deposit is roughly like a fan with the handle at the ravine; 
hence it is called an alluvial fan. Fig. 31. 

Similar alluvial fans are formed where a swift tributary 


emerges on a level valley or enters the parent stream, but 
these deposits in water are commonly finer material and 
are easily removed, so that alluvial fans are commonly 
features of arid- regions. 

66. Deposition in rivers. When a river changes its slope 
it must drop the coarser particles, and each time its velocity 
is checked, from any cause, some of the load will be dropped. 
Each deposit will contain particles of about the same size 
as shown in Fig. 36. Figure 37 also shows deposits due to 

C 1 D' 

FIG. 36. Deposition Due to Decrease in Velocity 

Longitudinal section of a stream. A BCD is the water surface, and 
A'B'C'D' the bed of the stream. 

change of velocity from other causes, and each deposit is 
assorted. At times when the river is in flood it will bring 
down coarser material and we may get a little admixture 
of the new and the old which will spoil the assortment. In 
this way we get boulders deposited in the very middle of 
the stream, mixed with gravel and some sand; on the sides, 
sandy deposits, and here and there, where the water is very 
quiet, we find mud. On the inner curve of a stream we 
always find a sandy deposit because the water's velocity is 
checked somewhat as it rounds the bend. 

Stratification of deposits. As long as the velocity of a 
stream remains the same, the character of the deposit at a 
given place will remain the same. But when the velocity 
changes, the coarseness of the deposit will change. Figure 38 
shows a finer deposit overlying a coarser one. This is called 



FIG. 37. Stream Deposits 

At A, fine material is deposited behind a boulder where the water is 
quiet. At B we find gravel, because the velocity of the stream is checked 
by the headland. The boulders were brought down when the stream was 
in flood. 

All river deposits are assorted and stratified, but the re- 
sult is not nearly so perfect as that produced by wave action 
on the shore. 

67. Flood plains. When a river in flood overflows its 
banks, the velocity of the flood waters is checked and much 

B 1 C 

FIG. 38. Stratification of River Deposits Due to Decreasing Velocity 

of the load must be dropped. This deposit, being level, forms 
a flat strip of ground, bordering the river on each side, which 
is called the flood plain, because it is subject to inundation 


during floods. An old river like the Mississippi has a wide 
flood plain, while a young river, like the Colorado, has al- 
most none. 

68. Natural levees. The greatest loss of velocity, as a 
river spreads over its flood plain, occurs where the water 
leaves the channel, that is to say, along the banks of the 
river; and here the coarsest and the largest deposit is formed. 
This builds up each bank into a low ridge called a natural 

FIG. 39. Cross Section of a Flood Plain 

levee (L and L' in Fig. 39). Natural levees are higher than the 
flood plain, some of them twenty feet high. They furnish 
good locations for home sites and roads. 

Artificial levees are often built to protect the adjacent 
region from flooding. A break in a levee is called a cre- 

69. The delta. When the velocity of the stream is suddenly 
checked at its mouth as it enters a large body of water, it 
must drop its entire load. This mass soon builds up and ob- 
structs the path of the river. The river is therefore forced to 
find a new path on the two sides of the obstruction and 
sometimes, in flood, it overflows the mass of material. This 
finally builds up above the surface and assumes the shape of 
the Greek letter delta, A; hence this name is given to the 
deposit. The name distributaries is given to the system of 
stream channels through which the main stream finds its 
way into the sea or lake. To prevent the distributaries from 
being choked by sediment, engineers often build jetties. These 
are obstructions which narrow the river, increasing its ve- 
locity, so that it scours out a deep channel. 

The material deposited at the delta builds out into the 
water and extends the length of the river. The Mississippi 




FIG. 40. The Delta of the Mississippi 

is increasing its length about 250 feet a year. Since 400 B.C., 
the Rhone River has been increasing about 40 feet per year. 

The Nile and the Danube 
increase only about 15 feet 
per year. 

The delta of the Nile is 
about 100 miles long and it 
is about 200 miles wide at 
the Mediterranean. The Mis- 
sissippi delta is about 200 
miles long but not so broad. 
Deltas form at the mouths 
of all rivers; but sometimes 
wave action and shore cur- 
rents spread the alluvium 
and prevent the formation of 
a perfect delta. The St. Lawrence, the Susquehanna, and 
the Amazon have imperfect deltas for this reason. 

Completion Summary 

A region is humid if there is enough rainfall to collect 
in pools and to run off. 

Running water erodes by solution, somewhat, but chiefly 

by . Increase of velocity makes it possible for running 

water . 

A stream cuts into its bed most rapidly at its - . 

Here it is therefore young. As soon as it begins to 

it is mature; and when it cannot cut any deeper into its 
bed - . Every stream is old at , mature - , 
and young at - . 

A river levels its valley with the help of . The 

stream pattern - dendritic. 

Whenever a decrease in velocity , a stream 

load. This deposit size. In an arid region, the deposit 

is not uniform, because 

-. We call it 

Rivers in humid regions 

assorted deposits. There 


may be some material of a different size in river deposits, 
because - . 

A flood plain is formed - . 

On the banks of the river - - a deposit called - . 

At the mouth of a river - - deposit, called - . 

Such a river enters the sea through its - , which in- 
crease their length by deposition. 


1. What is a humid region? 

2. What happens to the rainfall? 

3. How does solution aid in erosion? 

4. Name two substances carried into the sea in solution. 

5. How does weathering help running water in its work of 

6. If a stream moves pebbles, one inch in diameter, when it 
flows at the rate of two miles per hour, what size boulders will it 
move at four miles per hour? 

7. Why are streams muddy when they flow over shale? 

8. When does a river erode its bed most rapidly? 

9. When does a river become mature? 

10. When does a stream become old? 

11. Explain the phrase, "a river wears down mountains." 

12. What is meant by dendritic drainage? 

13. When does running water deposit part of its load? 

14. Explain how an alluvial fan is formed. 

15. Why are alluvial fans more common in arid regions? 

16. Why does a river assort its deposits? 

17. Why are river deposits not perfectly assorted? 

18. How do deposits become stratified? 

19. Explain how a flood plain is formed. 

20. What type of river has practically no flood plain? 

21. Explain how a natural levee is formed. 

22. What is a crevasse? 

23. Explain how a delta is formed. 

24. What are distributaries? 

25. How does a river increase its length at its mouth? 

26. Why do some rivers have imperfect deltas? Name one. 


* Optional Exercises 

27. The diameter of a particle moved by running water varies 
as the square of the velocity. Show that the size of particle varies 
as the sixth power of the velocity. 

28. If pebbles two inches in diameter are being moved by a 
stream whose velocity is two miles per hour, what size boulders 
could be moved at twenty miles per hour? 

29. If a river is young, mature, and old at the same time, at 
different parts of its course, how can we call a particular river 

30. What is the meaning of the expression, "the valley of the 

31. Why do we not have dendritic drainage, if rocks on the 
surface are not uniform in hardness? 

32. Does a stream deposit all its load when its velocity be- 
comes zero? What kind is not dropped? Why? 


*70. Rivers as highways. As civilization spreads to unknown 
lands, explorers find great forests covering all humid regions ex- 
cept in the vicinity of the poles. The eastern part of North America, 
for example, was a dense forest crossed only by trails along which 
the Indians traveled in single file, because the trails were too nar- 
row to allow them to do otherwise. Some of these trails, doubtless, 
may have allowed travel on horseback by the earliest settlers, but 
there were no wagon roads at all. 

The principal highway used by the Indian was the river, and the 
direction of travel, exploration, and settlement by white men was 
controlled by the locations and the courses of the rivers. 

The extent to which river highways expedite exploration is well 
illustrated by the relative progress of the French and English 
explorers of North America. The English who settled at Jamestown 
ast of the Appalachian Mountains found only short rivers, and 
in 150 years extended their domain only as far as these rivers 
enabled them to travel. 

The French, however, settling at the mouth of the St. Lawrence, 
quickly occupied the whole of the St. Lawrence Basin, including 
the Great Lakes, and crossed the height of land that separates it 
from the Mississippi Basin, at various " portages" marked x in 
Fig. 41. Each portage led to a tributary of the Mississippi and 
made it possible to reach any part of the great Mississippi system 
with its thousands of miles of navigable water. 

In 1750 the French claimed most of the territory between the 
lines A B and CD in Fig. 41, although they had only about 
80,000 inhabitants in the region, whereas the English, with a 
population almost 20 times as great, were confined to the smaller 
area between the line AB and the coast. 

Comparison of the census of 1790 with that of 1820 shows that 
our population advanced during that interval along the Ohio and 




Cumberland rivers to the Mississippi, and thence south to New 
Orleans and north to Quincy, Illinois; it indicates also that 

population followed the Mis- 
souri River to Kansas City, 
showing that the settlers used 
the rivers as highways. 

Many explorers, besides the 
French, used the rivers of 
North America as highways. 
The Spanish came up to Santa 
Fe from Mexico along the Rio 
Grande; the Fremont Expedi- 
tion of 1842 followed the South 
Platte, and the Lewis and 
Clark expedition followed the 
Missouri, the Yellowstone, and 
the Columbia. 

Besides the excellent high- 
way for boats which rivers 
provide, many river valleys 
furnish a graded location for 

FIG. 41. Illustrating the Influence of 
Rivers on Exploration 

wagon roads and railroads that 
engineers find better than any 
other location in the region, and hence roads usually follow rivers. 

As a rule, roads that cross the stream are located at shallow 
places that can be forded or narrow places that can be bridged 
cheaply; and in the case of canyons or steep-sided valleys, the cross- 
ings are at gaps in the valley wall. The " Spanish Trail," in western 
United States, crossed the Green River of Utah where there is a 
gap in the canyon wall, and our railroads now use the same gap. 

Where there is sufficient travel to warrant the expense of a 
bridge, neither the deep river, the rapids, nor the canyon is an 
insurmountable barrier. 

*71. Rivers and commerce. Just as the canoe was the chief 
means of travel in North America, in the early days, so the flat- 
boat was the chief means of transporting freight. The river was the 
first artery of commerce in all countries. Ancient histories tell us of 
boats propelled by oars, poles, or sails, carrying cargoes on the Nile 
and on the Euphrates. 


The transportation of commodities downstream on the Missis- 
sippi became so important in the early days of the last century as 
to lead to the " Louisiana Purchase." The journey upstream with 
a flatboat was slow and difficult, sometimes lasting several months; 
but the boats could carry many tons of freight, required small 
crews, and traveled downstream without power at the rate of four 
or five miles an hour. 

The appearance of the steamboat on the Ohio River in 1811 
decreased the time required for upstream journeys to about 5% 
of that formerly required, and caused a great increase in the 
volume of freight carried on the river. As a result, freight charges 
were reduced to about one fourth of the former rates. 

When railroads were built, transportation by boat in America be- 
gan to decline, because (1) river transportation is slower than rail 
transportation; (2) it is often obliged to follow roundabout routes; 
(3) it must be suspended (in the north) in winter; (4) it is sus- 
pended during the summer, on many streams, because of low water. 

Even with these disadvantages, river transportation has exer- 
cised a salutary influence in controlling the average rate per ton- 
mile for the transportation of freight. In 1837 the average charge 
made by railroads was seven and one third cents per ton-mile. It 
had dropped to less than one cent per ton-mile by 1905. Railroads 
and auto trucks are now carrying most of the passenger and 
express traffic of this country, including all perishables and all 
light articles; but they cannot compete with the river transporta- 
tion companies in handling heavy or bulky freight, especially when 
carried downstream. 

In 1925 the Ohio River and its tributaries carried about 
16,000,000 tons of coal, lumber, gravel, and sand, and more than 
75 per cent of it was carried downstream. On the Mississippi, 
coal, lumber, and sand are carried downstream; and some of the 
products of the delta regions, such as cotton, sugar, rice, and 
petroleum, are carried upstream. 

Exceptionally favorable conditions for transportation on the 
Great Lakes have enabled the companies operating there to con- 
trol much of the freight business of the region through the low 
cost of transportation. In 1909 the through rate for iron ore from 
Lake Superior was less than one mill per ton-mile, whereas the 
rate by rail was one cent per ton-mile. 


The "Soo Canal" around the falls of St. Mary makes it pos- 
sible for boats to pass from Lake Superior to Lake Huron; and 
although the canal is closed for about five months every year, it 
carries about four times as much freight during its seven months' 
period as the Suez Canal carries during its twelve months' season. 
Eastbound freight through the "Soo" is principally iron ore and 
grains; westbound freight is principally coal. Much lumber, how- 
ever, comes through the "Soo," and much more is shipped from 
the shores of Lake Michigan and Lake Huron. 

*72. River transportation in Europe. The rivers of Europe have 
been "corrected" and improved at great expense and are of much 
greater relative importance, in comparison with railroads, than 
are the rivers of the United States at the present time. This is 
partly due to the greater relative mileage of railroads in this 
country, but there is no doubt that business will be greatly stim- 
ulated and the cost of manufactured articles greatly diminished 
when the thousands of miles of navigable rivers and canals in the 
United States are developed as highly as they are in Europe. 

*73. Rivers and water supply. Man has always gone to streams 
for a large part of his water supply, both for household purposes 
and for irrigation. Practically all rivers furnish water suitable for 
irrigation, but great care must be exercised to secure water suit- 
able for drinking purposes, for the following reasons: 

1. Streams that are safe for drinking at their ordinary stage be- 
come unfit for drinking when at flood stage because of filth washed 
into them from the land. 

2. Water that appears and tastes as though it were absolutely 
pure may be loaded with the germs of certain contagious diseases. 

3. Sewage may be discharged into the stream above the point 
at which the water is used. 

Many towns empty their sewage into a river, thus endangering 
all towns that draw water from the river below them. Notable 
instances of this practice are found at many points on the Great 
Lakes and on the Mississippi River. Where most of the cities 
depend upon the river or the lake for their water supply and empty 
their sewage into the same body of water at a point some distance 
below their waterworks intake, such conditions always oblige 
cities to purify their water before it is safe to use. Most of the 
cities on the Mississippi go to the river for their water supply, and 


so do some of the cities On the Hudson, Connecticut, and other 

New York City derives most of its water supply from streams. 
The old Croton system depends upon the Croton River Basin, and 
the new Catskill system upon the upper portion of the Basin of 
Esopus and Schoharie Creeks and some near-by streams. 

It is obvious that densely populated stream basins are more 
likely to contain impure water than sparsely populated basins, 
and many states give their cities sanitary control over the basin 
from which they draw their water. In spite of such control, New 
York City has found it increasingly necessary to purify the water 
from the Croton system with chlorine, in order to prevent the re- 
currence of epidemics of typhoid fever. 

*74. Military advantages. The feudal castle was surrounded by 
a deep moat as a means of defence. Modern armies frequently 
select positions with a natural moat; i.e., with a river between them 
and the enemy. The degree to which a stream aids an army de- 
pends upon the characteristics of the stream. A frozen stream, a 
dry stream, or a shallow stream with a wide, open valley would 
only slightly retard the advance of an enemy. A deep river would 
require bridges, and these might be destroyed by artillery as fast 
as an enemy could rebuild them. 

A canyon like that of the Colorado and a gorge like that of the 
Niagara, or rapids like those near Niagara Falls, would be prac- 
tically impassable for an attacking army and would prevent a 
frontal attack upon an army defending them. Many illustrations 
of this use of rivers are to be found in history. In the Battle of 
New Orleans, 1815, the Mississippi, and in the World War, the 
Marne, the Aisne, the Dnieper, the Tagliamento, and the Piave 
were thus used. 

*75. Rivers as national boundaries. The fact that rivers can be 
easily described in deeds and treaties has led to their wide use as 
national boundaries. In our own country the Mississippi was once 
the western boundary; the Rio Grande is our present boundary 
for some distance on the south and the St. Lawrence on the north. 
The Missouri River separates Nebraska and Kansas from Missouri 
and separates Iowa from South Dakota. In none of the instances 
mentioned, excepting perhaps that of the St. Lawrence, has the 
river proved to be a satisfactory boundary, because of the tend- 


encies of the rivers concerned to shift their channels. There have 
been controversies between several states bordering the Missouri 
and Mississippi rivers and between the United States and Mexico 
because of the shifting channel. 

The only satisfactory boundary line for a nation is one possess- 
ing military advantages like a canyon or a mountain range. The 
Niagara River, from the Falls to Lewiston, is an ideal national 
boundary; its channel does not shift, it prevents smuggling, and 
in case of war it would need no defence except where the gorge 
was bridged. 

76. How a river grows. Every permanent river rises on 
high land and flows down grade, ultimately joining some 
large body of water, a lake or an ocean, but it grows or it 
lengthens in the opposite direction. Every rain forms a few 
new rills, possibly a gully, through which the runoff finds 
its way into the river. Each successive rainfall deepens some 
of these new channels until they become permanent tribu- 
taries of the main stream, which has increased its length 
as outlined above by headward erosion. 

FIG. 42. Headward Erosion 

The dotted lines show temporary streams which will become permanent 

In the meantime, other streams are developing in the 
same way until they divide up the entire region among 
them. The ridge which separates two drainage systems is 
called the divide (Fig. 43). The valley of a river is the de- 
pression cut out of the land by the river and all its tribu- 
taries, but in a broader sense it refers to the entire drainage 
area of the river. 



77. Stream capture. As rivers grow headward, one of 
them may succeed in cutting down faster than another, 
possibly because its slope is greater or because its bed is 


FIG. 43 

Stream 1 

Stream 2 

Stream 1 

FIG. 44. Stream Capture 

Stream 1 has cut through the divide and captured the headwaters 
of Stream 2, forming a new divide. 

softer. Ultimately it may penetrate the divide, diverting 
the headwaters of a rival stream. This is called stream cap- 
ture or stream piracy. It is shown in Fig. 44. Stream 1 has 


Photo by Ewing Galloway 

FIG. 45. The Ausable Chasm 

breached the divide, captured Stream 2, and formed a new 

78. A river in youth. The gradient of a youthful river is 



steep. It bites deeply into its bed; it flows in a straight 
course, overriding obstacles in its path. While the stream 
cuts down, weathering plays its part on the new surfaces 
exposed and the loose material thus formed is washed into 
the stream. 

This widens the valley at the top, giving it a V-shaped 
appearance. If the bedrock is hard and resistant, the weather- 
ing process may act much more slowly than the downcutting 
of the stream. In that case the river will cut a gorge with 
almost vertical sides (Fig. 45). 

Young rivers are not deep, because the water runs too 

FIG. 46. Diagram of Rapids Due to Erosion 

rapidly. In places the bedrock may be visible where the 
slope is very steep, while in other places, boulders interrupt 
the movement of the water. Such features are called rapids. 
Rapids are sometimes developed by a stream itself, because 
of difference in the rates of erosion of two kinds of rock 
in the stream bed. Figure 46 shows limestone and shale, 
which were in contact with each other along the line B'C' 
when the stream was at the level A BCD. The shale eroded 
faster than the limestone, developing the present bed of the 
stream A'B'C'D' with rapids from B' to C". 

If the strata dip downward toward the mouth of the 
stream, as in Fig. 46, mentioned above, the rapids will be 


quite permanent; but if they dip toward the source of the 
stream, as in Fig. 47, or if they are horizontal, the rapids 
will soon develop into a waterfall, unless the stream is a 
small one. 

79. Waterfalls. The typical waterfall is one over which 
the water tumbles vertically as it does at Niagara. Between 
this and the typical rapid, there are all degrees of slopes, 
which might be called cascades. Falls are due, as a rule, to 
differences in the resistance of the rocks forming the bed 
of the stream. Some, like Niagara, St. Anthony, and the 

FIG. 47. The shale erodes faster than the limestone, 
forming a waterfall at B. 

Great Falls of the Missouri River, are due to a cap of resist- 
ant rock that overlies a mass of weaker rock (Fig. 48) . 

Others, like the two falls of the Yellowstone, and the 
Passaic Falls at Paterson, New Jersey, are due to igneous 
rock, which offers much greater resistance to erosion than 
the rocks above and below it. 

Occasionally, falls are found at the point where a tributary 
joins the main stream. Such falls have been caused by a 
deepening of the channel of the main stream by glacial 
erosion (Fig. 49). The tributary stream which has been 
left hanging above the main stream is then said to have a 
hanging valley. There are many falls, due to hanging valleys, 
in the Finger Lakes of New York, like the one at the mouth 



of Watkins Glen. These were all formed by the gouging out 
of the lake beds by a great glacier. 

FIG. 48. Niagara Falls, Showing Layer of Limestone Overlying 
Weaker Rocks 

FIG. 49. Hanging Valleys 

In many instances, a mass of debris dropped by a glacier 
in the path of a river has dammed it up and caused the 
water to overflow the dam. 

Along our Atlantic coast, a number of rivers originating 
on the eastern slope of the Appalachian Mountains form 



falls or rapids. This has resulted from an uplift of the entire 
Atlantic coast region, which increased the gradient of all 
the rivers. These rivers naturally cut more rapidly in the 
softer sedimentary rock of the uplifted coastal plain than 
in the old crystalline rocks in the mountains, and therefore 
the fall line was developed close to the contact between the 

sedimentary and the crys- 
talline rocks. 

At the bottom of falls, 
potholes are frequently 
formed. They are caused by 
stones whirled around and 
around by the water, as at 
C in Fig. 48. 

80. Waterfalls are tem- 
porary features of streams. 
Erosion at a waterfall is 
much greater than elsewhere 
since the gradient is steeper. 
Hence, falls usually migrate 
upstream and finally disap- 

Many falls, like Niag- 
ara, are situated on the up- 
stream end of a gorge of 

considerable length which has been formed by the recession 
of the fall. It has been shown by careful surveys that the 
center of the Horseshoe Falls at Niagara is traveling toward 
Lake Erie at the rate of about five feet a year. Similar though 
less rapid recession takes place in all falls of this structure. 
Examination of Figure 48 will explain why the falls at 
Niagara recede. The falling water, as it strikes the bedrock, 
forms two whirlpools, / and K. These pick up the rock 
fragments and dash them against the face of the cliff with 
great force, cutting away the soft underlying shale and 
undermining the harder limestone cap rock. As the support 

FIG. 50. Map of the Fall Line 



is cut away from under the limestone, it finally breaks and 

*81. Locks. Both falls and rapids interfere with navigation, but 
the great value of their water power leads engineers to try to 
prevent their destruction and to build locks and canals, so that 
vessels may pass the falls or rapids safely. Figure 51 shows a vessel 
in the lock at St. Mary's Canal passing from Lake Superior to 

Courtesy of U. S. War Department 

FIG. 51. Locks of the St. Mary's Canal 

Lake Huron. In the illustration one gate is seen, but there is 
another one at the other end. When the vessel wishes to go from 
Lake Superior to Lake Huron, the gate at the Lake Superior end is 
opened, while the other gate remains closed, and the vessel enters 
the lock. The Lake Superior gate is now closed, and water is per- 
mitted to run out of the lock to the Lake Huron side. When the 
water in the lock is at the level of Lake Huron, the gate is opened 
and the vessel may leave. 

When the difference of elevation is too great, a series of locks is 
made use of. At Trollhatte, Sweden, three locks are used to over- 
come a fall of 77 feet. The two Gatun locks on the Panama Canal 
lift vessels a total of 85 feet. 

82. Water power. Satisfactory conditions for the devel- 
opment of water power may be obtained on any stream 
with a steep slope by building a dam across the stream, 
thus forming an artificial fall and providing a reservoir that 
will store water for use during a dry season. A dam across 


the Mississippi at Keokuk, Iowa, develops 300,000 electrical 
horsepower which is transmitted to St. Louis, 144 miles 
away. The Susquehanna River generates 120,000 horsepower 
which is sold in Baltimore and Philadelphia. But water- 
power plants are chiefly found on young rivers or in the 
young section of an older river, that is, nearer the source 
of the river. This is true for several reasons. Young rivers 

FIG. 52. Developing the Water Power of the Columbia River 

are not usually navigable and can therefore be dammed up 
without obstructing river traffic. Also it is easier to build 
a dam on a young river, because of its narrow profile. The 
water flows more rapidly in a young stream, developing 
more power per unit of volume. 

The estimated output of all the water-power sites in the 
United States is 80 million horsepower, of which about 16 
million (about 20%) was being utilized in 1936. Niagara 
Falls supplies power to a great portion of New York State, 
as far as Syracuse, 200 miles away. A large section of the 
South will be supplied by the Muscle Shoals hydroelectric 
plant on the Tennessee River, aided by the plants on the 
Coosa and Tallapoosa rivers. The Boulder Dam project on 
the Colorado River will supply power and water to Los 
Angeles and the neighboring territory, and the Grand Coulee 
Dam will develop the resources of the Pacific Northwest. 


San Francisco gets power from the Sierra Nevada Moun- 
tains at Colgate. 

Great Falls, Montana, is one of a number of important 
falls on the eastern slope of the Rocky Mountains. 

Falls that have played an important part in the develop- 
ment of the textile industry of New England and in the 
growth of the cities near them are located at Fall River, 
Fitchburg, Lawrence, Lowell, and Taunton, Massachusetts; 
at Manchester, N. H. ; at Lewiston, Maine; at Pawtucket 
and Woonsocket, R. I. 

Other cities, located along the fall line where they can 
make use of the water power, include Passaic, N. J. ; Phila- 
delphia, Pa. ; Baltimore, Md. ; Washington, D. C. ; Richmond, 
Va; Raleigh, N. C.; Camden and Columbia, S. C.; and Au- 
gusta, Ga. 

In New York State, water power is obtained from three 
groups of streams: (1) Streams flowing into Lake Ontario 
provide power at Niagara Falls, Rochester, Auburn, Oswego, 
and Watertown; (2) the Hudson and its tributaries at Troy, 
Glens Falls, and Cohoes; (3) rivers of southern New York 
at Jamestown, Binghamton, and Elmira. 

The city of Minneapolis obtains water power from the 
falls at Sault Ste Marie in Michigan, while in other parts 
of the country, power is obtained from rapids, notably those 
of the St. Lawrence River. 

83. A river in maturity. A mature stream no longer has 
steep sides, the slopes having been reduced by downcutting 
during youth (Fig. 53). Rock mantle produced by weather- 
ing is no longer swept down into the valley so rapidly. In- 
stead, it fills in the irregularities of the surface, rounding 
off the angles of youth into the gentle curves of maturity. 

Rapids and falls have largely disappeared and the streams, 
having lost much of their velocity, can no longer flow over 
or sweep aside every obstacle in their path. The water now 
lingers a bit longer at each level and does more sidewise 
cutting, widening the valley and developing a flood plain. 



*84. Influence of mature topography on man. Agriculture is 
confined chiefly to flood plains, although it has to contend with 
spring floods there. 

FIG. 53. A Mature Region with Its Gentle Curves 

Navigation is interfered with by sand bars and the low water of 
summer months. Roads and railroads are, as a rule, located in the 
stream valleys, and flood plains are the only areas for townsites or 
for farming. 

Mining is the most important industry of the interstream spaces 
in many mature regions, because of the ease with which we may 
discover valuable minerals which have become exposed by erosion 
of the bedrock. 

Other possible industries are forestry, hunting, and trapping; 
but in the absence of mining, the inhabitants of these regions 
usually live a life of poverty, hardship, and ignorance. 

85. Economic importance of flood plains. The soil of 
flood plains is often very fertile, and the level ground is 
easily tilled. The river furnishes water and provides an easily 
traveled highway on which the farm products can be carried 
to market. These two characteristics, fertile soil and ac- 
cessibility, have made flood plains so desirable that they are 
nearly everywhere densely populated. 

The flood plain of the Mississippi, below the mouth of 
the Ohio, is from 20 to 50 miles wide and about 600 miles 
long. The region is densely populated and fine crops of corn, 
cotton, and sugar cane are raised there. 


The flood plain of the lower Rhine in Holland is one of 
the most densely populated and carefully cultivated regions 
of Europe. The flood plain of the Yellow River, in China, 
probably has the densest population in the world. 

FIG. 54. An Old River, Showing Its Meandering Course and Wide 
Flood Plain 

The advantage derived from living on flood plains is 
shown in history. Egypt developed on the flood plain of the 
Nile, and Chaldea and Babylon on the plains of the Eu- 
phrates and the Tigris. These rivers were so important among 
the ancients that the period before 800 B.C. is sometimes 
referred to as the " fluvial (or river) period" of history. 

86. An old river. The velocity of a river in old age is 
so far reduced that it is turned aside by the slightest ob- 
stacle. Hence its course becomes very crooked and winding 
and the flood plain is very much widened. The river is wide, 
but deposition of material in its channel makes it shallow. 
Hence in the spring, floods are common, and these deposit 
alluvium on the banks, forming natural levees. Gradually 
the stream is built up on a higher level than its flood plain 
so that rain falling on the flood plain cannot flow into the 
main river and swamps develop all along the natural levees. 
For the same reason an old river has very few tributaries. 

87. Meanders. The winding streams characteristic of old 
regions are called meanders after a small river of that name 



FIG. 55. Development of Curves in a Stream 

FIG. 56 

in Asia Minor. When a stream is moving in a straight course, 
there is little erosion of the banks. But an ob- 
struction will cause slowly moving water to be 
deflected as shown in Fig. 55. The water will 
then be deflected back to the other side and 
begin undercutting at Y. In this way, once the 
river has started to curve it continues to do so 
and becomes more and more curved. 

On the outside of a river curve, there is usu- 
ally a vertical wall called the cut-bank, because 
here erosion is greatest. On the opposite side 
the velocity is least and deposits will be formed. 
This is called the slip-off slope (Fig. 56). 

It is evident that as the stream cuts away 
one bank, as at A and B, Fig. 57, and deposits 

material on the slip-off slopes at C and D, the course of the 

river will be more crooked than it was. 

88. Cutoffs and oxbow lakes. The meander, shown in 

Fig. 58, has developed to such a point that the two cut- 

FIG. 57. 
Growth of 
a Meander 



banks have been worn away, thus providing a shorter, 
straighter path for the water. The river now follows the 
new course, called a cutoff. Soon the entrances to the old 
channel will be blocked by deposits, since at those points 
the velocity of flow is less than in the main channel. This 
forms an oxbow lake (Fig. 59). 

89. Influence of old topography on man. The area adapted 
to agriculture is greatest in an old region because flood plains 
are very wide. Portions of the flood plains may be wet and 
require draining, but the fertility of the land amply repays 
the farmer for his labor. Because of the absence of steep 
slopes, it is easy to build roads and railroads and therefore 
many towns will be located in old regions. There are no 
falls or rapids to interfere with navigation, but the deposi- 
tion of silt is apt to obstruct the channel. 

90. Summary of the cycle of stream erosion. 






Flood plain 

Rather straight 

Scenic ; falls and 

Poor ; swamps and 
lakes in uplands 




Rounded; falls 
and rapids un- 

Good; no swamps 
or lakes 

Completely cov- 


Very many 


Flat; no falls or 

Good, except on 

flood plain 

Wide. River has 

natural levees. 
Very few 

91. The interrupted cycle of stream erosion. Many 
streams never complete their normal life cycle, from youth 


to old age, because of sudden change of slope or change of 

climate. Change of slope is brought about by raising or 

lowering of the land. 

92. Effects of depression. 
When the land is lowered, 
the region increases in age 
and the cycle of stream ero- 
sion is shortened. The chief 
evidences of depression are 
to be found at the mouth of 
the river. Here the entire 
flood plain is drowned, pro- 
ducing bays, which when 
narrow are called estuaries 
or fiords. Most of the rivers 
on the Atlantic coast have 
been drowned, and it is this 
feature which is responsible 
for the fine harbors found 
93. Effects of elevation. When the land is raised, the 

region becomes more youthful since streams begin to cut 

into their beds once more. Such 

a region is said to be rejuvenated. 

If the uplift is slow enough to 

permit the stream to cut down 

as fast as it is raised, then its 

course will not be altered by a 

mountain which rises across its 

path; instead the river will cut 

right across the mountains, main- 
taining its original course. This 

is an antecedent river, so called 

because it antedated the present 

topography. An antecedent river 

cutting across a mountain chain 

FIG. 58. A Cutoff 

FIG. 59. Oxbow Lakes, Sand De- 
posits, and Main Channel of the 
Mississippi River 


produces a water gap. Thus the Green River passes through 
the Uinta Mountains and the Hudson River through the 

FIG. 60. Water Gap Cut by an Antecedent Stream 

Highlands. The Delaware, the Susquehanna, and the Poto- 
mac rivers have cut water gaps across the Appalachian 
Mountains; hence they are antecedent streams. 

*The capture of an antecedent river may dry up a water gap, 
leaving what is then called a wind gap. A good illustration of a 
wind gap is shown in Fig. 61. The greater volume of the Potomac 

FIG. 61. Showing how a Water Gap Is Changed into a Wind Gap 

River enabled it to erode the rock more rapidly than Beaverdam 
Creek. This increased the gradient of the Shenandoah River so 
that it captured the head waters of Beaverdam Creek, leaving the 
wind gap instead of a water gap. The name wind gap is deceptive. 
Wind action has had nothing to do with its formation. 


When a region is uplifted, a stream is made younger and 
straightens its course, but if the uplift is so rapid that sidewise 
cutting cannot keep pace with downward erosion, a meandering 
river will maintain its course, forming a deep meandering gorge, 
called an entrenched meander. 

FIG. 62. An Entrenched Meander 

Rivers entrenching themselves in flood plains sometimes leave 
portions of the old flood plain, which remain as river terraces. 
These consist of rather flat beaches, paralleling the river on both 

FIG. 63. River Terraces 

sides and at the same elevation above the present flood plain. 
They are often composed of alluvial material or of rock thinly 
covered by alluvium. Sometimes we find several pairs of terraces 
at different elevations: good evidence of successive uplifts of the 


Completion Summary 

A river increases its length by - . 

The - - may be defined as the line which separates 
two river systems. When one stream cuts through the divide 
and drains the area occupied by another, we call that stream 
a - . 

A young stream - - valley, due chiefly to its velocity. 
It has waterfalls, but - - flood plain. Falls are destroyed 

*Falls and rapids obstruct - , but this may sometimes be 
overcome by - . 

Water power is usually developed on - - streams, be- 
cause - . 

A mature stream has no - . Its flood plain - , 
and the region which it drains is - , and forests 

The old river can be started on a - - course by any 
obstruction, because - . Once started, a meander - 
more and more - ; until a cutoff - . From the 
cutoff, an - - is easily developed. 

An old river - - flood plain, which is so called because 
. The sluggishness of an old stream causes it to de- 
posit - , and the bottom is - - higher than the 
. It is therefore not deep and - - floods. During 
the flood stage, deposits - - and natural levees - . 
An old river - - tributaries because - . From this, 
- swamps - - river. 

When a region is depressed in level, it becomes - . 

An important feature of the rivers of such a region - . 

If a region is uplifted, - - youthful. - - rivers 

may be formed which may - - water gaps. 

*If an - - river is captured, the water gap becomes - . 
Uplift may - - the meandering stream, but if downcutting 
keeps pace with uplift - - entrenched. 



1. State several reasons why rivers aid in exploration. 

2. Why may it be unsafe to use river water for drinking? 

3. What difficulty is sometimes met in using a river as a 
national boundary? What type of river would not have this dis- 

4. What is meant by headward erosion? 

5. Explain how a river grows. 

6. What is the divide? Explain by diagram. 

7. Explain stream capture. 

8. What are the characteristics of a young stream? 

9. Under what condition would a young stream not have a 
V-shaped profile? 

10. Why are young streams shallow? 

11. Explain the formation of rapids. 

12. Explain with diagram one way in which a waterfall de- 

13. What is a hanging valley? 

14. How are potholes formed? 

15. Why are waterfalls only temporary? 

16. Explain the recession of Niagara Falls. 

17. Explain how navigation may be maintained on a river with 

18. Explain the action of locks in river navigation. 

19. What type of river is best suited to the development of 
water power? Why? 

20. Name five cities located on the fall line of the Atlantic 
Coastal Plain. Give reasons for their location. 

21. Why do steep slopes disappear in maturity? 

22. State the characteristics of a mature river. 

23. Why is agriculture confined chiefly to flood plains? 

24. What are the characteristics of an old river? 

25. Explain the development of a wide flood plain. 

26. Show how the bed of an old river is built up above its 
flood plain. 

27. Explain the formation of natural levees. Why are they 
features only of an old river? 

28. For what two reasons are floods common on old rivers? 


29. How are swamps developed along the course of an old 

30. Why has an old stream few tributaries? 

31. Show by diagram how an obstruction starts a meander in 
an old stream. 

32. Make a diagram showing cut-bank and slip-off slope. 

33. Explain how a cutoff is formed. Why doesn't this happen 
on a young river? 

34. Why does an oxbow lake form on an old river? 

35. Why are old regions likely to be densely populated? 

36. In what kind of region do we have the best drainage? Why? 

37. What is the chief effect of depression on the cycle of stream 
erosion? Where does it show most? 

38. Explain the formation of a water gap. 

39. What is an antecedent river? Name one. 

40. Explain how a river can be young and old at different parts 
of its course. 

41. What effect would a change of climate, from moist to arid, 
have on the cycle of stream erosion? 

^Optional Exercises 

42. Write a short article on the effect of river transportation 
on railroad freight rates at the present time. 

43. Write a short analysis of the development of the steel 
industry in this country around the Great Lakes, because of the 
low rates for transportation. 

44. Explain the value of a river in a scheme of military defense. 

45. Write up the subject of water power in the United States 
from the standpoint of power supply, irrigation, flood control, and 
soil erosion. 

46. Explain the development of the flood plain from youth to 
old age. 

47. What would happen to an oxbow lake if the region were 

48. In paragraph 90, the drainage of a young stream is classed 
as poor. Explain how that can be so, in spite of the rapidly flowing 

49. Explain how a wind gap is formed. 


50. How is an entrenched meander formed? What does the 
presence of an entrenched meander tell about a region? 

51. What relation exists between river terraces and uplift? 

52. When a river is "born", it is old. As it grows, it becomes 
younger. Explain the paradox. 



*94. That the problem of floods is becoming more and more 
pressing is shown by the increasing property losses. In 1913 the 
losses in the Mississippi Valley were 162 million dollars while in 
1927 they amounted to 284 million dollars. It was in 1927, also, 

Courtesy Army Air Corps 

FIG. 64. A Flood on the Ohio River 

that exceptionally severe floods inundated New England. In 1936 
the spring floods, swelled by heavy rains, again caused untold 
suffering and damage and it begins to look as if we may expect 
frequent, if not annual, repetitions. 

*95. Where do we get floods? In young, mountainous regions, 
the problem of flooding is not very important. The rivers have 

"This entire chapter is optional. 


steep sides and therefore little flooding results. In the Rocky 
Mountain regions, especially along the Missouri River, cloud- 
bursts cause local flooding which results in washouts on railroads, 
damage to roads, and silting-up of irrigation ditches. But these 
floods subside rapidly. 

In an old region, on the other hand, a stream builds up natural 
levees, which confine it except during time of high water, when it 
breaks through with disastrous results. It is in the Mississippi 
Valley, therefore, where the streams are old and the flood plain 
wide, that we get most of our floods. 

Wherever the winters are severe, as in northern United States 
and in mountainous regions, the melting of snow in the spring will 
cause annual floods. On the Merrimack River at Lawrence, Mass., 
there have been 153 floods, between 1880 and 1933, in the months 
of March, April, and May. 

In western United States maximum flooding occurs in early 
summer while on the Pacific coast, late spring or early summer 
floods are caused by melting snows in the highlands. 

*96. Effect of the rainfall. If the precipitation of rain is well 
distributed through the year, as it is in New England, few floods 
will occur. In semiarid and arid regions, the rainfall is too small; 
but even there, the steep slopes and lack of soil will permit very 
rapid runoff which may result in local flooding. 

In the West, the maximum precipitation occurs in the late 
spring, and on the Pacific it is in the winter; hence flooding is 
common during the spring or early summer. 

*97. Relation of floods and soil. Porous soils permit very little 
runoff; most of the rain sinks into the ground. The Atlantic and 
Gulf coasts, therefore, seldom experience a flood, because of the 
porosity of the sands of the coastal plains, although the rainfall 
is fifty inches per year. In the Great Lakes region, the soil is 
glacial drift, which is likely to be porous. 

When the soil consists largely of clay, it is not porous. Hence the 
runoff is greater, soil erosion is increased, and flooding becomes 
more and more common. 

Needless to say, the surface runoff increases with the degree 
of slope. On a gentle slope it will be as little as 5% of the rainfall; 
but this increases up to 50% on the steep slopes of the Tennessee 
River. These figures are again modified by the nature of the surface 



cover. For example, on an 8% slope at Bethany, Missouri, there 
was about 30% surface runoff from a corn field while it was only 
3.5% where the ground was planted in alfalfa. 

At the same time, the soil erosion increases with the runoff; and, 
with the loss of soil, the land has less and less capacity for holding 
water. Hence the problem of floods is intimately tied up with soil 
erosion. Flood waters spread coarse deposits on good topsoil in 
the lowlands and often ruin the land for agriculture, while, by 


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%&B&FV f ?*. f ~7i'i ' 'JK^^^^^^SKif'SSSf'^ 


'^S; "^'I-u:: tr"";, . ^ -^i^^S 

^"^C ^ w '^*' v ''i''5 ?i o: : ;i;-''-''' - "' v f ^ ;; .^- * ' ; '*** 

'^^^m^.t &"* " ; !?S>A: 

< v^ "* $WL, " '*; "* ,>^ flff, ^VdL"^ 

FIG. 65. Cornfield Ruined by Coarse Deposits Spread on It by a 
Single Rain 

removing topsoil from the higher lands, they ruin that too (Fig. 
65). Here and there, this topsoil may be deposited on the flood 
plain, increasing its fertility. 

It was this fertility which probably attracted farmers to settle 
along the rivers. Then communities sprang up along the rivers, 
because of ease of transportation and water power. With the 
establishment of industry the railroads followed and they found it 
easy to build along the rivers, because the grades were gentle. 
And so the great cities of New Orleans, St. Louis, Louisville, 
Cincinnati, Nashville, and Pittsburgh sprang up on the flood plain 
of the Mississippi River and its branches. 


*98. Levees. Both natural and artificial levees are satisfactory 
devices for preventing small, local floods. At the same time it must 
not be forgotten that increasing the height of the river banks in- 


creases the height of the water above the flood plain, and if the 
levee breaks, the rush of water, and hence the damage, will be 
so much the greater (Fig. 66). 

Levees are usually built locally, in connection with straightening 
of the river, to confine the water to the new channel. They should 

FIG. 66. A Levee on the Mississippi 

not be erected too close to the stream, so that, in case of flooding, 
the capacity of the new channel will be sufficiently large. Local 
construction of levees is no real protection, for it is plain that, 
although the water may not overflow the levees, it can overflow 
where there are no levees and hence the flooding is not prevented. 
That being true, the false sense of security given by local levees 
prompts many to settle on the near-by lowlands, which, in times 
of unusual runoff, will become flooded. 

This was well illustrated on the lower Mississippi where levees 
were built by individual landowners, counties, or other local 
organizations. Wherever the river banks were not settled, or where 
they were inhabited by poor or shiftless people, no levees were 
built and it was soon found that the levees were no protection to 
their prudent builders. About 1850, the United States Govern- 
ment undertook the control of the lower river, and in 1879, the 
Mississippi River Commission undertook the control of floods. 

*99. Other measures for local prevention of floods. The me- 
anders of an old stream have a tendency to become silted up 
and choked with vegetation. This will holdback the water and when 
the level rises, the river will overflow at such points. Clearing the 



FIG. 67. Straightening a Meander 

channel of debris permits the water to flow rapidly past such points 
and prevents local flooding to that extent, especially when the 
banks are also raised and rein- 

Straightening out the mean- 
der by digging a new, straight 
channel will increase the velocity 
of flow, locally, and hence pre- 
vent flooding at this point; but 
it increases the height of the flood 
farther down the river (Fig. 67). 

If to all these measures is 
added the building of floodways 
protected by levees, in order to 
provide an additional channel for 
the swollen river, the local au- 
thorities will have done all they 

On the lower Mississippi 

River, in the Donaldsonville section, the levees are supplemented 
by spillways into the Atchaf alaya River and the Bayou Lafourche 
to Lake Pontchartrain and the Gulf of Mexico. This outlet is a 
much shorter course to sea level. It therefore gives adequate pro- 
tection to the river downstream and, to a limited extent, up- 
stream (Fig. 68). 

*100. Control of floods by a national commission. It should be 
apparent that floods cannot be controlled locally, and even where 
local measures are effective in protecting a particular community, 
they often increase the damage to others. Hence flood control must 
be undertaken by the Federal government with the cooperation of 
all other interested agencies. Only then can really adequate means 
be taken to protect life and property along a great river, if such 
protection is at all possible. And since this is intimately connected 
with soil erosion, the two problems must be placed under the control 
of one commission. The work of such a commission will include 
the following: 

1. Education of the farmer in methods of soil conservation. 

2. Reforestation, to provide adequate cover for the soil, par- 
ticularly on hilly land. 



3. Building of retarding basins or storage reservoirs. 

4. Cooperation with local authorities in building and regulat- 
ing levees, spillways, and floodways and in clearing and straighten- 
ing channels. 

*101. Reforestation. Where the land is hilly and the runoff 
rapid, there is nothing like a forest to hold the water. The ground 

FIG. 68. Spillways on the Lower Mississippi 

covered with leaves and decaying vegetation is like a sponge, while 
the soil is firmly held by the roots of the trees, preventing erosion. 
However, repeated heavy rains will finally saturate the ground 
and then the forest no longer retards the runoff. ' 

Reforestation, then, is an excellent auxiliary to other devices 
for flood control, to say nothing of the additional supply of lumber. 
Such areas can be made part of our national parks and in that way 
be made to minister to our recreation and health. 

*102. Retarding basins and reservoirs. The areas at the source 
of the river are usually hilly and it is there that reforestation is 


most effective. To supplement the forests, retarding basins should 
be built on the river where its profile permits. A series of dams 
behind which the flood waters can accumulate can be built so as 
to retard the flow of water and prevent damage to local flood 
control devices farther downstream. 

And finally, where feasible, large reservoirs must be built, 
capable of holding all the excess water. Such works are very- 
expensive except where they can be used for other purposes, like 
water power, irrigation, or recreation. For example, the Wilson 
Dam on the Tennessee River is used for water power, while it 
also controls the flow of the river. 

These basins tend to become clogged by sediment; but if the 
uplands are adequately covered by trees or grass, the soil erosion 
will be a minimum and this will protect the reservoir. 

^Completion Summary 

Floods are not very disastrous in , because a river 
has - - sides, so that the river channel can hold - . 
In - - regions, on the other hand, where the flood plain is 
and the population - , a flood causes untold misery 

and terrific losses. 

Floods are most common in spring, when . They are more 
common in regions whose soil is - , whose slope - , and 
whose surface cover - . 

The problem of floods is tied up with that of soil erosion; for 
the extraordinary runoff - - topsoil from - and often de- 
posits it in - - where it benefits no one. 

Sometimes coarse deposits from the uplands are , ruining 
the topsoil there, too. Occasionally the fine deposits are laid down 
on the flood plain, making it - . 

Levees are good - ; but natural and artificial levees may 
increase the disaster of a flood, because they permit more water 
- and if the levee does break - . 

Other local measures for preventing floods are - , - , 
and - . 

To be effective, flood prevention works must be under the 
control of - . Besides measures already mentioned for local 
control, floods can be prevented by - and . 



1. In what kind of region are floods frequent? 

2. How does a flood in a young region differ from one in an 
old region? 

3. Why is a flood in a young region more disastrous than one 
in an older region? 

4. Why do most floods come in the spring? 

5. Do we ever get floods in semiarid regions? Why? 

6. What is the relation between floods and type of soil? 

7. Show how artificial levees may be satisfactory for small, 
local floods, but for unusually high floods may be worse than no 
levees at all. 

8. Discuss the relation of floods to soil erosion. 

9. Discuss the protection rendered by artificial levees, touch- 
ing on their control by local, state, or federal authorities. 

10. What effect, on floods, has straightening a meander? 

11. What are floodways? How should they be built? 

12. What are spillways? In what part of a river can they be 

13. Discuss the effect of reforestation on soil erosion and floods. 
On what sort of land is this device particularly needed? 

14. What are retarding basins? 

15. Discuss the relation between retarding basins, water power, 
and irrigation. 

16. What danger is there to retarding basins when the uplands 
are not adequately covered by vegetation? 



103. The snow line. In almost every part of the United 
States, snow sometimes falls. In the southern lowlands it 
may last only an hour, whereas, in some northern states, 
it may last all winter and entirely disappear by summer. 
If we go far enough to the north, we shall find snow even 
in the summer. 

In any part of the earth, we may find snow on the moun- 
tains. Mount Washington has snow fields far into the sum- 

Diller, U.S.G.S. 

FIG. 69. The Snow Line 

mer and Mt. Hood is perpetually snow capped. Even under 
the equatorial sun there is snow on the highest mountains 
throughout the year. 

The lowest line in any region at which snow exists through- 
out the year is called the snow line (Fig. 69) . At the equator 



it is about 18,000 feet above sea level; in the northern Rockies 
it is about 10,000 feet; and in Greenland it is about 2,000 
feet. The nearer the poles, the lower the snow line. In other 
words, at high latitudes, the snow line is at low altitudes. In 
the temperate zone, the snow line is about two miles high. 

104. How a glacier is formed. Just as a snowball is made 
by compressing snow in the hands, so also the slight warmth 
of the sun and the pressure of successive snowfalls change 
the snow first into granular ice, called neve in the Alps, and 
finally into compact ice. As the weight of the ice increases, 
it begins to move slowly down the slope. This moving ice is 
a glacier. 

The Alpine type glaciers, which move along river valleys, 
are called valley glaciers. The Aletsch and Rhone glaciers 
are examples of valley glaciers. We find them at the tops 
of all high mountains. 

When several valley glaciers join together and spread out 
to form one continuous broad ice sheet, we call it a piedmont 
glacier. The Malaspina glacier of Alaska is of this type. 

A continental glacier covers a great area, of continental 
dimensions, like the Greenland and Antarctic ice sheets. It 
is usually thick enough to cover high mountains : 9,000 feet 
in Greenland. Continental glaciers today exist only in the 
polar regions and in Iceland. But in the geological past, 
continental glaciers covered large parts of North America 
and Europe, and existed also in South America, Australia, 
and South Africa. 

105. How a glacier moves. A piece of wax can be easily 
shaped by the application of pressure. Lead pipe is made 
by squeezing the hot but solid lead through a die. These 
illustrations, together with the experiment on "regelation" 
shown in Fig. 70, will make it easier for us to understand 
how a glacier moves. 

It is well known that if a cake of ice is placed upon another, 
the two will soon become one. What has happened? When 
anything is compressed, it must take up less room. When 



FIG. 70. Regelation 

The wire passes through the ice, 
but the ice remains one piece. 

ice is compressed, it takes up less room by changing to water. 
If the pressure is released, however, the water will change 
right back to ice, because its temperature is still below freez- 
ing. When one piece of ice is resting upon another, the pres- 
sure causes it to melt, but the water formed is squeezed out 
from between the two pieces 
of ice, the pressure is dimin- 
ished and the water freezes 
again. In the experiment on 
regelation (Fig. 70) the pres- 
sure of the wire causes ice, 
under it, to melt, and the 
wire sinks down under the 
water. Since there is no longer 
a pressure on the water, it 
immediately freezes again. 

It is by a method similar to the above that we believe 
a glacier moves. It does not slide downhill like a block of 
wood. We find it changing its shape to accommodate itself 
to the land over which it moves. It turns around curves in 
a valley, almost as if it were a liquid; it engulfs and carries 
along large and small boulders which it has encountered 
in its path (Fig. 71). 

Like a river, a glacier moves more rapidly in the middle 
than at its sides. This has been proved by driving stakes 
in rows across the glacier. In time the stakes at the middle 
were found to be in advance of those at the sides. 

Although ice will bend and twist without breaking when 
under pressure, it will crack, like other solids, when it is 
subjected to stresses without pressure. At the surface of a 
glacier, therefore, where there is no weight on it, the ice 
forms crevasses (Fig. 72). 

As a glacier moves downward, it slowly evaporates and 
there comes a place where it is warm enough to melt, es- 
pecially in summer. This marks the front of the glacier. 
Sometimes the glacier retreats, apparently moving uphill. 



Photograph by A. Klopfenstein, Addboden 

FIG. 71. A glacier follows the curves of the valley. 

But this is only because it is melting faster than it is moving 
forward. The rate of forward movement depends upon many 
factors: the slope, the weight of ice, the temperature, and 


the irregularity of the ground. Alpine glaciers have been 
shown to move about one foot per day, whereas some Alas- 
kan glaciers move about 50 feet a day. 

It is easy to comprehend how valley glaciers move down 
the slope; but the movement of a continental glacier is not 

S. R. Capps, U.S.G.S. 
FIG. 72. Glacial Crevasses 

well explained. One explanation has it that, as the ice ac- 
cumulates in great thickness, the weight squeezes the ice 
along the edges at the bottom, turning it to water. The 
liquid escapes from under the glacier and immediately freezes 
because of decreased pressure. In this way, the ice front ad- 
vances. In a similar way, a great mass of thick jelly poured 
on the middle of a table would pile up and finally begin to 
flow out from under, until it covered the table. If a glacier 
reaches the sea, as it usually does in the Arctics, it breaks 
up into huge floating masses called icebergs. 

106. Glacial erosion and transportation. By itself, ice 
has very little power to erode; but pieces of rock, which 
it pushes and drags along, become very effective tools that 
convert a glacier into a huge rasp (Fig. 73). 

A glacier carries its load, which is called moraine, in several 


ways. Pieces of rock falling from a cliff onto the top of the 
glacier are carried along and then dropped. Shelter Rock 
(Fig. 74) was carried about a hundred miles, from the 
Highlands of the Hudson River to its present resting place 

FIG. 73. Cross Section of a Glacier 

Showing lateral and ground moraine, crevasses, and ice 
table. Walther. 

FIG. 74. Shelter Rock, Whitney Estate, Manhassett, Long Island 

on Long Island. Some glaciers carry practically a forest on 
their backs (Fig. 75). 

The glacier pushes some of its load ahead, just as a broom 
would. This is called terminal moraine. Some of it is pushed 
along the sides. This is called lateral moraine. That which 
is dragged along underneath is the ground moraine. 



Boulders in the ground moraine are the chief tools of 
glacial erosion; and in the process, the tools themselves are 
marked and polished. They are not so perfectly rounded as 
stream- worn boulders, but faceted; that is, they have rather 

FIG. 75. Malaspina Glacier in Alaska has a forest growing on it. 

flat faces or facets, which are often striated (Fig. 76). These 
pebbles and boulders, dragged along by the glacier, make 
grooves in the bedrock, called glacial striae, which are parallel 
to each other because they are all in the direction of motion 
of the glacier. 

At the same time the bedrock is smoothed on its surface 
and in some cases large blocks of stone are quarried out. A 
mass of bedrock smoothed 
in this way is called a roche 
moutonne. This expression 
means, literally, muttoned or 
sheep-shaped rock. In the Dol- 
omite Alps, which have been 
thoroughly glaciated, glacial 

FIG. 76. Faceted Boulders, Showing 

striae are often so numerous 
as to give to some imagina- 
tive observer the impression 

that the white rock, smoothed and rounded and streaked 
with glacial grooves, is a flock of huge sheep. 

If the glacier moves from north to south, then the north 
side of an outcrop of rock will be smoothed because the ice 


rises and rides over the obstruction. But if the south end 
is free and unsupported, and especially if the strata project 
above the surface at an angle, huge masses will be broken 
off. Hence the south end of a roche moutonne often shows 
the effects of quarrying (Fig. 77). The Alps and the Ap- 

FIG. 77. Roche Moutonne" 
The glacier moved from left to right. Note quarrying on the right. 

palachians have been planed off by continental glaciers which 
rounded off the irregularities of their ridges and left many 
roches moutonnes. 

Frost action is an important factor in quarrying, especially 
at the source of the glacier. During a time of melting, water 
runs into the joints of the rock; and then when it freezes, 
it expands and cracks the rock, which is then removed 
by the glacier. This erosion forms great amphitheater-like 
basins, called cirques, near the tops of mountains. Some of 
the cirques are afterwards filled with water, forming moun- 
tain lakes or tarns. The jagged appearance of Matterhorn 
and other Alpine peaks is due to the development of cirques 
(Fig. 78). 

The water melted from a glacier in summer often finds 
its way into a crevasse. The eddying water whirls about, 
and, armed with the boulders and finer material it finds 



Photograph by Wehrli 

FIG. 78. The Matterhorn, Showing Cirques 

there, it grinds holes in the surface of the rock under the 
ice. These are called potholes (Fig. 84). 

107. The glaciated valley. Valley glaciers ream out their 
valleys, and give them a U-shape. Projecting spurs of bed- 
rock are ground flat (faceted spurs) or entirely cut off, 
smoothing the walls of the valley and deepening the channel. 



When, later, a river again flows in this channel, it is below 
where it used to be and its tributaries no longer join it on 
the same level, but remain hanging above it, forming a water- 
fall. Such a tributary is called a hanging valley (Fig. 49). In 

FIG. 79. A Glaciated Valley 

the northern regions of the earth, where glaciers are common, 
most of the valleys are glaciated; hence fiords are common 
in Norway, Alaska, and Labrador. 


1. Cirques at its source 4. Faceted spurs 

2. U-shaped 5. Roches moutonnes 

3. No sharp turns 6. Hanging valleys 

108. Glacial deposits. The load carried by a glacier, the 
moraine, is dropped wherever the ice melts. This material 
is not assorted, as is the waste carried by rivers, but is a 
mixture of large and small pieces of rock, some of them 
faceted, together with sand and silt all mixed up. This 
is called glacial drift or till (Fig. 30). The drift deposited 
at the end of the glacier's movement is called terminal 



It is easy to recognize a glacial boulder, for besides 
facets and striae, it is usually a different rock from that on 
which it rests. It is erratic, as we say. That is, it does not 
belong there. For example, on Long Island we find boulders 
of gabbro carried over from New Jersey. Here and there a 
specimen is found that hails from the Catskill Mountains. 
Pieces of copper ore from Michigan have been found in 

When a continental glacier melts, the great quantity of 
water carries away much of the finer till and deposits it 
elsewhere in strata of gravel, 
sand, and clay. Outwash plains 
are nearly level areas formed 
of this stratified till. Northern 
Long Island is almost entirely 
covered by a terminal moraine, 
while central and southern 
Long Island is an outwash 
plain, ten to twenty miles 
wide. Its chief characteristic is 
indicated by such names as 
Flatbush, Flatlands, and 
Hempstead Plains. 

When a valley glacier melts, 
the water that flows from it is 
known as glacier milk because 
of the rock flour it contains. The mass of material soon be- 
gins to deposit in an alluvial cone, containing chiefly coarse 
drift. The finer material is carried farther down the valley. 
Such a deposit of morainal material is called a valley train. 
The very fine material is often deposited as a delta when the 
stream enters a lake. Some of these deposits called varves show 
alternate coarse and fine bands: the coarse layer deposited 
in the spring and summer when the glacier was melting, and 
the fine layer deposited in the winter when the glacier was 
frozen and the lake water was quiet. Some of the other 

FIG. 80. Varves 



forms of deposit produced by running water acting on glacial 
drift are known as kames, eskers, and drumlins. 

Kames are more or less conical mounds of partially strati- 
fied drift deposited along the ice front by temporary streams 


FIG. 81. Formation of an Esker from the Deposits in a Subglacial Stream 

issuing from the glacier. Eskers are " serpentine kames." 
They are formed by streams which tunneled the glacier. 
The more or less stratified material was deposited along the 
bottom of the stream and when the glacier melted and the 
supporting walls of ice broke down, the deposit fell down too, 
forming a serpentine mound that looks like a railroad em- 
bankment before the tracks are laid (Fig. 81). 

FIG. 82. A Drumlin 

Drumlins look somewhat like inverted canoes. The axis 
of a drumlin is always parallel to the direction of ice move- 
ment. The word drumlin means little hill in Ireland. It is not 
definitely known how drumlins were formed, but apparently 
they were given their present shape by the last ice sheet 
that moved over them (Fig. 83). 



Sometimes the morainal deposits are so close together as 
to give the region an undulating appearance called knob 
and basin topography. The 
basins or kettles are of ten filled 
with water. Most of the small 
lakes and ponds of northern 
United States and Canada lie 
in kettles. These features can 
be explained by assuming 
that the glacier broke up in- 
to blocks of ice, the crevasses 
were filled up by drift, and 
when each block of ice, sur- 
rounded by its moraine, 
melted, a lake was left. 

Much of the discussion on 
glacial deposits will be under- 
stood by referring to Fig. 84. 

109. Former extension of 
glaciers. It is easy to recog- 
nize a glaciated region. We 
observe the work of present 
glaciers and by those same 

signs we recognize a region over which a glacier has passed. 
Polished and striated surfaces on the valley sides far above 
the present glaciers in the Alps tell us that those glaciers 
were once far above where they now are. A U-shaped valley, 


FIG. 83. Drumlins on a Topographic 

Note how they point in a north- 
south direction. 

:^V>STerimnal .x:v.V-.V -'.', >Pot Hole 
,.;.-; v^.v Moraine, ;/-; :^. Rock ^-,- ,-,'>..-^ ;, -. './*.7.-,; 

FIG. 84 


in a region now far removed from glaciers, can only mean 
that a glacier once moved through that valley, for we know 
no other force which could have reamed out the valley. 

Of much greater importance is the former existence of 
continental glaciers. The records tell us that in the not far 
distant geological past, about 25,000 years ago, much of 
North America and Europe was covered by continental ice 
sheets. That epoch is known as the Ice Age or Glacial Period. 

110. 'the Ice Age in North America. If we travel across 
the United States from north to south, we are impressed 
by the unlikeness of the topography in the north and in 
the south. 

In the north, the rivers are young, often with rapids and 
falls. Lakes are very numerous thousands in a single state 
like Minnesota. The uplands are level, or if uneven, the 
hills and ridges are covered with a cloak of unassorted and 
usually coarse rock mantle. Numerous boulders, wholly dif- 
ferent from the bedrock, are widely scattered. The moun- 
tains have numerous lakes and swamps in their valleys. The 
soils are all transported, being entirely unlike those formed 
from the bedrock. 

In the south, the rivers of the upland are mature, having 
long ago removed their rapids and falls. There are no lakes 
or swamps in the mountains or in the uplands; and the 
uplands are hilly and cloaked with residual soils. 

The line which separates these two types of topography 
follows roughly the Missouri and Ohio rivers to their sources 
in the Rocky Mountains and in southwestern New York; 
thence westward by an irregular line to Puget Sound and 
eastward and southeastward to the east end of Long Island, 
touching approximately these large cities: Boston, New 
York, Cincinnati, St. Louis, Omaha, and Seattle. 

*111. Causes of the glacial period. There has been much specu- 
lation regarding the causes of glacial epochs, but it seems clear single cause is sufficient to account for the facts. Most 
glacial periods in geological history have occurred when the 



continents were elevated. If the land were raised about two miles 
anywhere in the temperate zone, that region would undergo a 
glacial epoch. But this hypothesis is insufficient, of itself, to 
account for the repeated glacial and interglacial epochs of the 
Pleistocene Period. 

Another theory concerns the amount of water vapor and carbon 
dioxide in the air. These two constituents of the air act like a 

FIG. 85. Southern Limits of the Ice Sheet That Covered Northern North 
America During the Glacial Epoch in the Pleistocene 

The arrows show the directions in which the ice moved. 

blanket over the earth, preventing it from losing its heat rapidly. 
The sea water contains much more carbon dioxide than the air 
and the colder the water the more gas it will dissolve out of the air. 

If, then, the land is uplifted a little, it gets colder; the waters 
dissolve more carbon dioxide out of the air, making it still colder, 
and a glacial period develops. 

A slight rise in temperature would cause the sea water to give 
up its carbon dioxide to the air, the earth would be blanketed, it 
would get warmer, and the interglacial epoch would appear. 


But how get the increased heat? At present our winter occurs 
when we are three million miles nearer the sun than we are in 
summer. The shape of the earth's orbit changes, becoming less 
nearly a circle, and the position of the earth's axis changes too, so 
that there comes a time when we are farthest from the sun in winter, 
and hence our winter is longer than our summer. This hypothesis 
makes our glacial epochs the result of long aphelion winters. 

According to this hypothesis, glacial conditions now exist in 
the southern hemisphere; and the fact that the only extensive 
land area in the antarctic regions is covered by a thick sheet of ice 
seems to support this hypothesis. 

*112. Retreat of the ice sheet. As the ice sheet moved down 
from the north, it invaded a region probably as maturely dissected 
as Kentucky and Tennessee now are. River systems were widely 
branching and lakes had disappeared. When the ice began to 
melt, the ice front retreated and revealed a land with completely 
changed surface. Ridges were planed down, the surfaces of bed- 
rock gouged out to form long parallel grooves (glacial striae) which 
show the direction of movement, and valleys partially or wholly 
filled up with drift. Wherever the glacier paused for a time in its 
retreat, a terminal moraine was formed. If the ice advanced for a 
season, former moraines were obliterated and new ones deposited 
when once more the glacier retreated. The ice sheet did not ad- 
vance and retreat uniformly along its entire front, and records of 
these irregular movements can be found between the Ohio River 
and the Great Lakes, where we find many terminal moraines 
which are roughly parallel to each other (Fig. 86). 

In melting, the ice often left great erratic boulders perched in 
unstable positions (Fig. 87). They are called perched boulders, 
erratics, and sometimes rocking stones. 

By overrunning older ground moraine, long canoe-shaped hills 
called drumlins were fashioned. The formation of drumlins may 
be likened to the leaving of pointed strips of packed snow, when 
wet snow is swept from the pavement. 

The glacier in its advance swept the loose mantle from the 
surface wherever it was high and smooth, and filled in the low 
and irregular places. The effect of this was to flatten out the land, 
making it gently undulating (Fig. 88). 

*113. Effects of the ice sheet on drainage. The smoothing and 



flattening of the land reduced the gradient and made all streams 
sluggish. The great masses of morainal material, dumped all over 

FIG. 86. Map of Erie Moraines 

the land, dammed up rivers and formed lakes. Old streams were 

gouged out, making them deeper, but many new streams were 

formed on the surface of till. These streams carried large volumes 

of water from the melting ice 

much larger than they have ever 

had to carry since then ; and we 

find that many of the rivers of 

the glaciated region occupy 

valleys apparently too large for 


As the ice sheet advanced, it 
smoothed out the surface, but FlG 87 A Perched Boulder or 
as it receded, the moraines Rocking Stone 

formed an irregular surface 

called kettle moraine. Many of the kettles were filled with water. 
Those that were very shallow soon became choked up with vege- 
tation. In this way the numerous high-level marshes of northern 
United States and Canada were formed. 



*114. Formation of the great lakes. Rivers that flowed north, 
like the Red River, were dammed by the ice to form lakes, and the 
volume of water from the melting ice was so great that these lakes 

FIG. 88. Glacial Drift Filling Depressions and Making the Surface Flatter 

occupied very extensive areas. The one formed in the valley of the 
Red River, of which the present Lake Winnipeg is a remnant, is 
called glacial Lake Agassiz. It was several times the size of our 
present Lake Superior. With the passing of the glacial epoch it was 
drained, and its silt-covered bed is now one of the greatest wheat- 

FIG. 89. Glacial Lake Agassiz 

producing regions in North America. These lakes drained into the 
Illinois River, past Chicago, but later they found a lower outlet 
into the Mohawk River, which emptied into the Hudson River at 
Little Falls, New York. When, however, the ice-dammed St. Law- 
rence valley was cleared, the drainage found its present course 
(Fig. 90). 






The Great Lakes, in other words, occupied what were probably 
preglacial river valleys. At first, the vast amount of water pro- 
duced by the melting ice flooded the region, and for a time there 
was one vast lake. Then as the ice-dammed rivers were opened up, 
this lake was tapped and as its water dropped in height, the several 
lakes took their present form. The margins of these older lakes 
have been traced by their beaches and other shore-line features. 

Other large lakes, like the Finger Lakes of New York and most 
of the lakes of northern New York and New England, occupy 
basins produced by glacial drift, which dammed up ancient river 
valleys sufficiently to hold back the water formed by the melting 

115. Economic importance of glacial drift. The presence 
of vast deposits of glacial drift in northern United States 
has played an important part in determining the lines of 
its economic development. The general result of the deposi- 
tion was to leave this region more nearly level than before 
the coming of the glacier. This has favored the building of 
roads and railroads, which in turn promoted commerce. 

A thicker covering of rock mantle is found in the glaciated 
than in the unglaciated regions, and this favors the more 
even and constant flow of rivers. The numerous lakes also 
help to equalize the flow of streams, making transportation 
by water possible. River transportation in the south is very 
limited, whereas in the north, the lakes, the rivers, and the 
canals make carriage by water of importance, because it is 
very cheap. 

The soils of the north and south are very different. In 
the northeast, the soils are coarse and sandy and the surface 
is cumbered with glacial boulders. Farther west, the soils 
are of a finer texture, free from boulders, and very productive. 
It is probable that the difference in the character of the 
crops raised in the two sections, north and south, is due 
rather to difference of climate than to difference of soil. 

Clay, sand, and gravel are obtained from glacial deposits. 
The clays are used in the manufacture of bricks, tile, and 


earthenware. Sand is used in making glass, in building, and 
in many industrial enterprises. Gravel is used to make con- 

Completion Summary 

There is always snow above - . 

A valley glacier - - river - ; while a continental 
glacier overrides - . 

A glacier moves on a layer of - , formed by pressure 
of- -. 

A glacier carries rocks on top, , and . The 

glacial load is called - . The ground - - is the chief 
tool of erosion. Boulders grind and smooth off surfaces into 
. Grooves called - - are worn into the bedrock. 
The direction of these grooves tells us - - glacier. 

When the ice begins to melt, the water - boulders, 

and - - potholes. 

A glaciated valley has - - walls, has - - shape or 
profile and its tributaries join - - waterfalls. It is - 
hanging valley. 

Glacial - - is unassorted. It usually erratics, 

or specimens of rock - . 

When the glacier melts, it deposits - - in irregular 
piles. The end of the ice sheet is marked out by - 

Running water works over the moraine and forms mounds 
of various shapes. These are called - , , and 

Kames are - - stratified mounds of - , irregu- 
larly - - in shape. - - are kames that have been 
- by streams in and under the ice. They look like 

Drumlins are shaped like - , each one with its axis 
in which the glacier moved. 

About - - years ago, the northern part of Europe 
and North America was ice sheet. The glacier 


from the north, smoothing off , deepening and grind- 
ing - , carrying soil and loose boulders from - - to 
, where the moraine was deposited. 

*The causes of the glacial period are not fully understood. 
One theory - - elevation above the snow line. Another theory 
starts with a slight uplift which makes the temperature - . 
Colder water more carbon dioxide, removing it - air. 
Both moisture and carbon dioxide having been removed from 
the air, the earth loses - - by radiation and it gets - 
until - - glacial epoch. This cycle of changes would reverse 
itself, if the temperature - - slightly. 

The movement of the continental glacier changed - 
northern United States. The surface was left - , valleys were 
, many new lakes, like - , were formed. 


1. What is the snow line? How high is it in the United States? 

2. How does a glacier form? 

3. What are valley glaciers? Name one. 

4. What is a piedmont glacier? Name one. 

5. What is a continental glacier? Name one. 

6. How does a glacier move around a curve? 

7. Where are crevasses formed? 

8. What is meant by the retreat of a glacier? 

9. How are icebergs formed? 

10. What are the tools of glacial erosion? 

11. What is moraine? 

12. State several ways in which a glacier transports material. 

13. What is a terminal moraine? 

14. What is a lateral moraine? 

15. What is ground moraine? 

16. What is a faceted boulder? 

17. What are glacial striae? 

18. What is a roche moutonne? 

19. Explain how frost action produces a cirque. 

20. What is a tarn? 

21. How are potholes formed? 

22. What is the profile of a glaciated valley? 


23. What is a hanging valley? 

24. Where are fiords commonly found? Why? 

25. Explain how faceted spurs are formed. 

26. What are the characteristics of a glaciated valley? 

27. What is glacial drift? 

28. What is an erratic? Give an example. 

29. How are out wash plains formed? 

30. What is glacial milk? 

31. What is a valley train? 

32. How are varves formed? 

33. What are kames? 

34. What is an esker? 

35. What is a drumlin? 

36. What is meant by knob and basin topography? 

37. In what ways does the topography of the north differ from 
that of the south? 

38. Name five cities which are nearly at the southern limit of 
the great continental ice sheet that covered northern United States 
in the Glacial Period. 

39. What effects has the continental glacier had on commerce 
in northern United States? 

40. In what way do soils of the north differ from those of the 
south? Why do we find stone walls used to mark out fields in New 
England and not in Virginia? 

41. Why do we find many lakes in Wisconsin, Minnesota, and 
New England, and none in Kentucky, Tennessee, and the Caro- 

* Optional Exercises 

42. Explain regelation and apply it to the movement of a 

43. How does a continental glacier move? 

44. Why are cirques formed only near the source of the glacier? 

45. Would a glaciated valley look young or mature? Discuss. 
Does glaciation have any effect on the cycle of stream erosion? 

46. How are kettle lakes formed? 

47. What evidence have we that glaciers once covered much of 
Europe and North America? 

48. What relation exists between elevation and a glacial period? 


49. Discuss the effect of changes in the carbon dioxide and 
water content of the air on a glacial epoch. 

50. What are perched boulders? 

51. Why are glaciated valleys in the region of the continental 
glacier apparently too large for the present streams? 

52. How did the continental glacier affect the drainage of north- 
ern United States? 

53. What was glacial Lake Agassiz? 

54. Why were rivers that flowed north dammed by the ice 

55. How were the Great Lakes formed? 

56. Where did they drain at first? Why? 

57. Why did they change their outlet? 



116. In previous chapters we considered the effects of 
running water of that part of the rainfall which runs over 
the surface into rivers, and finds its way to the sea. Now 
we shall take up the study of that part of the rainfall which 
sinks into the ground. The percentage of the rainfall which 
supplies the ground water depends upon the temperature 
and dryness of the air, which control the rate of evaporation ; 
upon the slope of the land; upon the nature of the surface 
cover; and upon the porosity of the mantle and bedrock. 

Loose mantle may contain as much as 30% water by 
volume, sandstone about 15%, shale about 5%, and igneous 
rock about 1%. 

In the study of soil erosion it was shown that the surface 
runoff from sloping ground planted in corn was much greater 
than from similar ground covered with grass. 

117. The water table. In moist regions the rain usually 
wets the soil as far down as it is porous and then accumulates 
on the first impervious layer (Fig. 91). Near the surface 
the soil is not soaked or saturated with water but it is moist 
or damp because the surfaces of the grains of sand and clay 
are wet. It is important that the spaces between particles, in 
this region of vadose water, are filled with air. If they are 
filled with water the soil is not fertile. It is this moisture, 
called vadose water, which plant roots absorb ; and the capil- 
lary action of clay, due to its enormous surface, is continually 
drawing water up against the force of gravity. 

Farther down the water collects over a layer of impervious 
rock and wets the soil thoroughly so that there are actual 
drops of water between the grains. This is ground water. The 




line separating the zone of vadose water from that of ground 
water is called the water table (Fig. 91). 

In general the water table is more or less parallel to the 
surface of the ground, but if the ground slopes, or at least, 

Zone of 
Vadose Water 

FIG. 91 

if the impervious layer slopes, then gravity will cause the 
water to move down toward lower levels, but it will move 
very slowly. If there were no movement, the water would 
be level; if movement were quite free, as it is on the surface, 

FIG. 92. Water Table on Sloping Ground 

the water would be parallel to the surface, but actually it 
is hi between these two positions (Fig. 92). Wherever the 
water table intersects the surface, a body of water will col- 
lect : a spring, a pool, a lake, a swamp, or a river. 

Sometimes the impervious rock has unusual shapes, and 
therefore the water table will be unusually irregular and fan- 



tastic. Figure 93 shows a " perched water table " caused by 
such a condition. 

The water table is not fixed in its position but varies 
greatly. When it rains, the water table rises; during periods 
of drought, it falls and tends to become level. When a gully 
cuts into a hill, it frequently taps the ground water and the 
water table falls. It can, from this cause, fall so low that 
the piece of ground is made useless for farming. 

Surface waters have been found in rocks as far down as 
two miles; this is due, no doubt, to cracks rather than 
porosity. But it seems hardly possible that water can pene- 

'/ Perched- Water Table ' .' .' 

FIG. 93 

trate much farther than that, because, with the great pres- 
sure of the overlying rocks, it has been calculated there can 
be no cracks or pores in rocks at those depths. 

118. Relation of water table to agriculture. The varia- 
tion in the level of the water table has a marked influence 
upon the usefulness of the region for agriculture. If the water 
table is above the surface, ponds and lakes are numerous; 
if it is at or near the surface, swamps and marshes occur. 
Some of these wet lands can be reclaimed by draining. 

Much of the land now under cultivation is too wet to 
give a good yield. When the ground is soaked, air cannot 
penetrate the soil and it becomes infertile. 

If the water table is too high, the surface is continually 
wet. Minnesota is said to have 8,000 lakes, Connecticut has 


about 1,500, and in general the northern states, east of the 
Mississippi, have much of their surface covered with water. 
Shaler says there are 64 million acres of swampland east 
of the Appalachians that can be reclaimed. In addition, 
large areas on the flood plains of old rivers, like the Mis- 
sissippi, are swampland, at present useless. One sixth of 
the entire state of Arkansas is in this condition. When these 
lakes and swamps are drained, the reclaimed land will be 
very fertile. 

In Holland, it is prescribed that on pasture lands, the 
water table shall be kept 1| feet below the surface; and 
that on land used for general farm crops, it shall be kept 
2| to 3^ feet below the surface. 

If the water table is low, agriculture is impossible unless 
water can be obtained for irrigation. 

*119. Irrigation. There are a few regions where the water table 
is so near the surface that crops do not depend upon local 

The sections of Holland that have been reclaimed are probably 
the most important region of this type. A few other regions 
situated on flood plains have similar advantages. 

For many centuries, man has raised farm crops in arid regions 
by the aid of irrigation. The early Egyptians pumped water from 
the Nile as early as 3400 B.C. The ancient civilizations in the 
valley of the Tigris and Euphrates also depended upon extensive 
systems of irrigation. In the Americas it was first practiced by the 
Indians of Arizona and New Mexico and by the ancient Peruvians 
and Bolivians. 

Irrigation systems cost large sums of money, but when com- 
pleted, they bring two important advantages: an increase in the 
value of the land and an increase in the number of people who can 
make a living from the land. In Oregon, some very cheap land was 
changed, by irrigation, to orchards worth $1,000 an acre. In some 
of the well-developed irrigation districts of China and America, 
as many as 500 people per square mile have been able to live on 
the land. 

There are about 170 million acres under irrigation throughout 





the world, the largest area in India, 50 million acres, and the next 
largest area in the United States, 20 million acres. 

The Roosevelt Dam, built across a canyon in the Salt River of 
Arizona, forms a lake with an area of 25.5 square miles. It is esti- 


FIG, 95 

mated that the water in this great reservoir will irrigate 300 square 
miles of farm land. 

About four tenths of the United States is too dry to produce 
crops without artificial watering, since the entire annual rainfall is 
sufficient to irrigate only about 10% of the arid land. Most of these 


lands are in the west, but even in the east there is much land that 
could profit by irrigation. The capital investment is about $36 per 
acre, cost of preparing the land $18 per acre, and the value of the 
annual crop $41 per acre. 

The program of the United States Reclamation Service contem- 
plates placing about 45 million acres of land under irrigation, an 
area capable of supporting 45 million inhabitants. 

Some of these irrigation projects, like that at Boulder Dam, 
were undertaken for several purposes water supply, water power, 
etc. and many of them in the near future will be entered upon 
in connection with flood control. 

*120. Dry farming. In its literal meaning, dry farming means 
farming without water, which is absurd. The expression has come 
to mean farming in arid or semiarid regions without irrigation, by 
methods that preserve the limited rainfall for use during the 
growing season. A layer of dry dust called dust mulch is formed 
over the cultivated land in order to prevent evaporation of water, 
and in this way the water is stored in the soil for a year or more, 
so that a crop can be produced, perhaps, every other year. 

Dry farming has resulted in almost complete depopulation of the 
lands so cultivated, since most of the enterprises have failed. It has, 
moreover, resulted in dust storms and soil erosion from wind and 
water, causing destruction of the agricultural lands on a grand scale. 

121. Wells. If a hole is dug below the water table, water 
will run into it and any water drawn out will be replaced 
as long as the water table remains above the bottom of the 
hole. Such a hole is an ordinary well (Fig. 96). B is a per- 
manent well because the water table never drops below its 
bottom, while A is a temporary well, since in dry weather 
the water table is below it. Since it has become known that 
typhoid fever, diphtheria, and cholera are transmitted by 
drinking water, laws have been passed by the legislatures 
of many states to protect the water supplies of the cities. 
But about three quarters of the population of the United 
States depends for its drinking water upon wells, and it 
seems certain that thousands of deaths and innumerable 
cases of disease owe their origin to contaminated well water. 



FIG. 96. Temporary and Permanent Wells 

An examination of a large number of farm wells in one 
of our western states showed that about three quarters of 
all the wells less than 25 feet deep, and half of those be- 
tween 25 and 50 feet deep, contained germs of one or more 
of the diseases mentioned, whereas only one eighth of those 
between 50 and 100 feet deep were contaminated, and every 
well over 100 feet deep yielded pure water. 

The average well is a hole in the ground covered with 
planks. Chickens and geese walk over these planks and con- 
taminate them. The pumped well water runs over, cleans 
the planks, and drips back into the well. There is often a 
trough under the pump spout, where horses, pigs, and cows 
come to drink. The filth that they leave near the well is 
often washed into the well by the rain. 

The principal sources of contamination of wells are : surface 
water, manure heaps, sewers, cesspools, barns, chicken coops, 
hog pens, laundry drains, swamps, gas works, slaughter 
houses, starch works, and certain other industrial plants. 

122. Proper location of wells. Wells may be unsanitary 
because of improper location with respect to the flow of 
ground water from sources of contamination, or because of 
improper construction. 

In porous soils the flow of ground water is almost parallel 



to the slope of the ground. Hence the well should be located 
above all possible sources of pollution (Fig. 97). There are 
cases where the flow of ground water is not parallel to the 

FIG. 97. Wells should be placed above all sources of pollution. 

FIG. 98. Where should the well be located: at A or at B? 

surface, however (Fig. 98). In such a case the usual rules 
cannot be followed, and it is essential to get expert advice 
before locating the well. 

123. Construction of wells. 
A type of well that has proved 
very satisfactory is shown in 
Fig. 99. It is lined with brick 
or stone, laid in cement. It has 
a cement or stone top, with an FlQ 99 The Mason Well 

iron manhole on it. An apron 

of cement with a radius of about ten feet surrounds the well, 
and a tile drain carries away the waste water. 


The following general laws of sanitation of wells should 
never be violated: 

1. Locate a well so that the natural flow of the ground 
water cannot bring filth into it from any source whatever. 

2. -Construct a well so that no water can get into the 
well at any point above the water table. 

124. Artesian wells. In some deep wells, the water rises 
to the surface and overflows or is projected into the air 
(Fig. 100). The first well of this sort seems to have been 
located at Artois, France, and all wells like those at Artois 
are called artesian. 

The pressure which is responsible for the flow of artesian 
water owes its origin to the collection of water in a porous 
layer surrounded by two impervious layers (Fig. 101). Rain- 
fall over the collecting area sinks into the porous rock, called 
the aquifer (water-bearing rock), in which it is trapped by 
the impervious layers above and below. If the aquifer is 
tapped anywhere below the water table, water will flow out 
under the pressure of the water above it. It should be noted, 
however, that an artesian well does not depend for its supply 
of water upon the local water table, but upon a distant water 

These aquifers are usually sandstone or loose beds of sand. 
Artesian wells are common on the Atlantic Coastal Plain 
from New York to Texas because of the presence of underly- 
ing loose sands. Underlying the middle western states there 
are several aquifers which can be tapped for artesian water, 
but in New England and eastern Canada there is no possibil- 
ity of artesian wells, because of the presence underneath of 
metamorphic rocks. Boise City, Idaho, Memphis, Tennessee, 
San Antonio and Houston, Texas, and Brooklyn, New York, 
are a few of our great cities that have artesian well-water 
supply. Some of these wells are as much as 4,000 feet deep. 

Artesian well water is usually of exceptional purity, be- 
cause the impervious layers keep surface impurities out, 



N. H. Darton, U.S.G.S. 

FIG. 100. Artesian Well at Lynch, Nebraska 
Flow, 60 barrels a minute. 

while the great depth of the well insures thorough filtering 
of the water. 

125. Springs. Wherever the water table intersects the 



surface, water comes out of the ground (Fig. 91). If the 
water flows out in a current it is called a spring. 

Fissure springs. Ground water frequently comes to the 
surface through a fissure in the bedrock, forming a fissure 




FIG. 101 

spring (Fig. 102). The structure of such a spring is prac- 
tically identical with that of an artesian well, except that 
the spring is natural, while the artesian well has to be drilled. 
The water from fissure springs is usually wholesome, like 



FIG. 102. A Fissure Spring 

artesian well water. It is usually rather cold, because it 
comes from great depths where the sun's heat does not 

Mineral springs. Certain springs, like those at Saratoga, 
New York, and Vichy, France, contain carbon dioxide in 



solution, which causes them to effervesce, or bubble, as the 
water flows from the spring. Others, like the White Sulphur 
Springs of Virginia, the Fountain of Youth in Florida, and 
those near the Finger Lakes in New York State, contain 
hydrogen sulphide, an ill- i^,^,^,,^,.^^^ 
smelling gas, in solution. 

Nearly all springs, coming 
from deep sources, contain 
much dissolved mineral salts 
which give the water medicinal 
properties. Among these salts 
the most common are sodium 
chloride and sulphate, calcium 
sulphate and bicarbonate, 
magnesium sulphate, chloride, 
and bicarbonate, and iron bi- 
carbonate. These salts have 
undoubted effects on the body, 
but their curative properties 
are very much overrated and 
they should not be taken in 
quantity without the advice 
of a physician. 

Hot springs. The heat of 
some spring waters is probably 
due to their contact with 
heated rock, since most hot 
springs occur in volcanic re- 

FIG. 103. A Geyser in Action 

gions, near Fujiyama in 

Japan, near Vesuvius in Italy, 

near Lassen Peak hi California. There are great springs of 

boiling water in Yellowstone Park and in Arkansas and 

other regions showing evidence of volcanic activity in the past. 

*126. Geysers. A gushing hot spring in Iceland is called a 
geysir. The Giant Geyser of Yellowstone Park throws a column of 



hot water 250 feet high and continues for an hour or more. Some 
geysers are irregular but most of them discharge regularly. Old 
Faithful in Yellowstone Park erupts every 65 minutes. Geysers 
occur in Yellowstone Park, Iceland, and New Zealand (Fig. 103). 
The action of a geyser depends upon the effect of pressure on 
the boiling point of water. Water at atmospheric pressure boils at 

212 F. Increasing the pressure 
causes water to boil at a tempera- 
ture higher than 212 F. For ex- 
ample in Fig. 104, the boiling points 
of water are shown at the different 
depths, 250 and 275 F., and, where 
the pressure due to the weight of 
water is greatest, the boiling point is 
shown to be 300 F. The tempera- 
ture actually found at a depth of 
406 feet, in Old Faithful, was 
338 F. 

If the fissure connecting the 
source of hot water to the surface 
is large, the water will rise by convection as it gets hot, and we 
have a hot or boiling spring. But if this fissure is narrow, then we 
get the effect of a coffee percolator. Convection is prevented in a 
narrow tube because the overheated water down below cannot rise 
and mix with the cooler water above; it gets hotter and hotter 
until it reaches the boiling point (300 F. in Fig. 104). At that 
temperature it changes to steam which pushes some of the water 
out of the tube. That reduces the pressure and hence decreases 
the boiling point to less than 300 F., let us say 250 F. Therefore 
the entire mass of water which is at or near 300 F. boils instan- 
taneously, changing to steam; that is to say, it explodes, expell- 
ing the water above it with violence. The explosion lets the pent-up 
steam escape, and hence the water settles back into the fissure to 
be heated over again. 

127. Destructive work of ground water. Where the under- 
ground water is in motion, we get the same results as in 
stream erosion; but that is rare. The chief processes by 
which subsurface water acts are oxidation and solution. 

J 300F 
FIG. 104. Diagram of a Geyser 



Pure water alone has little effect on rocks the only 
two which are soluble being rock salt and gypsum. But 
water containing dissolved oxygen and carbon dioxide 
readily attacks rocks containing particularly the elements 
calcium, magnesium, and iron. Water containing dissolved 
iron wets the soil and the bedrock, and, by oxidation, it 
is often changed to rust. This accounts for the rusty color 

FIG. 105. A Large Sink Hole 

N. H. Darton, U.S.G.S. 

of most soils and rocks. Water containing dissolved calcium 
or magnesium salts is called hard water. It forms insoluble 
precipitates with soap and, when used in boilers, it deposits 
boiler scale. Such water may be softened by boiling or by 
adding washing soda. 

The principal rocks affected by the solvent action of the 
ground water are limestone and marble, both of which are 
calcium carbonate. There are few regions on the earth which 
do not contain some limestone beds either on the surface 
or along the underlying rock formations. 


If the limestone is on the surface, the water charged with 
carbon dioxide from decaying vegetation slowly dissolves 
it, aided often by fissures in the rock. These cracks are 
enlarged until great holes are formed, called sinks (Fig. 105). 
If these sink holes are very numerous, the drainage of the 
entire region may be underground, and there are no surface 
streams. This is true in the Karst region, east of the Adriatic 
Sea, and the term karst topography refers to that kind of 

Many of these sinks are above the water table; hence 
they are dry. But there are others, like the small lakes of 

FIG. 106. Sink Hole, Caverns, and Natural Bridge in Limestone 
Can you find each of these features in the figure? 

northern Florida, which are below the water table and hence 
filled with water. 

128. Caverns. When the hole dissolved out by the water 
and carbon dioxide is entirely below the surface, a cavern 
is the result. This will be permanent if the roof is an in- 
soluble layer of rock, like shale (Fig. 106). If it is all lime- 
stone, the roof ultimately falls in and we have a sink. But 
sometimes part of the roof is left as a natural bridge (Fig. 106). 
Caverns are found in all parts of the world. Examples are: 
the Mammoth Caves of Kentucky, the Luray Caverns of 
Virginia, Howe Caverns in New York, Carlsbad Caverns 
in New Mexico, and others in Florida, Cuba, the Philippines, 
Indo-China, and Switzerland. The Mammoth Cave has a 
network of galleries and passages which cross and recross 


one another, with a total length of over two hundred miles. 
It has rivers and lakes. The Carlsbad Cavern is about 1,000 
feet deep. One of its rooms is 200 feet wide by a half mile 

Caverns often contain fossils of animals and men. The 
best-preserved skeletons of prehistoric man, as well as 
samples of his handiwork, are found in caverns in France, 
Belgium, and Spain. 

*129. Calcareous deposits from ground water. When water con- 
taining calcium bicarbonate (formed by the solution of limestone 
in water and carbon dioxide) finds its way through a fissure to the 
roof of a cavern, it often hangs there in drops. If the pressure de- 
creases slightly or the temperature rises, the calcium bicarbonate 
decomposes into its original components : calcium carbonate (lime- 
stone), carbon dioxide, and water. This is shown in the following 
equation : 

Ca(HCO 3 ) 2 -> C0 2 + H 2 + CaC0 3 

calcium carbon water limestone 

bicarbonate dioxide 

The calcium carbonate collects on the ceiling of the cavern 
and increases in size, as drop after drop of the liquid under- 
goes the change. The form of the deposit is like icicles 
hanging from the roof. They are called stalactites. 

Some of the drops fall to the floor of the cavern and there 
build up a deposit of calcium carbonate, called stalagmites 
(Fig. 107). 

if Travertine is the term applied to limestone deposited from 
waters, so that stalactites and stalagmites are forms of travertine. 

When travertine is deposited rapidly on plants growing near 
springs, it forms a soft porous mass with holes in it, through which 
grasses grow; and often it has impressions of leaves and twigs in it. 
This is called calcareous tufa or petrified moss. 

Several dense varieties of travertine are formed by slow evapo- 
ration of hard water (calcium bicarbonate), and show bands of 
different colors, due to impurities. The so-called Mexican onyx is 
the most attractive form of banded travertine, and it is much used 
for ornamental stone work. 



Photo by Russell, U.S.G.S. 

FIG. 107. Stalactites and Stalagmites, Carlsbad Cave, New Mexico 

130. Other deposits from ground water. Alkali is formed 
in arid regions by the evaporation of ground water. This 
material has been dissolved out of the rocks, but since there 
are no permanent streams, it is not carried into the ocean, 
as it is in humid regions. Hence it collects in the soil and 
as the ground water moves to the surface and evaporates, 
it leaves an incrustation of alkali. The chief constituents are 
sodium chloride, sodium sulphate, and sodium carbonate, 
the latter having alkaline properties. 

If there is not too much of it, the alkali may be removed 
by flooding the ground; but often this remedy makes the 
ground worse, by bringing so much alkali to the surface 
that the land is ruined. 

Ground water containing dissolved mineral matter often 
deposits material in the pores or cracks of rocks. It is in 


this way that loose mantle, like sand or gravel, is often 
consolidated into compact rock like sandstone or conglom- 
erate. The cement is most commonly calcium carbonate; 
but often it is iron carbonate, and sometimes it is silica. 

*Material deposited in a fissure, from solution, is called a vein, 
and most often veins are made of calcite or quartz (silica); but 
sometimes gold, silver, copper, and other metallic compounds are 
deposited in veins, and many of these deposits are worked as 
valuable ores. It is believed that most sulphide ores owe their 
concentration to a process called secondary enrichment. The pri- 
mary ore deposit is often not workable, but as the ground waters 
take the oxidized surface ores into solution they often precipitate 
them out in some deeper zone in more concentrated form, making 
it worth while to extract them. 

Silica deposited from solution in rhythmic layers forms agate, 
in which the colors are due to impurities. 

Opal is a kind of silica formed in this way. In some cases, trees 
have been changed to petrified wood by a deposit of silica which has 
replaced the woody material, as fast as it was removed, cell by cell. 

Geyserite is another form of silica deposited in and about the 
crater of a geyser. 

Material in solution silica, calcium carbonate, etc. some- 
times precipitates around a solid like a piece of bone or stone to 
form a concretion. When these are broken open, we may find fossil 
insects, fern leaves, or the shells of marine animals. 

Sometimes quartz, calcite, or other minerals are found in a 
fissure or cavity with their crystals growing out from the walls. 
If a cavity is only partially filled by such crystals, we may get a 
geode. These often look like rounded stones which are found to 
be hollow and lined with crystals. 

131. Summary. Solution and deposition by ground water 
tends to 

1. Collect metallic minerals that were widely scattered, 
to form valuable veins of ore. 

2. Repair breaks and fill cavities in bedrock. 

3. Cement mantle into compact rock. 

4. Form sinks and caverns. 


132. Deposits formed by ground water. 






a. Surface of soil 
b. Bottom of lake 

Gypsum, salt, 


Changes in 
and pressure 

a. Cavities and fissures 

Ores, silica, cal- 
cite, etc. 


b. Porous rock 

Silica, calcite 


c. Geysers 


Craters and 

Loss of car- 
bon dioxide 

a. Ceiling of cavern 
b. Floor of cavern 
c. Near springs 

Calcareous tufa 


Chemical ac- 
tion, precip- 

a. Fissures 
b. Porous rocks 

Sulphide ores 
Calcite, silica, 
iron oxide, etc. 


Ground water 

Completion Summary 

water table. 

A body of water, like a river or lake, is formed wherever 
the - surface. 

In humid regions, the water table , whereas in arid 

regions - . 

A well - water table. Wells should be located . 

An artesian well - aquifer as well as im- 
pervious - . 

If the intersection of the water table with the surface 
causes a small stream of water - - spring. 

*A geyser owes its intermittent action to water being heated 
- boiling point in tubelike . The action resembles 

that of - . 

Subsurface water containing dissolved limestone. 

If the limestone - , sinks are formed. If the limestone 
is capped by a layer of - , a cavern . 


If the water containing dissolved limestone is warmed, the 
limestone will be redeposited. 

The deposits hanging from the roof - ; those grow- 
ing up from the floor - . 

in arid regions, by evaporation of ground water. 


1. What factors control the amount of rainfall that supplies 
the ground water? 

2. How does vadose water differ from ground water? 

3. State the approximate per cent of water contained in differ- 
ent kinds of rock. 

4. Draw a diagram, different from the one in the text, to show 
the water table. 

5. Why is the water table not permanent? 

6. What is the maximum depth to which ground water can 

7. What effect has the water table on agriculture? 

8. How does the water table in a humid region differ from 
that in a dry region? 

9. What is a well? Explain with diagram. 

10. Explain how wells become contaminated. 

11. WTiere should a well be located? 

12. Show by diagram, different from the text, that a well may 
not be safe although it is above the source of contamination. 

13. State the laws of sanitation for wells. 

14. What is an artesian well? Show by diagram. 

15. Define aquifer. What kind of rock does it usually consist of? 

16. Why is artesian water superior in quality to ordinary well 

17. What is a spring? Show by diagram. 

18. What are mineral springs? 

19. What evidence is there that hot springs have some connec- 
tion with volcanic activity? 

20. What elements are dissolved out of rocks by water contain- 
ing carbon dioxide? 

21. Why are most soils brown? 

22. What are sink holes? How are they formed? 

23. What is karst topography? 


24. A great number of small lakes in a limestone region indi- 
cates what origin for the lakes? 

25. What conditions are necessary for the formation of a 

26. What conditions are necessary for the formation of a 
natural bridge? 

27. How are stalactites formed? stalagmites? 

28. What is alkali? 

29. How is sandstone formed from loose sand? 

* Optional Exercises 

30. Show how each of several conditions determines the amount 
of ground water. 

31. Explain with diagram why the intersection of the water 
table with the surface produces a swamp, in one case, and a lake 
in another. 

32. What is a perched water table? 

33. Show with diagram how a gully can ruin a piece of ground 
for farming. 

34. Discuss irrigation, its relation to the water table and to 

35. What is dry farming? What effect has it had on the land? 

36. Explain the impossibility of locating an artesian well in 
igneous or metamorphic rock. 

37. Explain the resemblance between an artesian well and a 
fissure spring. 

38. Explain the action of a geyser. 

39. Explain the chemistry of the formation of hard water and 
its relation to limestone caverns and stalactites. 

40. If a topographic map showed no streams in a humid region, 
what kind of rock would that indicate for the surface? 

41. Why is it reasonable to expect to find fossils of men and 
animals in cavern deposits? 

42. What is travertine? 

43. How are agate, opal, and petrified wood formed? 

44. What relation, if any, exists between the water table and 
an artesian well? 

45. What reasons are there for believing that the water supply 
of your town is not contaminated? 



133. Origin of lakes. Lakes are relatively large bodies 
of water, filling basins or depressions in the land. We have 
already studied the origin of glacial lakes, sink holes in 
limestone, and oxbow lakes. Most lakes are above sea level, 
but some are at sea level coastal lagoons. The great 
majority of lakes are a result of glaciation and therefore we 
find them very numerous in regions that have been glaciated, 
and these regions are found in high latitudes and altitudes. 
That being so, if drainage is good, the lake is bound to 
disappear, and it is only in humid regions, where drainage 
has been interfered with, that a lake is developed. 

Lake basins have the following origins: 

1. Movements of the earth's 4. Rivers 

crust 5. Marine erosion 

2. Volcanoes 6. Sink holes (solution 

3. Glaciers of limestone) 

7. Artificial 

134. Lake basins formed by movements of the earth's 
crust. Lakes may be formed by uplift of the land so as to 
trap an arm of the sea, enclosing it on all sides. This is 
believed to be the origin of the Caspian Sea, which is salty. 
Depression of the land surface is an occasional cause of a 
lake basin. In 1811, a large but shallow basin of this type, 
known as Reelfoot Lake, was formed in the Mississippi 
Valley after an earthquake. 

Other examples of this type of lake basin are Lake Baikal 
in Siberia and the thirty or more lakes of the Great Rift 
Valley, extending 4,000 miles from the Dead Sea to the 




African lakes, Tanganyika and Nyassa. Many of these lakes 
are almost a mile deep. 

135. Lake basins formed by volcanoes. Lakes some- 
times occupy the craters of extinct volcanoes. Crater Lake, 
Oregon (Fig. 108) and Lake Avernus, near Naples, Italy, 

J. S. Ditter, tl.S.G.S. 

FIG. 108. Crater Lake Showing Steep Sides 

are fine examples. Such lakes have very steep sides and are 
often very deep. Crater Lake is 2,000 feet deep. Lava flows 
from volcanoes sometimes dam up a river and form a lake 

in that way. Lake Tahoe in 
California and several lakes 
around Mt. Hood and other 
former volcanoes have this 
origin. They are called coulee 

136. Glacial lake basins. 
Glaciers quarry out large 
basins at their source, called 
cirques, and these are some- 
times filled with water. They are often called mountain lakes 
or tarns. Iceberg Lake in Glacier National Park has walls 
almost 3,000 feet high. It is a cirque lake (Fig. 109). 

Most glacial lakes are the result of deposits. Ninety per 
cent of all known lakes are of this type formed by ob- 

FIG. 109. A Cirque Lake 


struction of drainage by glacial till. Many small glacial 
lakes are formed in kettle holes by the melting ice (Fig. 110). 
Such lakes always occur in groups. When a glacier blocks 
the natural drainage of a region, the water accumulates in 
the basin, one side of which is the glacier. Such lakes are 
found only near the polar region; but during the Glacial 
Period they were large and numerous in the temperate 

FIG. 110. How Kettle Lakes Are Formed 

The glacier on the right once covered the entire land. As it receded, it left 
blocks of ice in holes. One of these, on the extreme left, has formed a kettle lake. 

zone. Glacial Lake Agassiz, one of this kind, was the largest 
lake that ever existed (Fig. 89). 

The Great Lakes, the Finger Lakes of New York, and 
thousands of lakes in Minnesota and other northern states 
and in Canada are of glacial origin. 

137. Lake basins formed by rivers. 

Oxbow lakes. Figure 59 is a map of the flood plain of the 
Mississippi River near the mouth of the Big Black River. 
It shows five oxbow lakes. Lake A shows an early stage in 
the formation of an oxbow basin. A cutoff has been formed 
recently, but the lake is still connected with the cutoff at 
both ends. Lakes C and D have been separated from the 
river at one end by a deposit of sand. At the other end, 
water connection with the river is still possible. Lakes B 
and E are entirely disconnected from the river by sand 
deposited in the ends of the former meander. 

The three stages in the formation of an oxbow lake basin 

1 . Formation of a meander 

2. Formation of a cutoff (A) 

3. Separation of cutoff from river (B and E) 


Basins at the mouths of tributaries. We have seen that the 
banks of an old meandering river are higher than its flood 
plain. When the river is in flood, it occupies the depressions 
on the flood plain, between the natural levees and the valley 
walls, and forms lakes. As the flood recedes, much of this water 
will find its way back into the river; but when a tributary 
enters the main channel it may furnish sufficient water to 
form a permanent lake, which may collect until it flows 
over the top of the levee. Lake Maurepas near New Orleans 
is of this type. 

Delta lakes. Since tributaries are younger than the parent 
stream, their gradient is steeper. When, therefore, they 
join the main stream, their velocity is reduced and deposition 
takes place. If this change of velocity is sufficient, a delta 
will be formed, blocking the main channel and forming a 
lake. In this way the Chippewa River deposited more sedi- 
ment in the Mississippi than it could carry away, and the 
delta thus formed dammed up the Mississippi, which ex- 
panded into Lake Pepin. 

Lake Pontchartrain (Fig. Ill) was formed when the delta 
of the Mississippi, by wave action and shore currents, was 
distributed so as to cut off an arm of the sea. 

The Salton Sea in California is a former arm of the sea, cut 
off by the delta of the Colorado River. In recent geological 
time, its basin has sunk so that it is now below sea level. 

138. Lakes formed by marine erosion. Lake Pontchar- 
train, mentioned above, is an example of a lake formed by 
the action of waves and currents on delta deposits. 

On irregular shores, marine erosion often traps a small 
part of the sea by building barriers. The tide usually main- 
tains a connection through such barriers, but the water of 
the lagoon, as such a lake is called, is quiet (Fig. 112). On 
shore lines of emergence (see page 514), the regular coast 
soon becomes broken up by these lagoons, formed by wave 
action. There is a succession of them on the Atlantic Coast 
from New York to Florida. 




FIG. Ill 

FIG. 112. This lagoon is entirely cut off from the ocean. 

139. Lake basins formed in limestone. In the study of 
ground water we mentioned the formation of caverns and 
sinks in limestone regions. If the bottom of the sink is 


below the water table, it will be filled with water; other- 
wise it will be dry unless the bottom is choked with imper- 
vious material. There are hundreds of these small lakes in 
Kentucky, Indiana, and Florida. 

140. Salt lakes. Some of the salt lakes, like the Salton 
Sea, were at one time arms of the sea, but others started as 
fresh-water lakes. They are all situated in arid regions where 
evaporation exceeds inflow and there is no outlet to the 
sea. The volume of water diminishes and evaporation there- 
fore slows down until an equilibrium is established. At this 
stage the loss by evaporation equals the inflow and the lake 
remains the same size. But since the inflowing waters con- 
tain dissolved salts, while the evaporated water is pure, the 
water of the lake increases in saltiness year by year. This 
is the history of the Salton Sea, the Dead Sea, the Great 
Salt Lake, and the Caspian Sea. Lake Champlain, on the 
other hand, started as a part of the sea, but since it had an 
outlet and was in a humid region, the salt was gradually 
washed out until today it is a fresh-water lake. 

The Great Salt Lake contains about 20% salt, chiefly 
sodium chloride (common salt), sodium sulphate (Glauber's 
salt), and magnesium sulphate (Epsom salts). 

Some lakes contain considerable Glauber's salt, soda, 
borax, or Chile saltpeter. These are called alkaline lakes. 
White deposits of some of these chemicals may be seen on 
the shores and upon all objects that project above the water 
of Soda Lake. Searle's Lake contains all of these chemicals, 
and a number of lakes in the Great Basin are important 
sources of one or more of them. 

The Great Salt Lake is the shrunken remnant of Lake 
Bonneville, which was at one time as large as Lake Huron. 
The wave-cut terraces of the ancient lake, showing the differ- 
ent levels of the lake shore in times past, may be seen in 
Fig. 113. 

141. Playa lakes. Where rainfall is slight, in arid and 
semiarid regions, lakes appear with every rain, only to dis- 


FIG. 113. Terraces of Ancient Lake Bonneville 

appear before the next rain. Evaporation is rapid and the 
water table is so far down that most of the streams are not 
permanent. Such intermittent lakes are called playas. They 
are common in Nevada, Utah, and Arizona. The soils that 
make up the basins of playa lakes are usually alkaline. 

142. Destruction of lakes. Some lakes, like playas, dis- 
appear by evaporation, but in a humid region the inflow 
will exceed the evaporation and hence the level of the lake 
will rise until it overflows its rim. That starts a river which 
begins erosion and, by this process, the rim of the lake is 
cut deeper and deeper until the lake is drained. Rivers are 
the mortal enemies of lakes. 

At the same time the water entering the lake brings sedi- 
ment (Fig. 114), which is deposited and helps to fill up the 
lake. As it becomes more and more shallow, plants begin 
to grow near the edges and help to fill it up until the lake 
is nothing but a swamp, and even this is ultimately filled 
with silt and converted into a plain. 

Certain kinds of moss and some other plants sometimes 
grow on the surface and hold wind-blown sand and dust 
which gradually spread over the lake, forming a floating 
bog. A railroad line in Minnesota crossed such a bog. Cattle 
grazed upon it before the line was built; but the engineers 
discovered that the floating bog was a mass of vegetable 
matter and dust, four feet thick, and that beneath it was 



W. H. Jackson, U.S.G.S. 

FIG. 114. Delta Deposits in a Lake 

FIG. 115. A Lake Being Destroyed by Vegetation 


twenty feet of water. Prairie Tremblant, Louisiana, is a 
floating bog through holes in which fish may be caught. 
Eel grass and wild rice also assist in filling in lakes. 


These methods of filling gradually convert a lake into a 
swamp or marsh, and many of our fresh-water marshes are 
former lakes which have been destroyed in this way. 

143. Importance of lakes. Lakes regulate the flow of 
rivers and so prevent floods. If the Mississippi and its 
branches had their sources in large lakes, there would be 
no great floods in the Mississippi Valley, just as there are 
none in the St. Lawrence Valley. The building of such reser- 
voirs, artificially, has been recommended as one means of 
preventing the disastrous floods which occur in the Missis- 
sippi Valley. 

Lakes moderate the climate of the neighboring lands be- 
cause the water warms up more slowly than the earth and 
cools the land in summer, while it does not cool out rapidly 
in winter and therefore warms the land. 

Lakes that are high enough furnish water power, while 
others are used as sources of drinking water for cities and 
for irrigation in dry regions. New York City has provided 
an available storage capacity of 150 billion gallons in the 
Ashokan and Schoharie reservoirs in the Catskill Moun- 
tains. These are in addition to the reservoirs at Kensico, 
Hill View, and Silver Lake. 

The Roosevelt Dam forms a lake of more than 16,000 acres, 
250 feet deep, and supplies water for irrigation to the inhab- 
itants of that section of Arizona. Boulder Dam will im- 
pound the waters of the Colorado, making a large artificial 
lake of 230 square miles (Fig. 116). It will furnish water for 
irrigation to the inhabitants of several of our southwestern 
states and Mexico, and drinking water for the city of Los 

The Great Lakes are important arteries of commerce over 
which the products of the middle west for example iron 
ore and wheat find their way to the great commercial 
centers of the east, New York, Buffalo, Cleveland, and Pitts- 

Lakes also have great value as pleasure and health resorts. 



U. S. Bureau of Reclamation 

FIG. 116. Boulder Dam 


*144. Swamps. In paragraph 142, it was shown that a lake is 
destroyed by filling and draining and that near the end of its 
existence it becomes a swamp. Most swamps are of this origin. 

Swampland is saturated with water that is, the water table 
intersects the surface but it is not covered with water. If the 
surface is covered with water, it is either a pond or a lake. 

The flood plain of an old river is usually swampy because the 
rainfall is trapped between the natural levees and the valley wall 

FIG. 117. A Scene in the Dismal Swamp of Virginia 

and the land is very flat. There is much swampy ground along the 
lower reaches of the Mississippi. Glaciated regions are full of lakes 
and swamps, because of interference by the glacial drift with 
natural drainage. 

Coastal plains, which are very flat, often are swampy. Such is 
the origin of the Dismal Swamp in Virginia and the Everglades in 

Swamps have a great deal of vegetation, particularly of certain 
kinds which like saturated soil. In the cooler climates, sphagnum 
moss is a common swamp growth. It grows out from the sides of a 
pond along the surface. At the same time, decaying plant material 
helps fill in the water under the sphagnum, until in time the entire 


surface may be covered with a thick mat under which there is a 
mass of decaying vegetation mixed with water. This is a quaking 
bog. As an animal walks on it the floating mass moves and some- 
times breaks through, so that we find fossils in these bogs. 

*145. Formation of peat. When a lake is destroyed by the en- 
croachment of plants from the shore, the dead plants decay through 
the activity of microbes, but this very process produces some com- 
pounds which are antiseptic and ultimately kill the bacteria. From 
then on, the partially decayed plant material will be preserved. 
This is peat. It is dark brown in color, and roots and other parts 
of plants may often be recognized in it. Other plants grow on top 
of the peat and push it down by their weight, and in time the entire 
lake becomes a peat bog with the lower layers of peat more and 
more consolidated. Peat is the first stage in the formation of coal. 

In some of the past ages of the earth's history, particularly the 
Pennsylvanian, large areas of the land were low and swampy and 
the climate was warm. Conditions were ideal for peat making and 
as a result most of our coal was laid down during that time. The 
trees, however, were quite unlike those we know today: no oak, 
beech, or maple, but gigantic ferns and scale trees. (See page 276 
for description of a Pennsylvanian forest.) 

Where coal is not found, as in Ireland, the inhabitants often 
use peat as fuel; and no doubt, when our extensive coal deposits 
are depleted, we also shall use the enormous amount of peat in 
our swamps. 

*146. Economic aspects of swamps. Certain diseases, like 
malaria and yellow fever, are prevalent in swampy regions and 
are carried by mosquitoes which breed in swamps. This is par- 
ticularly true of tropical regions. 

Few plants will grow in saturated soil; but when swamps are 
drained the soil is very fertile. There are 64 million acres of swamp- 
land east of the Appalachian Mountains that can be reclaimed. 
This is being done on a small scale in this country, where land is 
plentiful ; but in the Netherlands such reclamation of land has been 
going on for centuries. About half of the present agricultural lands 
of the Netherlands (the word means lowlands) has been reclaimed 
from the sea by building dikes and pumping out the water, and 
pumps are constantly in operation in order to keep these lands 


Completion Summary 

A lake is formed wherever - drainage. Most lakes 
are of - - origin. 

lakes are formed on old river basins. 

Lagoons are lakes formed by - , which cuts off - . 

Sink holes in regions become lakes, if their bottoms 

- water table or if - - choked. 

Salt lakes develop in - - when evaporation - . 
Intermittent lakes, called - , exist in - - regions. 
Rivers are the mortal enemies of lakes. They - - sedi- 
ment, and by cutting down the lip of the lake basin - . 
Lakes - - floods, are useful as - - drinking water, 

- irrigation, and some - - water power. Some lakes 

- navigation. 

* Where the water table is just at the - - swamp. Swamps 
are therefore common - and on the flood plains of - . 

Peat is formed in swamps because decay brought about by 
microbes produces and preserves . Peat is the 
in the formation of coal. 

When swamps , - - very fertile agricultural lands. 


1. Explain why we find lakes only in humid regions, where 
drainage has been interfered with. 

2. Make a table showing the origins of lakes and, for each one, 
state briefly how drainage was interfered with, to produce the lake. 

3. Name a lake which is believed to have been formed by 
crustal movement. 

4. Name a lake of volcanic origin. 

5. What is a coulee lake? 

6. State three ways in which a glacier may form a lake. 

7. Name lakes of glacial origin. 

8. How and where is an oxbow lake formed? 

9. Show how lakes may be formed where tributaries enter an 
old parent stream. 

10. How is a delta lake formed? 


11. What is a lagoon? 

12. Under what conditions do sinks become lakes? 

13. Explain how a salt lake is formed. 

14. What is an alkaline lake? 

15. What is a playa lake? 

16. In what two ways do rivers destroy lakes? 

17. Mention an artificial lake. 

18. State several functions performed by lakes. 

if Optional Exercises 

19. Distinguish between a lake and a swamp. 

20. Where are swamps very common? Why? 

21. What is a quaking bog? 

22. How is peat formed? 

23. What dangers lurk in swampy regions? 

24. Why are reclaimed swamplands fertile? 

25. Explain how glaciers form lakes. 

26. Name lakes of glacial origin, not mentioned in the text. 

27. Trace the life history of an oxbow lake. 

28. What is the origin of Lake Pontchartrain? 

29. Explain the formation of lagoons on emerging shores. 

30. Trace the life history of a lake. 



147. What are plains? The surface of the earth is, in 
most regions, rather uneven; but here and there we find it 
relatively flat with few inequalities. Such a region is called 
a plain. 

Plains are usually lowlands, some of them as smooth as 
a table, relatively, while others are slightly rolling. The 
smooth surface of a plain is due to deposition of sediments 

FIG. 118. The layers of sediment deposited in the ocean have a 
smooth surface. 

in water (Fig. 118). In a lake or in the ocean the sediments 
brought down by rivers are spread out on the bottom, in 
layers which are practically horizontal. When these under- 
water layers emerge, by either the drainage of the lake or 
the uplift of the ocean floor by earth movements, we have 
a plain, unless, in the uplift, the layers are sharply tilted 
or folded. 

Plains are also formed by erosion. This process gradually 
wears down the higher places and carries away the material 
formed, until there remain only a few places higher than the 
general level. Such a plain is never quite smooth and is 
therefore called a peneplain, which means " almost a plain. " 

148. Peneplains. The peneplain is the final stage of the 
cycle of erosion. In the beginning, erosion increases the re- 



lief or unevenness of the land because weaker rocks are worn 
down faster. Streams soon carve their courses in these weaker 
rocks and carry away the products of weathering. But as 
soon as the erosion of the weak rock has reached base level 
it can no longer continue, because at base level water does 
not flow. 

While the weaker rocks have been wearing down rapidly, 
the harder ones have also been weathering, but more slowly; 
and, in fact, wherever they stand higher than the weaker 
ones, they will wear rapidly. Both hard and soft rocks will 
wear down, then, the harder ones always remaining as out- 
standing features of the land, like ridges. 

When the region has reached old age, the rivers have 
broad flood plains, with here and there an isolated hill, 
usually composed of very resistant rock. These remains 
of the former highland are called monadnocks, after Mt. 
Monadnock in New Hampshire (Fig. 119). 


FIG. 119. Mount Monadnock stands out from an otherwise level 
sky line. 

*In sufficient time, monadnocks would also be worn down to 
base level; but as the region gets closer and closer to base level, 
the rate of erosion becomes slower and slower and rejuvenation 
of the region always interrupts the last stage in the erosion of the 
monadnocks. When a peneplain is uplifted it presents a very even 
sky line (Fig. 120). Soon the work of erosion begins again to cut 
up the surface and make it uneven, but there usually remain 
enough evidences to enable one to detect the former peneplain. 
The Appalachian Mountains were once peneplaned but they have 
since been uplifted again and, although deeply dissected, the 
highest points are at the same general level. 

The rock mantle of a peneplain may consist of sediments de- 
posited in former lakes and rivers or of drift deposited by a con- 


tinental glacier; but these local deposits do not indicate the true 
history of the region. It is the erosion of the bedrock, rather than 
the differences in the thickness of the mantle, which gives the 
region its plainlike character. 

G. W. Stose, U.S.G.S. 
FIG. 120. An Uplifted Peneplain 
The sky line is straight, except where a river has cut its valley. 

149. Plains formed by deposition. Many plains owe their 
smooth, flat upper surface to deposits of rock mantle. 

Most of the agents that transport and deposit rock man- 
tle form flat-topped deposits. The characteristics of these 
deposits will depend upon (1) the agent transporting the 
material and (2) the conditions under which it is depos- 

There are four classes of plains of deposition. 

1. Marine or coastal plains, formed by uplift of the sea 

2. Alluvial plains (formed by rivers) 

a. Flood plains along the river course 

b. Deltas at the mouth of the river 

c. Piedmont alluvial plains 

3. Lake plains, left after a lake has been destroyed 

4. Glacial plains 


150. Coastal plains. Coastal plains are composed of rock 
waste cut from sea cliffs by wave action or carried to the 
ocean by rivers. The movements of the water spread the 
loose material out into smooth, gently sloping deposits such 
as we see on all beaches; but the beach is only the part of 
the deposit that is above the water, while the rest sometimes 
extends a hundred miles from shore. 

The smooth, gently sloping deposit is called the conti- 
nental shelf and when it is uplifted, it is called a coastal 

*The surface material of a newly uplifted coastal plain is as- 
sorted and stratified into nearly horizontal layers, unless, in the 
uplift, the strata have been tilted or folded. 

Coarser layers of gravel will usually be found underneath, with 
sand on top of gravel and silt and clay on top of all. This is in 
perfect agreement with the cycle of erosion, since, when the land 
near by was high, the streams were capable of carrying gravel, 
while later, after erosion had reduced the elevation, the streams 
could carry only fine material. 

Frequently the rock mantle is quite loose and unconsoli- 
dated. Coastal plains are therefore very porous and permit 
ground water to flow through them readily. But a layer of 
clay, which often tops the others, makes an impervious 
covering and hence the conditions for artesian wells are often 
found on coastal plains. 

When a coastal plain is young, the surface is smooth, 
drainage is simple, and the shore line is regular. The greatest 
coastal plain in the world forms the northern and western 
part of Siberia. It has a maximum width of more than one 
thousand miles. On the shores of the Bay of Bengal is a 
narrow coastal plain not more than fifty miles wide. On 
the western coast of the United States, coastal plains are 
unimportant, although there are some narrow plains, like 
that at Los Angeles. 

*151. The Atlantic Coastal Plain. This interesting plain lies 
along the Atlantic coast from New York to Florida. An extension 



of the Atlantic Coastal Plain called the Gulf Coastal Plain extends 
from Florida around the Gulf of Mexico. Its width varies from 
half a mile to five hundred miles. 

The rock waste forming this vast deposit, which in places is 
several hundred feet thick, came mostly from the eastern slope of 
the folded Appalachian Mountains. During the period of deposi- 
tion, the region which is now the coastal plain was of course sub- 
merged, the ancient shore line being near the present "fall line" 
(Fig. 121). 

As soon as it was uplifted, the streams flowing into the Atlantic 
cut deep valleys in the soft deposits of the new coastal plain. 

FIG. 121. The Atlantic Coastal Plain, Showing the Fall Line 

This period of erosion ended, for the eastern half of the coastal 
plain, when it was again submerged; this moved the shore line to 
about its present position, drowned the valleys that had been 
eroded, and gave us New York, Delaware, and Chesapeake bays, 
as well as the bays and sounds south of them, and many smaller 



bays like those of the Navesink, Toms River, and Great Egg River 
of New Jersey. 

The old river valleys, which extended farther east than they 
do now, can still be traced by soundings. It is said that the old 
mouth of the Hudson River is more than a hundred miles out to 

The shore of the coastal plain is low and marshy, but the land 
rises gently to the fall line. The strata beneath the soil are similar 
to those of the continental shelf and contain many marine fossils 
that prove the former submergence of the region. 

L. W. Stephenson, U.S.G.S. 

FIG. 122. Much of the Atlantic Coastal Plain is sandy. 

In the Carolinas, rice is raised in the marshes. Between the 
marshes are wide areas of sand, of little value for agriculture, which 
are chiefly occupied by pine forests. Farther inland, the soil is 
fertile and much cotton is raised, while in some localities garden- 
ers maintain successful truck farms. 

At the western border of the Atlantic Coastal Plain, the land 
rises somewhat abruptly to the Piedmont Plateau. The rivers of 
this region usually have falls or rapids where they descend from 
the plateau to the plain. These falls furnish water power. 

152. Ancient coastal plains. Geikie well says, "Only where 
the sediment is strewn over the sea floor beyond the limit 


of breaker action, is it permitted to accumulate undisturbed. 
In these quiet depths are now growing the shales, sandstones, 
and limestones which, by future terrestrial revolution, will 
be raised into land, as those of the past have been." 

The continent of North America has grown to its present 
size through the emergence of successive continental shelves, 
each one of which was for a time a coastal plain. Much of 
the present continent is therefore underlaid by sedimentary 

Some of these former coastal plains still retain enough of 
their original characteristics to enable us to recognize them 
as "ancient coastal plains." 

Other portions of them are now great mountain ranges, 
and still others have been great mountain ranges but are 
now so worn down that we call them plains. 

153, Plains formed by rivers. Old streams have wide 
flood plains. During floods, when the river overflows its 
banks, it deposits alluvium on the flood plain. Most of this 
sediment is deposited near the river, because it is there that 
the water loses most of its velocity as it leaves its channel. 
This results in a smooth, nearly level surface that slopes 
away from the river, and ends in marshy "back swamps" 
at the outer edge of the flood plain. 

*The materials forming flood plains are not arranged in con- 
tinuous horizontal strata, but are exceedingly irregular, owing to 
the meandering of the river. As the flood subsides, the depressions, 
caused by new channels eroded by the river in flood, are filled with 
sediments which often differ from the others in fineness. 

The flood plain of the Mississippi is about 80 miles wide at 
Greenville, Mississippi; it gradually decreases toward the north 
until at Helena, Arkansas, it is but 20 miles wide. The total area 
of the Mississippi flood plain is about 30,000 square miles. 

*154. Delta plains. At the mouth of a stream, or wherever its 
velocity drops, an alluvial deposit will be formed, and it will 
usually have a flat surface. Alluvial deposits therefore form plains. 
The great delta plains are those of the Mississippi, the Po, the 
Rhine, the Ganges, and the Hwang Ho. 



Delta plains have a fine mud surface, especially near the sea 
where they are often flooded. No part of the delta is much above 
sea level; the maximum, at the natural levees, is no more than 
50 feet. For this reason, houses, towns, and roads are built on the 
natural levees. 

Delta soils are often very fertile and easily tilled. With the in- 
creasing need for farm land, it is not surprising to find many deltas 
occupied by dense populations. 

U. S. Department of Agriculture 

FIG. 123. Farming on the Delta Plain of the Mississippi 

The Dutch have not only taken possession of the deltas of the 
Rhine, the Meuse, and the Scheldt rivers, but they have built 
extensive systems of dikes and have pumped out the sea water in 
order to reclaim wide areas of the delta plains, some of which lie 
as much as 15 feet below sea level. 

155. Piedmont alluvial plains. Where tributary streams 
with steep slopes come out of the mountains to join the 
main stream, their velocity is suddenly arrested and they 
drop most of their load, forming alluvial fans (Fig. 124). 

In semiarid regions, we find alluvial fans particularly well 
developed, because the main streams have no water except 
when it rains. When it does rain, the tributaries, rushing 



Part of the Cucamonga Sheet, U.S.G.S. 

FIG. 124. The lower half shows a piedmont alluvial plain. 

down the mountain sides and carrying a heavy load, come 
out on the main valley floor, where they spread out and drop 
their entire load. In time, neighboring fans coalesce and 
form one continuous piedmont plain. The name is derived 


from a region in northern Italy, called Piedmont (foot of 
the mountains), where the tributaries of the river Po have 
formed an alluvial plain of this type. Such plains are usually 
several hundred miles long and very narrow. Our own San 
Joaquin and Sacramento rivers in California have formed 
piedmont plains. 

Figure 124 shows the piedmont plain formed by the in- 
termittent streams of the San Bernardino Mountains, in 
California. Note the fanshape of the contour lines curving 
outward from the point where each stream comes out of the 
mountains, and note how neighboring fans have coalesced. 

*In arid regions, such plains are usually the sites of villages, be- 
cause water can be obtained from the mountain streams or from 
wells. Most of the settlements in Utah are situated on piedmont 
plains west of the Wasatch Mountains, a range about 200 miles 
long that borders the Great Basin on the east. 

On piedmont plains, there is little slope on a line parallel to the 
mountains, and roads and railroads usually follow such lines. 

The surface of piedmont plains is often full of gravel and boulders 
near the mountains, but farther away it becomes fine and is good 
for farming wherever water is available. They furnish excellent 
sites for irrigation projects and some of them are famous for their 
agricultural produce. Such, for example, are the valleys of the San 
Joaquin and Sacramento rivers of California, where much of our 
fruit is grown. 

156. Glaciated plains. Glaciers have produced plains of 
two types. In one, the rock surface has been rubbed flat by 
glacial erosion, while in the other, glacial deposits have filled 
in irregularities. The latter are known as till plains (Fig. 125). 

Plains formed by glacial erosion are the Laurentian plain 
of Canada and those of Sweden and Finland. They are 
rolling plains, with many lakes, very little soil on the higher 
places, and swamps in the lower regions. Much of the land 
is covered by forest. 

Glacial drift usually smooths the surface of the land and 
hence forms plains, especially in front of the glacier, where 


streams work over the drift and assort it. These are out- 
wash plains. The plains of northern Ohio and of southern 
Long Island are good examples. 

Wisconsin Geological Survey 

FIG. 125. A Till Plain 

FIG. 126. Stone Wall Built of Glacial Drift 

The soil of a glacial plain is composed of glacial till: large 
boulders mixed with small pebbles and sand. Much of this 
material is erratic, having been dragged from some distant 



region by the glacier. These boulders are often so numerous 
as to interfere with the cultivation of the soil. In New Eng- 
land, where this condition prevails, stone walls built of these 
boulders are a common sight (Fig. 126). 

157. Lake plains. Glacial lakes, some of them of vast 
area, have been drained naturally, in recent geological his- 
tory; and others have been artificially filled or drained to 
form extensive farm lands. These lands are called lacustrine 
or lake plains (Fig. 127). 


FIG. 127. Farming on a Lacustrine Plain 

The black muck soil of these lake bottoms, in Michigan, 
New York, and elsewhere in northern United States, is es- 
pecially good farm land. 

Lacustrine plains are usually small, though some of the 
larger ones, like that of former glacial Lake Agassiz, have 
an area of thousands of square miles. It is stated that there 
are about 8,000 glacial lakes in Minnesota and that about 



one half of them will become farm land, by natural processes, 
in about 50 years. 

*Michigan, Wisconsin, New York, Connecticut, and other 
states, once covered by the glacier, have similar lakes which will 
eventually become rich farm lands. 

The floor of ancient Lake Agassiz is one of the levelest regions 
in the world. It covers the valley of the present Red River of the 
North, in Minnesota, North Dakota, and Canada. (See page 124.) 
The soil of this lacustrine plain is fine-grained and rich, so that it 
produces enormous quantities of excellent wheat. 

Ancient Lake Bonneville, which has shrunk to the present 
Great Salt Lake, once occupied the eastern portion of the Great 
Basin. The sediments deposited in this lake, which was as large 
as Lake Huron, filled the valleys between north and south moun- 
tain ranges, forming many small lacustrine plains (Fig. 128). 

G. K. Gilbert, U.S.G.S. 

FIG. 128. Plains of Ancient Lake Bonneville, in Utah 

During the Glacial Period, the Great Lakes occupied different 
basins because the ice to the north blocked the natural drainage 
of that region. In this way, many lacustrine deposits were formed 
on the margins of the present lakes, which were ultimately exposed 
when the lakes finally receded to their present basins. 


The prairies of northern Illinois are lake plains which were once 
covered by the waters of ancient Lake Chicago, an extension of 
Lake Michigan. The city of Chicago is built on this plain. 

In New York State, a southern extension of Lake Ontario gave 
us the lacustrine plain that extends from the Mohawk Valley to 
Syracuse. This plain provides a favorable location for the Erie 
Canal and the New York Central Railroad. It was also used by 
the early settlers of the West as the main highway toward their 
new homes. 

158. Lake plains in dry regions. Occasional heavy rains 
in dry regions cause great torrents to flow over the lower 
land and to accumulate in large, shallow, temporary lakes, 
called playas. The largest of these is in the Black Rock 
Desert, Nevada. It has an area of 500 square miles, but 
it is hardly a foot in depth. When the water evaporates, 
the basins are covered with fine sand and clay, sometimes 
mixed with crystals of salt and gypsum. 

As a rule these plains are not fertile, but some of them 
contain valuable deposits of salt, soda, or borax, as in the 
Great Basin and the Imperial Valley. Sometimes these al- 
kali plains can be made fertile by irrigation. 

159. The Great Plains. The Great Plains occupy about the 
middle portion of the United States and Canada, between the 
Rocky Mountains and the Mississippi. This entire region was, in 
past geological history, repeatedly submerged by the sea, only to 
be finally uplifted ; therefore it is of the coastal plain type, but very 
much modified by erosion and, more recently, by glaciation in 
the north. 

The region of plains between the Mississippi and the Appa- 
lachians, known as the prairies, is characterized by tall, deep- 
rooted grasses, but is devoid of trees (Fig. 130). 

The Great Plains are about 6,000 feet above sea level near the 
Rocky Mountains, and slope down gradually to the Mississippi, 
800 miles away. This slope is so gradual that it is hardly notice- 
able. The plains east of the Mississippi rise to about 200 feet 
elevation near the Appalachians. 

The surface covering was brought down by streams from the 




mountains on the east and west. Much of it is wind-blown and, in 
the north, it is to a great degree of glacial origin. 

The soil is rich and easily cultivated since there is no forest, but 
lack of sufficient rainfall, which perhaps accounts for the absence of 
trees, makes it difficult or impossible to farm portions of the plains. 

Forest Service, U. S. Dept. of Agriculture 

FIG. 130. Scene on the Prairies 

*160. Erosion of the Great Plains. In parts of the Dakotas and 
Montana, the soft surface deposits are being gullied so badly as 
to make travel in these regions extremely difficult. These regions 
are known as the "Bad Lands" (Fig. 131), "mauvaises terres pour 
traverser" as the French traders called them. A similar process of 
soil erosion is taking place in parts of the semiarid plains, where 
dry farming has removed the natural surface cover or where over- 
grazing has been practiced. (See Soil Erosion, page 57.) 

161. Economic importance of plains. The comparatively 
even surface of plains makes the building of roads, rail- 
roads, and canals easier than in hilly regions and decreases 
the labor of the farmer. Airports are practically always 
established on plains. 

The soil of the plain is likely to be finer, deeper, and more 
fertile than the adjoining higher land, since the rains carry 
down soluble plant food and fine sediment from the high lands. 


The shallow valleys of the plain tend to keep the water 
table near the surface, thus rendering crops less likely to 
suffer from drought. It is warmer on plains than in the up- 
lands, and the crops have a longer growing season. 

All these conditions favor agriculture and development 
of trade, and therefore contribute to progress in civilization. 

N. H. Darton, U.S.G.S. 

FIG. 131. The Bad Lands 

Much of the land of the earth is unsuited for large popu- 
lations. In tropical and subtropical lands, tropical vegeta- 
tion flourishes on the lowlands, whereas at higher altitudes 
the crops of the temperate zone, such as wheat and barley, 
can be grown. Again, a plain on the wrong side of a moun- 
tain range may have a fertile soil, yet not be suited to agri- 
culture because of deficient rainfall. Similarly, plains in the 
trade-wind belts suffer from lack of rainfall; and plains in 
the polar regions cannot be important agricultural lands be- 
cause of the cold. 

As a rule, the well-watered plains of the temperate zones 
are the great agricultural regions of the world and we are 
not surprised to learn, therefore, that the vast majority 
of the people of the world live on plains in the temperate 


Completion Summary 

Plains are of two types: plains of - - and plains of 

*A peneplain is never as even as a plain of - , because 
erosion - , but leaves monadnocks. 

Coastal plains are uplifted - . 

*The surfaces of coastal plains are usually fine-grained, be- 
cause - , while the layers underneath - . 

Artesian wells - - coastal plains. 

*When the Atlantic Coastal Plain was uplifted, the streams 
. Then part of the region was submerged, forming - . 

The flood plains of - - are wide. 

*The great - - plains of the world have the densest popula- 
tion because - . 

Piedmont plains are formed by streams . Such 

alluvial deposits are common in semiarid regions. 

Outwash plains - - glacial drift, which has been 

- running water. 

Lake plains are the bottoms of - . Many former 
glacial lakes have , leaving the glaciated region . 

*Ancient Lake Agassiz ; ancient Lake Bonneville 
Great Salt Lake. 

*The Great Lakes occupied larger basins right after the Glacial 
Period, - - lacustrine deposits - . 

- are temporary lakes in dry regions. The plains 
they leave - - fertile. 

*The Great Plains are the result of uplift of - . The soil is 
, but - . The soft surface has been badly gullied in 
places, forming - . In other places, dry farming - and 
wind action - . 

The - - of the temperate zone are the most impor- 
tant agricultural regions of the world. 



1. How are plains formed by deposition? 

2. What kind of plain is formed by erosion? How? 

3. How do peneplains differ from plains of deposition? 

4. Explain the origin of a coastal plain. 

5. Why are the rocks of a coastal plain usually horizontal? 

6. Why is a layer of clay often found on top of a coastal plain, 
while underneath we find sandstone and conglomerate? 

7. Why are artesian wells frequently found on coastal plains? 

8. Explain the presence of marine fossils and wide areas of 
seashore sand far inland in North Carolina. 

9. Why do we find sedimentary rocks in nearly every part of 
the earth? 

10. What kind of flood plains do we find in young, mature, and 
old river valleys? 

11. What is a piedmont plain? Name one. 

12. What two types of glacial plains are there? 

13. What is an outwash plain? Where is there one? 

14. What is a lake plain? Name one. 

15. Why are lacustrine plains very fertile? 

16. Most of the plains of northern United States are lacustrine. 

17. Why are playa lake basins unfertile? 

18. State several reasons why plains are more densely inhabited 
than is hilly country. 

if Optional Exercises 

19. How can an uplifted peneplain be detected? 

20. What kind of rocks are always found on a coastal plain? 

21. The shore line of a young coastal plain is very regular. 

22. What is the extent of the Atlantic Coastal Plain? 

23. What is the origin of the Atlantic Coastal Plain? 

24. What is the "fall line"? Explain its significance. 

25. What happened to the rivers of the east coast when the 
Atlantic Coastal Plain was uplifted? 

26. What is the history of the Atlantic Coastal Plain? 


27. Explain the presence of swamps on flood plains of old 

28. Explain the dense population of delta plains. 

29. Why are alluvial fans very common in arid regions? 

30. To what class do the Great Plains belong? 

31. What is the origin of the Bad Lands? 



162. What is a plateau? Plateaus, like plains, have a 
broad and relatively even surface, but the plateau is a high- 
land high in contrast to some near-by lowland. 

Sometimes the even surface is due to the horizontal strata 
underlying the surface, but in other cases, the strata are at 

N. W. Carkhuff, U.S.G.S. 

FIG. 132. Horizontal Strata of the Colorado Plateau 

an angle, the flat surface having been peneplaned by erosion 
before it was uplifted as a plateau. 

The sedimentary strata of the Colorado Plateau are hori- 
zontal, as shown in Fig. 132, while the Appalachian Plateau 
has underlying rocks which are tilted, and only the surface 
is relatively flat because the entire region was peneplaned 
before it was uplifted. 



Although plateaus are, as a rule, higher than plains, it is 
not possible to distinguish between them on the basis of 
altitude. For example, the Piedmont Plateau, between the 
Appalachian Mountains and the Atlantic Coastal Plain, is 
much lower than the plains of the Mississippi Valley. The 
Appalachian Plateau has an altitude of about 3,000 feet, 
whereas the Great Plains, east of the Rockies, reach an alti- 
tude of 6,000 feet. They are called plains because they are 
relatively lower than the Rocky Mountains which are on 
their west, while the Piedmont Plateau is so called because 
it is higher than the Atlantic Coastal Plain which borders 
it on the east. If it were near the Great Plains, it would not 
be called a plateau. Low plateaus, then, are often called 
plains, while high plateaus, after erosion has cut them up, 
resemble mountains. 

We may then define a plateau as follows : A plateau is a 
region of broad summit area that is conspicuously higher 
than adjoining land or water on at least one side. 

The Appalachian Plateau has an elevation of about 3,000 
feet, the Colorado Plateau about 8,000 feet, while the Ti- 
betan Plateau is about 15,000 feet above sea level. 

163. Erosion of plateaus. The cycle of erosion on a plateau 
is similar to that on a plain, except that the streams at the 
edges of the plateau are younger than on top, because of 
the greater slope. These young streams cut gorges into the 
plateau and these are the most striking features of the to- 
pography. The Colorado River has cut the Grand Canyon, 
which is 125 miles long and, in places, more than a mile 
deep. This region is still youthful. 

The Appalachian Plateau (Fig. 133) is mature. Its hills 
are rounded, its valleys are no longer V-shaped. Why do 
we call it a plateau, since it is not flat like the Colorado 
Plateau? We have evidence that it was once flat and con- 
tinuous like a plain: the tops of its numerous ridges form a 
straight and nearly level sky line (Fig. 133). At one time it 
was an erosion plain. It was uplifted into a plateau, and 



M. R. Campbell, U.S.G.S. 

FIG. 133. The Appalachian Plateau 

Before the valley was formed, the sky line was continuous and almost hori- 

when its old streams were rejuvenated they cut it up into 

In old age, a plateau would be completely reduced to 
base level and would become a plain, showing no evidence 
of its former elevated condition. This can happen only when 
erosion is quite uniform. Since it is not, we find monadnocks 
as a feature of old regions. In humid regions these monad- 
nocks have rounded forms like little hills, because weather- 
ing tends to wear off the edges first. But in arid regions, 
where weathering is at a minimum, these monadnocks re- 
tain a youthful, angular outline (Fig. 134). Such a feature 
is called a mesa (table) and a small mesa is called a butte. 
The top stratum of a mesa is usually very resistant. 

164. Climate of plateaus. Because of their elevation, 
plateaus are usually cold. For example, in Mexico it is 
tropical in the lowlands near the coast, but temperate on 
the plateau. In Tibet, even in midsummer it is cold. 

Most plateaus are arid. One reason for this condition is 
that plateaus, situated between mountain ranges, are cut 
off from moist winds on all sides. The winds precipitate 



FIG. 134. A Butte 

their moisture on one side of the mountains, so that when 
they pass over the plateau, they are dry. 

The depth of the river valleys gorges lowers the 
level of ground water and still further accentuates the 
aridity of plateaus. Agriculture therefore does not usually 
flourish in such regions. 

165. Economic importance of plateaus. The cool, dry 
climate of plateaus is an advantage in tropical regions. For 
example, the plateau of Mexico furnishes grains such as 
are grown in temperate regions, whereas the lowlands fur- 
nish only tropical products. 

In temperate climates, however, plateaus are arid. In the 
plateau of Tibet, the climate is almost arctic and much of the 
region is abandoned to wild animals and the inhabitants are 

In our own southwestern plateau region, there are a few 
farms near the mountains, where streams may be used for 
irrigation, and some other sections are fair grazing lands; 
but, as a whole, the region is almost unoccupied. With 
the growing threat of soil erosion in other sections of our 
country, some of these lands may be reclaimed by a more 
ambitious program of the Soil Conservation Service. 


Completion Summary 

Plateaus resemble - - in having a flat surface; but 
they are like mountains because - . 

A plateau - - might be called a plain. In youth, it 
looks exactly like a plain, except that it is elevated - . 

Rivers on top of the plateau may be old, but on the 
edges - . These - - streams dissect - , - 
mountains. When these mountains are rounded off and the 
slopes are reduced - - maturity; and in that stage, the 
plateau resembles - . The former plateau can be dis- 
tinguished only by - . 

In old age, a plateau - - plain. 

In a dry region, - - buttes. 

A plateau is usually arid, because - . 


1. Explain how a plateau can have a flat surface if its bedrock 
consists of strata which are tilted. 

2. Show how a plateau and a plain might have exactly the 
same elevation. 

3. The Appalachian Plateau looks like mountains. Why do we 
call it a plateau? 

4. What are monadnocks? 

5. How do monatinocks differ, in humid and in arid regions? 

6. What is a butte? 

7. What is a mesa? 

8. Explain why a plateau is usually arid. 

9. Why is a plateau cold? 

* Optional Exercises 

10. Describe the cycle of erosion of a plateau. 

11. Explain why monadnocks in humid and in arid regions 
look different. 

12. Show by diagram how a gorge in a plateau lowers the 
water table. 

13. Why is it that a plateau in a tropical region may not be 


*166. Evidences of crustal movement. There is much evidence 
in the hands of geologists that every part of the earth's crust has 
moved at one time or another, and there is a little evidence pre- 
sented by history. Pliny tells us that Pompeii was on the seacoast, 
whereas today the ruins are miles inland. Apparently the land 
has risen since Pliny's time, or the sea has dropped. 

The ancient temple of Jupiter Serapis at Pozzuoli, near Pompeii, 
is known to have been on dry land A.D. 235. When it was redis- 
covered in 1749, the bases of its remaining upright columns were 
buried in marine sediments to a depth of twelve feet above the 
floor of the temple. For a distance of nine feet, the lithodomi, or 
stonehouse animals, had bored holes in the columns and lived in 
them, causing the dark bands seen in the columns (Fig. 135). 

The double caves shown in Fig. 136 were carved by waves. 
When the upper cave was cut out, it was in about the same position, 
with respect to sea level, that the lower one now occupies. 

On the island of Crete there are some old docks which are now 
on dry land. It is evident that these must have been built in the 
water and that the land has risen recently. 

The finding of the skeleton of a whale in the glacial gravels near 
Lake Champlain and the remains of other marine animals coral, 
fishes, and others too numerous to mention in the sedimentary 
rocks of our mountains can mean only one thing : these areas were 
once below sea water. 

Some of these changes must have been due to sinking of the 
ocean bottom, which would permit the water to run off the land; 
others to the accumulation of vast amounts of sediment, eroded 
from the land and deposited in the sea, which would cause the 
water to overflow the land; still others to a glacial epoch, which 
precipitated the water, evaporated from the ocean, as snow instead 

* This entire chapter is optional. 




Ewing Galloway 

FIG. 135. Ruins of the Temple of Jupiter Serapis 
Note the rough surface of the columns due to the lithodomi. 

FIG. 136. Double Sea Caves, an Evidence of Crustal Movement 



of water. It has been estimated that if the ice sheets of the earth 
today were all melted, it would raise the level of the oceans about 
80 feet. And when we realize that there was much more ice on the 
continents during the last glacial epoch than there is today, the 
statement that the sea at that tune was 150 to 300 feet lower 
than it is today is not surprising. 

As we have seen in the study of rejuvenated rivers, there have 
been many times when entire continents were uplifted or depressed. 



FIG. 137. Normal Succession of Sedimentary Rocks 

We have furthermore the following evidence in the succession 
of sedimentary strata. We know that coarse material like boulders 
and gravel is carried only by rapidly moving streams. It follows 
that if we find boulders and gravel in the rocks, conglomerates, 
their source must have been a near-by highland. Now, the normal 
succession of sedimentary rocks is conglomerate, sandstone, shale, 
and limestone, as shown in Fig. 137, because, as the highlands wear 
down, the streams are unable to carry large particles and therefore 

the deposits get finer and finer. 
The limestone, which is made 
principally of the shells of ma- 
rine animals, is formed only in 
clear water; this shows that the 
streams were so sluggish that 

they no longer carried even fine 
FIG. 138. Conglomerate Resting on . . , 

Limestone particles. 

What then can it signify that 

we find a conglomerate on top of a limestone, as in Fig. 138? It is 
apparent that the land near by has been raised considerably to 
enable the rivers to carry the gravel which makes up the con- 

And suppose we have a shale on top of a conglomerate? This 



must mean, since the sandstone is missing, that the land was 
suddenly depressed. And when we examine the succession of 
sedimentary formations, we find all kinds of relations indicating 




Limestone and 


Sandy Shale 

Shale and 




Sandy Shale 




Shale & Limestone 

Sheets & Dikes 

of Lava 

Schist, Granite & Gneiss 

FIG. 139. The succession of strata in the Grand Canyon is evidence of many 

earth movements. 

that the land has moved either up or down many times during the 
earth's history (Fig. 139). 

These are all evidences of diastrophism, or movements of the 
earth's crust. These movements are usually imperceptible, re- 
quiring millions of years; but occasionally, during an earthquake, 
the movement is rather sudden. 




Ti i if i ii 



*167. What is the cause of earth movements? We find the 
layers of sedimentary rock in most regions, not horizontal, as they 
were laid down, but tilted, twisted, and bent. Since the bent 
layers occupy less area than they did before, it follows that there 

^^^^^__-^^^_ ^ nas been a shortening and that 

the earth must have shrunk 
(Fig. 140). It may be, then, that 
the folding is due to shrinking 
brought about by cooling of the 

Another theory to account for 
periodic uplifts is the theory of 
isostasy developed on page 7. 
The sediments eroded from the 
mountains are deposited by 
rivers on the continental shelf. 
This causes the water to rise and 
flood the land. At the same time 
the removal of the load from the land segment makes it lighter, 
while the sediments make the ocean segment heavier. Slow ad- 

FIG. 140. Folded strata occupy a 
smaller area of the earth's surface 
than the original rocks. 


made Lighter 

by Erosion 

[ Deposit 
f Section/ 

FIG. 141. The Theory of Isostasy 

justment takes place, pushing down the oceanic segment and forcing 
up the granitic land segment. 

This theory accounts for the existence of mountains parallel to 
the continental margins and for their elevation and re-elevation 
again and again. It also explains the sidewise pressure, which 
crumples the rocks, as due to the sinking of the oceanic wedges 
which squeeze the continents. 



FIG. 142. An Anticline 

FIG. 143. A Syncline 

G. W. Stose, U.S.G.S. 

*168. Folds. It seems apparent that the crust of the earth 
cannot be elevated without being tilted unless a block is thrust 
straight up. On the Colorado Plateau we find nearly horizontal 
strata. At least they seem to be quite horizontal until followed for 



miles, when it is found that there are warps or irregularities, but 
they are rather broad and gentle. The entire region is one broad 
warp or blister on the earth. 

But much more often the strata were crumpled into folds when 
earth movements took place. Some of these folds are small enough 
to notice in a single view (Figs. 142 and 143), but more often one 
has to map the region, over many miles, before the nature of the 
folds is revealed. 

We distinguish two parts of folded strata, anticlines and syn- 
clines (Figs. 142 and 143). When the region is first folded, the anti- 
clines are the hills, the synclines 
are the valleys; and even after 
the region is peneplaned, we still 
call the structures underneath, 
anticlines and synclines. 

Anticlines of large dimensions 

FIG. 144. Folds in Wax 

FIG. 145. A cylinder of rock 
confined in a close-fitting steel 
jacket changes its shape, without 
breaking, when subjected to great 

are called geanticlines; great downwarps or troughs are called 

Folds resembling those in nature have been produced in the 
laboratory by squeezing layers of wax or clay in a vise. Some of 
these are shown in Fig. 144. 

Ordinary rocks are brittle and when subjected to compressional 
forces they change their shape slightly, but finally break and fly to 
pieces when the pressure becomes excessive. But when the speci- 
men is confined in a steel jacket, it cannot break but apparently 
flows (Fig. 145). The conditions of this experiment are much like 
those in the crust of the earth where the underlying rocks, con- 
fined on all sides, are under enormous pressures. 



*169. Joints, fissures, and faults. Near the surface of the earth, 
therefore, we have cracks in the rocks just as we should expect, 
while down below, because of the pressure, cracks become smaller 
and smaller and finally disappear entirely. 

We find fissures developed very often in anticline structures 
and rarely in synclines. The anticline is under tension, which pulls 


FIG. 146. Joints are formed in anticlines during folding, because 
of tension. 

the rock apart, while the syncline is under pressure, which closes 
up any joints rather than develops new ones (Fig. 146). During 
erosion, therefore, the anticlinal structures are more easily breached 
and worn down, and it follows 
that mountains are often syncli- 
nal in structure. 

If the crack is merely a small 
separation of the walls of rock, 
it is called & joint; if there is a 
distinct separation (an actual 
gap), it is called a fissure. If be- FlG 147 A Fault 

The block on the left has dropped, 
developing a waterfall. 

sides separating, there has also 
been motion parallel to the crack, 
it is called & fault (Fig. 147). 

Many igneous rocks are jointed into large blocks which give 
the impression of stratification (Fig. 19). The joints were formed 
by uniform cooling of the mass, and they are always at right 
angles to the cooling surfaces. Such jointing is called columnar 
structure. It accounts for the Palisades of New Jersey and the 
Giant's Causeway of Ireland, where the igneous mass was hori- 
zontal, making the joints vertical. 



Faulting often produces a repetition of strata on the surface 
(Fig. 148), and when we find such a repetition, we suspect a fault. 

Folding also produces a repetition of strata on the surface, but 
the arrangement is different (Fig. 149). After erosion has planed 

down the fold structures, the 
anticline shows a repetition of 
strata like ABCBA; and the 
syncline, CBABC where A is 
the youngest stratum and C the 

When the displacement on a 
fault is vertical, the one side 
stands higher than the other, 
leaving a fault scarp or cliff 

FIG. 148. Repetition of Strata Due 
to Faulting 

The upper block shows the rock 
before faulting. The middle block 
shows the result of faulting. Erosion 
then wears down the fault scarp on 
the left, leaving the peneplaned sur- 
face, as in the lowest block, with a 
repetition of strata. 

FIG. 149. Repetition of Strata Due 
to Folding 

Compare the order of repeated 
strata here, ABCBA, with that in 
Fig. 148, ABCDABCD. 

(Fig. 147). This forms one kind of mountain, called a block moun- 
tain, examples of which are found in the Great Basin of western 
United States. 

It seems well established that the cause of folding and faulting 
is sidewise pressure. On the surface the rocks crack and fault, but 
in depth they fold, especially when the overlying pressure is great 
and when it is applied very slowly so that the rocks can adjust 


Sudden movements of the earth's crust manifest themselves as 

Completion Summary 

There is evidence that the land has risen or the sea - 
many times in geological history. There is even - - human 

The normal succession of sedimentary strata is conglomerate, 
, - , with limestone at the top. This is so because, 
starting with a highland, the streams can carry - , which 
ultimately forms - . As the highland is worn down, the sedi- 
ments become finer, until at last, when the highland is entirely 
worn down to a peneplain, - - sediment is carried, the waters 
become clear, and - - is formed on top of all the sediments 
brought down from the former highland. 

We believe earth movements are an - adjustment, caused 
by deposition of great weights of sediment on the continental shelf. 

Crustal movements cause - - of the strata into anticlines 
and - . When the forces are too great, the rocks are broken, 
forming - . 

Joints are - - in the rocks, whereas - - are actual 
separations. Fissures are usually developed in anticlines, while in 
synclines, - - because - . 

Uniform cooling of a mass of igneous rock forms joints, at right 
angles to the cooling surface. This is called - . 

A fault scarp is caused by - . On a large scale, a fault 
scarp - - block mountains. 


1. What historical evidence is there that the land has changed 
its elevation? 

2. Explain how a glacial epoch would cause a difference in 
elevation of the land with respect to sea level. 

3. Why do we estimate that the sea was over 150 feet lower 
during the Glacial Period than it is today? 

4. How would the deposition of vast amounts of sediment in 
the sea affect the sea level? 

5. What does a rejuvenated river prove about crustal move- 


6. Show how the character of sedimentary strata changes 
from coarse to fine with the elevation of the source of the sedi- 

7. Limestone on top of sandstone is evidence of what crustal 

8. Shale on top of sandstone indicates that something has hap- 
pened to the near-by highland. Explain. 

9. What does a limestone stratum indicate regarding the 
near-by land? 

10. What is indicated by a great thickness of limestone? 

11. Explain why we have much thicker layers of limestone 
than of conglomerate. 

12. What is diastrophism? 

13. State and explain the theory of isostasy. 

14. What is a fold? Show by diagram. 

15. What is a broad warp? How does it differ from an ordi- 
nary fold? 

16. What is an anticline? a geanticline? 

17. What is a syncline? a geosyncline? 

18. Explain how rocks may fold without breaking. 

19. Why are joints developed near the surface? 

20. Explain why there are often fissures in anticlines but not in 

21. Explain why mountains are often synclinal. 

22. How does a fissure differ from a joint? 

23. What is a fault? 

24. What is columnar structure? 

25. How can we tell a fault by repetition of strata on the 

26. How can we tell that the underlying strata are folded into 
an anticline? a syncline? 

27. What is a fault scarp? 

28. What is believed to be the cause of folding and faulting? 



170. What are mountains? When plateaus are dissected 
they become mountains. The plateau has a large summit 
area, while the mountain has a small area at the top, forming 
a peak. On the one hand, then, plateaus resemble plains in 
that each has an extended flat top; on the other hand, we 
find plateaus resemble mountains in that each is at a high 

Some mountains, like Pikes Peak, consist of a single 
sharp summit or peak; others consist of long ridges. A ridge 
has much greater length than breadth. 

A mountain range is a ridge or a group of parallel ridges. 
A chain of mountains is a group of parallel ridges. A cordillera 
is a group of mountain chains; for example, the Cordilleras 
of western United States include the Rocky Mountain chain, 
the Sierra Nevadas, and the Coast Range. 

There is a widespread belief that mountains are very, 
very steep, with slopes approaching 90. The average slope 
is not more than 20 except near the top, where it may be 
as much as 40. The impression of steepness is probably 
given by the fact that the incline is not continuous but is 
broken up into steps, each of which may have a very steep 
slope. In climbing up the mountain, one can see only one of 
these steps at a time and that leaves the impression of 90 

171. Distribution of mountains. As we have already 
seen (page 196), all great mountain ranges are formed near 
the borders of the continents, parallel to the oceans, and 
the highest mountains are near the deepest ocean, the 
Pacific: for example, the Andes and the Rocky Mountain 




FIG. 150. The Four Great Cordilleran Regions 

systems in South and North America, and the Himalayas 
in Asia. 

There are four great cordilleran regions. 

1 . The North American Cordillera 

2. The Andes of western South America 

3. The southern European Cordillera 

4. The Asiatic Cordillera 

The North American Cordillera includes the Rockies, the 
Sierra Nevadas, the Cascade and Coast Ranges, the Basin 
Ranges, and the coast ranges of British Columbia and 
Alaska. The Andes are really the natural continuation of 
the North American Cordillera. 

The cordillera of southern Europe includes the Alps, the 
Pyrenees, the mountains of Spain and northern Africa, and 
the Carpathians. 

The Asiatic Cordillera includes the Himalayas, the Kunlun 
and Hindu Rush, and Caucasus Mountains. 

172. Origin of mountains. The more important processes 
by which mountains have been formed are shown in the 
following table. 






1. Eruption 
2. Intrusion 
3. Erosion 

Volcanic cone 
Dome mountain 
Dissected plateau 

Single peak 
groups of peaks 

4. Faulting 
5. Folding 
6. Combined 

Block mountains 
Folded mountains 
Complex mountains 



173. Volcanic cones. During the eruption of one type 
of volcano, a large amount of rock material is hurled into 
the air, where it cools rapidly and is already solid by the 
tune it falls. This volcanic ash is very irregular in shape and 
forms a steep conical hill around the crater (Fig. 151). This 

B. Willis, U.S.G.S. 

FIG. 151. A Steep Volcanic Cone 

volcanic cone is gradually built up, higher and higher, by 
successive eruptions until a mountain is formed. Mt. Rainier 
in Washington, Lassen Peak in California, Vesuvius in Italy, 
Fujiyama in Japan, and Aconcagua in Chile are good ex- 
amples of volcanic cones. Aconcagua is 23,000 feet high. 

Some of the Hawaiian volcanoes rise 30,000 feet above 
the ocean floor, with a circumference of 100 miles at sea 
level, and are very symmetrical. These volcanic cones, in 


other words, are not steep like the former, because they do 
not hurl their ash into the air; but the lava flows out quietly 
as a liquid, and therefore spreads out much more before it 

174. Dome mountains. Some igneous intrusions, called 
laccoliths, never reach the surface but bow up the strata of 
overlying rocks by reason of the great pressure from be- 
neath. The strata are not folded but simply raised up into 
a dome. 

The Henry Mountains of Utah (Fig. 152), the Black Hills of 

FIG. 152. Section of the Henry Mountains, Showing How the Igneous 
Intrusion, Forced up from Below, Domed up the Strata 

These strata have since been eroded, exposing the igneous mass. 

South Dakota, and the Elk Mountains of Colorado are 
dome mountains. 

The surface strata above the dome are usually cracked 
during the uplift. Since they are sedimentary rocks, which 
are not very resistant, and because they are at a greater 
altitude, they are eroded rapidly, exposing the igneous 
mass laccolith underneath (Fig. 152). 

175. The dissected plateau. We have already considered 
the cycle of erosion on a plateau, page 188. As the young 
streams cut gorges into the plateau, weathering of the edges 
along the tops of the gorges widens them until the region 
loses its plateaulike appearance and is cut up into mountains 
(Fig. 153). The Catskill Mountains of New York and the 
Appalachian Mountains are both dissected plateaus. 



The final stage of the dissection of a plateau changes the 
region into a plain, with monadnocks in humid regions, and 
buttes and mesas in arid regions. 

Lookout Mountain is a remnant of an old plateau and 
the " temples of the Grand Canyon" (Fig. 9, page 13), 

FIG. 153. Mountains Formed by Dissection of a Plateau 

some of which are over 5,000 feet high, were formed by 
dissecting a plateau. 

176. Block mountains. In the Great Basin there are 
ranges of such simple structure that their origin is clearly 
shown. They were formed by a series of great faults. Figure 
154 shows the structure of three of the ranges, A, B, and D. 

FIG. 154. Diagram of Block Mountains 

The bedrock was evidently faulted, perhaps by a stretching 
force. Since the entire region is bowed up into one broad 
warp without much folding, the top of the anticlinal struc- 
ture would be subject to stretching and would therefore 
crack. This is shown by the dropping down of the wedge- 
shaped block, C. 

Block mountains have a short, steep slope on one side 
and a long, gentler slope on the other side, with the strata 
parallel to the gentler slope. 

In southern Oregon there are many block mountains that 


were formed so recently (geologically speaking) that ero- 
sion has not yet notched their summit lines; and the fre- 
quent earthquakes of the region are further evidence that the 
faulting which accompanies mountain making is still going 

The block mountains of Nevada and Utah are older and 
much eroded. The summits are notched and uneven and 
the slopes are scarred with deep ravines. Some of the ranges 
are 80 miles long and 20 miles wide and their summits rise 
from 2,000 to 7,000 feet above the valleys. 

177. Folded mountains. The great mountain chains of 
the world are records of former crustal movement or dis- 
placements of the rocks resulting from enormous compress- 
ing or uplifting forces, already studied (page 196). 

The surfaces of young mountains are always covered by 
thick layers of sedimentary rocks, indicating that it is always 
a great trough, filled for a long time by an arm of the sea, 
which is finally uplifted and folded into mountains. 

*Underneath the sedimentary rocks we always find meta- 
morphic and igneous rock, chiefly the former, making up the core 
of the mountains. This would seem to indicate the enormous 
forces engaged in the mountain-making process: great pressure 
and heat, causing violent movements and accompanied by igneous 
activity from the depths of the earth, are some of the factors that 
bring about the change of rocks which were originally igneous into 
rocks which are now metamorphic. Often, too, this igneous activity 
has brought up, from the depths, metals and their ores, so that 
mining as an industry is confined chiefly to mountains or areas 
that were once mountains. 

*178. The Jura Mountains. One of the best examples of simple 
folded mountains is the Jura Mountains of France. They consist 
of a series of ridges of sedimentary rock containing marine fossils. 
The rocks were originally horizontal, since all sediments are laid 
down that way; but they are now bent and folded into a series of 
anticlines and synclines (Fig. 155). Most folded mountains also 
show faulting. 

The Jura Mountains have been only slightly eroded. The upper 



layers on the tops of the ridges have been removed and spread over 
the valley floors; but the mountains are still young. 

*179. The Appalachian Mountains. The folds of these moun- 
tains are not at all like the simple curves of the Juras. In the 

FIG. 155. Cross Section of the Jura Mountains 

By the U.S.G.S. 
FIG. 156. Cross Section of the Folds and Faults of the Southern Appalachians 

southern Appalachians the folds have been crowded together and 
faulted. In some of these faults the displacement is several thou- 
sand feet (Fig. 156). 

Examine Fig. 157, which is a cross section of the Appalachian 

\ \ \Mountain 

15pO v Ft.---X-\ - 


Second Blue 
Mountain Mountain 

FIG. 157 

folds. The Pocono sandstone, a resistant rock, forms the summit 
of Peters Mountain, and of Second Mountain, and, from the dip 
or inclination, it is apparent that the Pocono sandstone forms a 
synclinal fold underneath Third Mountain. 

If the strata were continued, as is indicated by the dotted lines, 
we should have a diagram of the anticlines that formed the ances- 


tral Appalachian Mountains, a range which probably resembled 
the Alps in grandeur and elevation (Fig. 158). 

If the continuation of these lines were actually carried out, to 
scale, it would appear that the ancestral Appalachian mountains 
were perhaps ten miles high, which is not borne out by other 
evidence. We must remember that the process of mountain mak- 

Former Appalachians 


Present Appalachians 
FIG. 158 

ing may take many millions of years and while it is going on, 
erosion is also going on and cutting down the highest places. These 
two processes, working against each other, produce the actual 
mountains, whereas, if there were no erosion at all, it is possible 
that some of our mountain ranges would be ten to fifteen miles high. 
Although three and one half miles of rock have been eroded 
from the top of the Uinta Mountains, it is probable that they have 
never been much higher than they are at present. 

It may seem strange that the tops of the present moun- 
tains are synclinal in structure rather than anticlinal, because 
when they are formed, the anticlines form the mountains, 
while the synclines are the valleys, as is the case with the Jura 
Mountains (Fig. 155). But we must remember that during the 
folding the tops of the anticlines are cracked, and hence 
easily eroded ; while the synclines consist of firm, dense rock, 
because there the strata are compressed and all cracks closed 
up. Erosion of running water and frost action quickly wear 
through such anticlines, while the synclines, having few, if 
any, cracks are worn much more slowly. If this process con- 
tinues long enough, the synclines will be at the same level 
as the anticlines, and we shall have a peneplain (Fig. 159). 

If this peneplain is elevated, the former anticlines will 


again begin to wear more than the synclines, and we shall 
in time have a dissected plateau in which the synclines are 
the mountains and the anticlines are in the valleys. 

If we were standing on the top of Blue Mountain (Fig. 
157), looking west over Second, Third, and Peters Mountains, 
we should see that these 
mountain tops are at about 
the same level. If the valleys 
between the mountains were 
filled, a plain would be formed 
which would slope gently to- 
ward the east. (The dotted 
line in the figure is horizon- 
tal.) The evidence shows that 
the ancestral Appalachian 
Mountains were originally 
uplifted in great folds; they 
were then peneplaned, up- 
lifted as a plateau, and it is 
this plateau which has since 
been dissected to form the 
present mountains. 

FIG. 159. Development of a Syn- 
clinal Mountain 

Starting with the uppermost block, 
the anticlines are eroded rapidly, and 
even when the region is peneplaned 
and then re-elevated, the anticlines 
continue to erode faster than the syn- 
clines, until in the lowest block we 
have a synclinal mountain. 

The ancestral Appala- 
chians were folded mountains, 
formed by a lateral thrust 
that acted westward from the 
Atlantic coast. The present 
Appalachians were formed by 
erosion made possible by a vertical force which elevated the 
region, after it had been peneplaned, into a plateau. 

*The ancestral Appalachians were uplifted at the end of the 
Paleozoic Era, about 180 million years ago, and the peneplaned 
mountains were re-elevated at the end of the Mesozoic, about 60 
million years ago. 

The top of Third Mountain was once the bottom of a 
great valley or syncline ; hence it is called a, synclinal mountain. 


The Appalachians are mature mountains with rounded 
and well-forested tops. 

180. The Rocky Mountains. Like all great mountain sys- 
tems, the Rocky Mountains were formed from thick sedi- 
ments that accumulated in a great synclinal trough in the 
bottom of the sea, which then covered interior North Amer- 
ica. During the uplift there was some folding, but it was 
not nearly so intense as in the case of the Appalachians 
(Fig. 160). Apparently there was some pressure from the 

By the U.S.G.S. 

FIG. 160. Cross Section of the Rocky Mountains 

side, but the chief force was vertical, so that the strata are 
not badly folded and crushed. 

The core of the mountains is made of granite and other 
similar igneous rocks. On either side of this core are layers 
of sedimentary rocks which have been pushed up by the 

*The Rocky Mountain uplift occurred at the end of the Mesozoic 
Era, about 60 million years ago, and has continued into recent 
times. It is probably still going on. 

The Rocky Mountains are young mountains, with rugged 
peaks which make a very irregular sky line. The name comes 
from the rocky tops of the mountains, many of which are 
above the timber line and therefore bare (Fig. 161). 

181. The life history of mountains. The life history of 
mountains follows closely that of rivers; and we may sum 
up the characteristics of young, mature, and old regions as 
follows : 

Example: Rocky Mountains 

1. Lofty elevation 

2. Rugged irregular sky line. Good scenery 


3. Steep slopes with little talus 

4. Young streams, often torrential, with deep ravines 

5. Avalanches, landslides, and earthquakes occur. 


B. Willis, U.S.G.S. 
FIG. 161. Scene in the Rocky Mountains 

Example: Appalachian Mountains 

1. Elevation not very high 

2. Rounded tops, well covered with vegetation 

3. Uniform slopes with much talus 

4. Mature streams. Water gaps appear. 

5. Avalanches rare. Earthquakes unknown 

Example: New York City 

1. Low elevation, approaching the peneplain 

2. Monadnocks stand out. 

3. Region rather flat with low, rolling hills 

4. Streams old 

There is, of course, no sharp line of division between the 
three stages in the life history of mountains and it is possible 
to add other subdivisions such as early youth, late youth, 
early maturity, etc. 


The uplift and decline of mountains takes place so slowly 
that the change produced during one's lifetime passes un- 
noticed, and men have come to think and speak of moun- 
tains as everlasting. History gives us no assistance either. 
Polybius's description of the Alps as they were when Hannibal 
crossed them in 218 B.C. is practically a description of the 
Alps today. They are still young mountains and the lapse 
of 2,000 years has not changed them noticeably. It is evi- 
dent, then, that the life history of mountains cannot be ex- 
pressed even in thousands of years. The Rocky Mountains, 
which were formed, we believe, about 60 million years ago, 
are still young. 

182. Climate of mountains. The snow-capped mountains 
of the torrid zone exemplify, upon their slopes, all the cli- 
matic changes that one would experience in traveling from 
the torrid zone to the polar regions. As one ascends, the 
palms and bananas of the torrid zone gradually disappear 
and are replaced by the deciduous trees and wild flowers of 
the temperate zone. These, in turn, are replaced by the 
cone-bearing trees, which, as the ascent is continued, be- 
come low and dwarfed; finally all trees disappear farther 
up, and the snow-clad top is a frigid zone in miniature. In 
a similar manner the forms of animal life that inhabit the 
bases of such mountains gradually disappear and are re- 
placed by forms resembling those that characterize the colder 
parts of the earth. 

The great variety in mountain climate is due to the fact 
that the temperature gradient in air at rest is more than 
1,000 times as great as the horizontal temperature gradient. 
That is to say, the temperature, as one ascends, decreases 
more than 1,000 times as fast as it does when one travels 
toward the pole. 

The timber line and snow line are more or less irregular, 
being higher on the sunny side than on the shady side. In 
the equatorial region, the snow line is about 18,000 feet 
above sea level, but its altitude diminishes as the distance 


from the equator increases, reaching sea level at the polar 

Many ranges are subject to excessive rainfall or snowfall 
on the windward side, and where they cross prevailing winds, 
the climates of the opposite slopes are quite different. For 
example, on the western slope of the Sierra Nevadas the 
moist wind is chilled as it rises, and thus it produces abun- 
dant rainfall, which supports forests, whereas the same 
wind on the eastern slope, having lost much of its moisture 
and being heated as it descends, becomes a drying wind 
which takes moisture from the land and makes it arid. 

183. Influence of mountains on man and history. Be- 
cause of the difficulty of crossing mountain ranges, the dif- 
ference in climate on the different sides, and the military 
advantages which they afford, mountain ranges are the 
natural boundary lines for nations. The Himalayas, which 
separate different races; the low Pyrenees, crossed by few 
roads and railroads; the Caucasus, the Alps, and the Andes, 
all illustrate the tendency of nations to select mountain 
ranges for their frontiers. 

As the Indian and the pioneer gained a measure of se- 
curity within their stockades, so a nation surrounded by 
mountain ramparts feels secure from outside interference. 
Their elevation enables scouts to see an approaching enemy 
who would be invisible on a plain, and this diminishes the 
chance of surprise. Narrow passes, well fortified, can be suc- 
cessfully defended against vastly superior numbers because 
the invading army cannot approach the pass in line of battle 
and is met in small parties. The famous defence of Ther- 
mopylae by the ancient Greeks illustrates this advantage. 

The soldier on the mountain meets a tired foe, and in 
hand-to-hand conflict this is an important aid. Artificial 
avalanches of boulders have frequently decimated armies 
attempting to cross mountain passes. When Hannibal crossed 
the Alps, his losses through this kind of warfare contributed 
in no small measure to his ultimate defeat. 


Because of the security afforded, conquered races usually 
make their last stand in mountains and have frequently 
been able to maintain their position through long periods. 
Some of these peoples have maintained their individuality 
even to the present day, as the Basques, the Welsh, and 
the Scotch Highlanders. 

With the military advantage comes a degree of isolation 
which favors the development of a distinct type of civilization 
and an individual language, or dialect, in the region thus set 
apart from the rest of the world. This tendency is illustrated 
in the many small principalities which developed in Europe 
during the Middle Ages, several of which exist today; and 
by the fact that, in the California valleys, there were almost 
as many tribes of Indians having characteristic languages 
and customs as there were valleys between the mountains. 

The same isolation limits commerce and knowledge of the 
outside world, and compels the residents of mountainous re- 
gions to depend upon themselves for their wares and for 
their progress. If their number is small, as it is apt to be 
on mountain slopes where the struggle for existence is so 
strenuous, there is rarely progress in the ways of civiliza- 
tion, but instead there is often a retrograde movement. 
Mountaineers are proverbially conservative, using the same 
processes and following the same customs that their ancestors 
used and followed. In the southern Appalachians we find 
excellent illustrations of this effect; here are peoples follow- 
ing habits and customs of the eighteenth century. Mining 
cities in mountains are exceptions. To them, the sudden 
wealth brings all that is good and all that is bad in our 
modern civilization. 

184. Mountains as barriers. Several conditions make it 
difficult even for man to cross high mountains. (1) The 
steep slopes at low elevations are hard to climb; those at 
high elevations are worse because of the thin air. (2) More 
serious conditions are the low temperature, the penetrating 
wind, and the driving snowstorms, followed by the blinding 


glare of the sun. (3) Avalanches and landslides sometimes 
bury whole caravans in the Himalayas. In our own Cascades, 
an avalanche carried away an entire train of Pullman cars. 
(4) Doubtless the most dangerous mountain road in the 
world is the caravan route from India into western China. 
For several days' journey the route is above the timber 
line, where no grass can grow and where it is so cold that 
there are no inhabitants. Thousands of horses have been 
lost in this section through cold and hunger. 

Even the minor mountain ranges retard the exploration 
and settlement of a region. 

Mountain ranges are watersheds and rivers rarely cross 
them. The explorer who wished to cross them was obliged 
to obtain horses or carry his supplies on his back. This re- 
quired complete reorganization of a party that was equipped 
to travel by canoe, and often caused the explorer to turn back. 

If an explorer follows rivers, shelter and food for many 
weeks may be carried in a canoe by one man; if he journeys 
over plains, he must carry food for his horses and the men 
who care for them 'as well as for himself; if he is to cross 
mountains, pack animals must be substituted for wagons, 
with further increase in the size of the party. 

There is no better illustration of this retarding action 
than that found in the early history of this country. Before 
the year 1600 European explorers had discovered the mouths 
of the St. Lawrence, the James, the Mississippi, and the 
Rio Grande, and had visited California. During the next cen- 
tury the English explored and settled the Atlantic Coastal 
Plain, but made few attempts to cross the low ridges of the 
Appalachians; the French, during the same period, explored 
the St. Lawrence and followed the Mississippi to the Gulf. 
They established settlements along these routes, which grew 
into towns still bearing French names, such as Detroit, 
Sault Ste Marie, Fond du Lac, Prairie du Chien, St. Louis, 
and Baton Rouge. The Spanish settlers on the Gulf of 
Mexico, during the sixteenth and seventeenth centuries, 


extended their missions toward the north as far as Sante Fe, 
where the Rocky Mountains checked further progress in 
this direction. They therefore pushed westward to south- 
ern California. From there they followed the Pacific coast 
toward the north, establishing missions in the narrow area 
between the Coast Range and the Pacific. Their trail is 
now marked by cities still having Spanish names, such as 
San Antonio, Sante Fe, and, along the coast, San Diego, 
Los Angeles, San Francisco, Sacramento, San Jose, and 
San Luis Obispo. 

The Berkshire Hills, in Massachusetts, exerted an im- 
portant influence in settling the contest between Boston 
and New York City for commercial supremacy. Freight 
brought from the west through the Mohawk Valley to Al- 
bany could be brought to New York by boat more cheaply 
than it could be hauled over the Berkshires by teams, and 
much of it was naturally deflected to New York. When 
railroads were built along the Hudson and through the Mo- 
hawk Valley, New York City acquired further advantage 
over Boston because of the Berkshires. Before a railroad 
line from Albany to Boston was completed, the position of 
New York as the chief seaport of the United States was 
fully established. 

Mountains are not absolute barriers. They are difficult 
to cross; but when sufficient incentive is provided, men al- 
ways succeed in crossing them. 

In the case of the English colonists the necessary incentive 
came in the demand for more room and more virgin soil 
and in the increased importance of the trans-Appalachian 
fur trade. During the French and Indian War the possession 
of the best passes through the mountains was stubbornly 
contested, as is shown by the large number of battlefields 
between the Hudson and Lake Champlain, and between the 
Mohawk and Lake Ontario. 

The Rocky Mountains retarded the settlement of Cali- 
fornia more effectively than the Appalachians confined the 


colonists to the Atlantic coast, and for a longer period, be- 
cause of their greater height and breadth; but the necessary 
incentive came in the discovery of gold in 1848. Before the 
close of 1849 there were 100,000 people in California. 

185. Mountain industries. The excessive cost of trans- 
portation in mountainous regions is so serious a handicap 
as to render manufacturing unprofitable when in competi- 
tion with manufactories located on plains, unless the moun- 
taineer can find his raw material on the mountain and can 
convert it into a light finished product of greatly increased 
value. Thus lumber grown on the mountain is made into 
carvings and souvenirs and sold to tourists, entirely elimi- 
nating all cost of transportation. 

In mining regions the handicap is less important than 
elsewhere, because the product is so valuable. The few dollars' 
worth of gold obtained from a ton of ore can be carried 
down the mountain in one's pocket. 

186. Mining. One of the reasons why mining is so im- 
portant among mountain industries is that igneous rocks 
are so often found in the center of the mountain mass 
as shown in the cross section of the Rocky Mountains 
(Fig. 160). Certain valuable ores are found in these deposits 
of igneous rock, others occur in rocks metamorphosed by 
contact with the igneous rocks, and still others are found 
in fissure veins or in porous rocks through which ground 
water has circulated. 

A second reason for the importance of mining in moun- 
tain regions is the ease with which the kinds of rock form- 
ing the mountain are discovered. On plains, the kind of 
rock underground can be determined only by boring. On 
the mountain, where streams cut gorges across the strata, 
the structure and the kinds of rock are revealed. In the 
United States we obtain a large percentage of the supply 
of gold, silver, copper, iron, lead, zinc, and anthracite coal 
from mountain mines. Much marble, slate, and granite are 
quarried in mountains. 


Bituminous coal, salt, rock phosphate, and some iron ores 
occur in sedimentary rocks. These ores are mined in plains 
as well as in mountains. 

Figure 162 shows the location of the gold and silver mines 
of the United States. Gold mines are found in the Appa- 
lachian Mountains, in the Black Hills, and in the western 
mountains. Silver mines are all located among the ranges of 
the Western Cordillera. Compare this map with Fig. 150 to de- 
termine the position of the mines with respect to mountains. 

Can you suggest a reason for the large number of mines 
along the eastern boundary of California? 

187. Water power. The water power of mountain streams 
has long been utilized in cities along the fall line and by 
the miners of the western mountains, but only a small per- 
centage of mountain streams can be thus utilized. The pos- 
sibility of electric transmission of power has greatly increased 
the value of these streams; and the public interest in the 
" white coal/' as water power is called, speaks for its rapid 
development. It is destined soon to become a second im- 
portant industry in the mountain regions and a great stim- 
ulus to our manufactures. 

The amount of power that can be obtained from a given 
fall depends upon the weight of the water per minute and 
upon the vertical distance that the water falls. The product 
of these two numbers gives us the theoretic number of foot- 
pounds of work that the fall can produce; but this number 
can never be obtained since no water wheel is 100 per cent 

If the flow of a stream were uniform throughout its course, 
its best water power would be where the highest fall was 
located. It is estimated that the total power that could be 
developed by the Mississippi River between St. Louis and 
New Orleans, where the slope is gentle, is only about 150,000 
horsepower. A smaller quantity of water, flowing in the steep 
portion of the river near its source, could develop 6,430,000 




Our best water-power opportunities are located in regions 
of rugged relief, generally at falls or steep rapids. There 
are numerous falls and rapids on mountain slopes, and the 
streams often flow in deep canyons that may be converted 
into great reservoirs for storing water until it is needed. 
These two advantages have led to an increase in the de- 
velopment of mountain power stations, and it is hoped that 
the millions of horsepower now being wasted in mountain 
regions will soon be utilized. 

188. Irrigation. The high mountains of the United States, 
like the Sierra Nevadas, the Cascades, and the Rockies, 
have heavy rainfall on their western slopes, and an arid 
region east of them, because they lie across the path of the 
prevailing westerlies. 

The mountains are the cause of the arid regions, but they 
also enable us to restore fertility to the region through irri- 
gation, as artificial watering is called. Through their forests, 
glaciers, and lakes, mountains are great reservoirs of water 
which give rivers rising in mountains a much more uniform 
flow of water than that of rivers rising in plains. Then, too, 
mountain valleys of such streams can be converted into 
additional reservoirs of great capacity by building dams 
across the valleys. From these reservoirs, canals can carry 
water to each farm in the district. 

*189. Agriculture. Mountain slopes are not well adapted to 
agriculture, but there are regions where the number of inhabitants 
is so great that every foot of land must be made to yield its share 
of the food supply or someone will go hungry. In China, for ex- 
ample, steep slopes have been terraced by building stone walls, at 
intervals, on the slopes and by carrying earth up the mountain to 
fill the space on the upper side of the wall, to produce a nearly level 
field. This requires labor that doubtless makes the fields thus built 
much more costly than the same area of level land; but when the 
work is done, the owner has a field that will furnish food as long as 
he lives. The Igorots in the Philippines also raise rice on such 
terraces. In Europe there are many terraced vineyards on moun- 
tain slopes (Fig. 163). 


Terracing prevents erosion of the soil, which so often destroys 
slopes as soon as the forests are cleared away. The roots of trees 
tend to hold the soil in place ; and in some places mountain slopes 
have been slowly converted into orchards of apples, oranges, or 
olives, by replacing the forest trees with fruit trees. In other sec- 

FIG. 163. Farming on Terraces 

tions there are groves of nut-bearing trees that yield as great a 
return per acre as our best wheat land. In all cases of mountainside 
farming, the cost of raising the crop and of transporting it to market 
is much greater than it would be if the land were level. The in- 
centive that leads the farmer to incur the extra expense is usually 
the scarcity of level lands; but occasionally such conditions as the 
unusual fertility of a certain slope or its favorable exposure to the 
sun's rays may lead him to cultivate the hillside. 

Stock raising is an important industry in the western mountains 
of both North and South America; also in Switzerland, Germany, 
and Norway; and in Asia. It is about the only industry that can 
be carried on at elevations much above a mile; grasses, however, 
grow nearly up to the snow line, and herds are driven up the 
mountainside as spring climbs upward, to move down again as 
autumn climbs downward. The land in the valleys, near the lower 
limit of the stock-raising belt, are all used to supply hay for winter 


190. Forest reserves. The increasing population of the world 
demands greater and greater supplies of food, leads to the clearing 
of more and more of the level land each year, and drives the forests 
to the rugged lands. This increase threatens our supply of lumber 
and has already used up practically all of the timber that was 
growing on the plains of Europe. In the United States we still 
have several large areas of level land, like the pine barrens of the 
coastal plains, that are forested because of their low fertility, but 
it is only a matter of time till these will also disappear. As soon as 
the demand for food has become great enough, these areas will 
be cleared and fertilized, and then they will be turned to agricul- 
tural use. 

The United States Forest Service is now developing about 
200,000,000 acres of forest land belonging to the government and 
is employing hundreds of men to patrol the reserves and enforce 
the rules of the service. It is its policy to prevent loss of trees 
through forest fires, tree diseases, and wasteful lumbering. It sees 
that all trees showing signs of deterioration are cut and sold for 
lumber or firewood, and that every tree that is cut is replaced by 
a young tree. This plan will eventually permit the sale of six or 
seven billion board feet of lumber annually without injury to the 
forests. It builds roads and maintains 1,500 camp grounds that are 
open to the public. 

Completion Summary 

A chain of mountains usually includes several 


- usually has several roughly parallel chains of 

The four great are the North American Cordillera, 

the Andes, the , and the . 

A single mountain may be either a , or a . 

A - cut up by streams becomes a chain of moun- 

Faulting during an uplift mountains. 

Most of the great mountain systems owe their origin to 


*The great forces engaged in mountain making meta- 
morphosed condition of the cores of the mountains, often exposed 
by erosion. These rocks - - metals and their ores. 

The - - mountains show simple folding, while the Appa- 
lachians have very complex and faulted . 

Mountains are often synclinal because . 

The Appalachians are mature . 

The Rocky Mountains are not - , but consist rather 

of one broad warp, showing vertical , rather than 

pressure. They are - - mountains. 

The life history of mountains corresponds to 

streams. In youth, - - grandeur, with forested, 

and deep ravines. In maturity, the tops - , waterfalls 

. In old age, - with monadnocks. 

The climate of mountains is - and usually at least 

one side - . 

Mountains - - barriers to civilization. 

Industries often found in mountainous areas include 
, - , and - -. 

Water power - , and in connection with water power, 
irrigation '. But in spite of all these advantages, agri- 
culture . 


1. How does a mountain or group of mountains differ from a 

2. What is the average slope of mountains? Why do we think 
it is much steeper? 

3. Where do we find the great Cordilleras with respect to the 

4. Name the great Cordilleras. 

5. Name the ranges included in the North American Cordillera. 

6. Explain how a volcanic cone is built up. 

7. How are dome mountains raised? Name one. 

8. Explain why dome mountains are not eroded as fast as 
volcanic cones. 

9. What is a dissected plateau? Name one. 


10. How are block mountains formed? Explain. 

11. Where are there block mountains in the United States? 

12. What are synclinal mountains? Explain how they are 

13. What are folded mountains? How do we believe they were 

14. Name a range of folded mountains. 

15. Why are the Rocky Mountains not badly folded? 

16. Choose an appropriate illustration in this chapter and state 
which of the characteristics of young mountains you find there. 

17. Repeat question 16 for mature mountains. 

18. Explain the great variation in the climates of a mountain. 

19. Select one example to show how mountains prevent the 
spread of civilization. 

20. In a brief outline, show how mountains prevented the 
spread of colonization in the early history of the United States. 

21. Explain how mountains prevented Boston from developing 
as fast as New York. 

22. What are the chief industries of mountains? 

23. Why do we discover mines in mountainous areas more 
readily than elsewhere? 

24. Where are the best water-power sites located? 

25. Why is irrigation usually necessary on one side of a moun- 
tain range? 

26. Why is it usually easier to develop irrigation projects near 

if Optional Exercises 

27. Why is stock raising often followed by mountaineers? 

28. Explain the difference between the two types of volcanic 

29. In what way does a dome mountain differ from the others? 
Which type does it most resemble? How could these be dis- 

30. Describe the cycle of erosion of a plateau. 

31. Explain why block mountains have a long, gentle slope on 
one side, and a short, steep slope on the other. 

32. Why are young folded mountains always covered by thick 
layers of sedimentary rocks? 


33. Why are the cores of mountains usually metamorphic? 
What were they originally? 

34. Why do we often find metallic ore bodies in mountainous 

35. Show by diagram that the Appalachian Mountains were at 
one time much higher than they are at present. 

36. If there were no erosion, some of our mountains would be 
fifteen miles high. Explain. 

37. Outline briefly the history of the Appalachian Mountains. 

38. State features of the topography of New York City which 
class it as an area of old mountains. 

39. State one reason why our water power is not more fully 

40. Explain the difficulties of agriculture on mountains. How 
can some of these difficulties be overcome? 

41. Why are the forest reserves confined to mountainous areas? 

42. Discuss briefly the work of the Forest Service. 



191. What is an earthquake? We often experience a 
shaking of the earth when a heavy truck rumbles over the 
pavement. We do not call that an earthquake, but the same 
intensity of movement due to natural causes would be called 
an earthquake. Our instruments, seismographs, record several 
thousand earthquakes a year. In 1923 the Tokyo earth- 
quake caused the destruction of 150,000 people and three 
billion dollars in property. It is estimated that during human 
history about 15 million people have been killed by earth- 

*192. The Ischian earthquake. On July 24, 1883, the island of 
Ischia, near Naples, Italy, was shaken by an earthquake which 
lasted only 15 seconds. Violent detonations accompanied the 
tremors, fissures were opened, landslips occurred, 1,200 houses were 
destroyed, and 2,300 persons were killed. Survivors tell us that the 
whole town seemed to "jump into the air and fall in ruins." 

On this island is the great crater of Epomeo, a volcano which 
was in eruption in 1302, after at least 1,000 years of slumber. This 
earthquake was not accompanied by an eruption but it is believed 
that the underground explosions which caused the earthquake were 
of volcanic origin. 

*193. The Charleston earthquake. In 1886 slight earthquake 
shocks occurred, at intervals, at Charleston, South Carolina. 
Their violence gradually increased, culminating in one of the great 
earthquakes of the century. There was first noticed a distant 
rumble which increased in intensity as though an enormous rail- 
way train were approaching through a tunnel beneath the city. 
As this rumble became a roar, the ground seemed to rise and fall 
in visible waves. The disturbance lasted about 70 seconds and was 
repeated, with the same violence, about 8 minutes later. 




During these tremors men could not stand, chimneys were 
thrown down, and every building in the city was damaged. Great 
fissures were opened in the earth and both underground and surface 
drainage were disturbed. Railroad tracks were twisted and bent 
and 27 persons were killed. The shock was felt as far as Canada. 

The earthquake was succeeded by several less severe shocks 
during the night and slight shocks were observed in the region 
for several months. 

194. The San Francisco earthquake. About 5 A.M. on 
April 18, 1906, an earthquake occurred on the California 
coast which lasted 67 seconds. During this short interval 

W. C. Mendenhall, U.S.G.S. 

FIG. 164. Building Wrecked in the San Francisco Earthquake of 1906 

many buildings in San Francisco were wrecked, gas mains 
were broken, and the water pipes breached so that the fire 
which followed destroyed a large part of the city (Fig. 

At the same time, landslides occurred in the near-by moun- 
tains, fissures were opened in the earth, and some districts 
settled several feet. 



This earthquake was due to slipping along an old fault, 
the San Andreas Rift, which has been traced 600 miles 
(Fig. 165). The displacement was entirely horizontal, as is 
shown in Fig. 166. 

Gilbert, U.S.G.S. 

FIG. 165. The San Andreas Rift 

FIG. 166. Displacement of a Road and Fence along the San Andreas Rift 


*195. The Messina earthquake. At 5:23 A.M., December 18, 
1908, the region about the Strait of Messina, in southern Italy, 
experienced one of the most disastrous earthquakes in the history 
of the world. The cities of Messina and Reggio were reduced to a 
shapeless mass of ruins, several smaller towns were more or less 
damaged, and more than 200,000 persons were instantly killed or 
imprisoned in the ruins, so that rescue was impossible. 

The ground seems to have been suddenly raised and then 
dropped, causing the buildings to collapse. Great fissures opened. 
The wharf sank to the level of the sea and a sea wave, from six to 
ten feet high, swept over the lower portions of the region. The 
earthquake was preceded by several slight shocks and the seismic 
activity continued for several weeks. The rupture of telegraph 
cables indicates a submarine disturbance whose center was a line 
through the Strait of Messina. It is probable that the earthquake 
was due to slipping along the old fault plane which runs through 
the strait. This fault plane has probably been the seat of many 
earthquakes. The total vertical displacement of one side of the fault 
is known to be several thousand feet. 

196. Distribution of earthquakes. No portion of the 
earth is entirely free from earthquakes, although most of 
them occur either in the vicinity of active volcanoes or near 
young mountains. The borders of the Pacific Ocean are 
particularly subject to earthquakes. Another belt crosses 
Eurasia, beginning at Gibraltar and following the general 
direction of the Alps, the Caucasus, and the Himalaya 
Mountains (Fig. 167). 

There have been, in recent times, rather few earthquakes 
among the older mountains which border our eastern coast. 
Professor Shaler has called attention to the fact that, in 
New England, there has been no violent earthquake since 
glacial times; for if there had been one, it would have dis- 
placed the numerous balanced rocks, perched boulders (see 
page 122), which are found there. 

It appears from the record, then, that regions of old 
mountains and wide continental shelves, like those on the 
Atlantic seaboard, are practically free from earthquakes, 



whereas they are very common in regions of young moun- 
tains, like those on our western coasts. 

Any natural phenomenon which results in a heavy blow 
to the bedrock might throw a portion of it into a vibration 

FIG. 167. The earthquake belts are shown in black. 

which would be classed as an earthquake. Slight tremors 
may be caused by great landslides, by the fall of the roof 
of a large cave, or by similar accidents. 

The earthquake belts correspond rather closely to the 
belts of volcanoes; from which it might be inferred that all 
earthquakes are caused by volcanoes. But these belts are 
also the regions in which there are young mountains. We 
believe that all major earthquakes are the result of faulting. 

Stresses set up within the crust of the earth by lateral 
pressure cause deformation of the rocks up to the limit of 
their elasticity. But when these stresses become too great 
the rock breaks and slips, making a fault (Fig. 168). After 
the break the two surfaces are held by friction, and again 
when this is overcome another slip will occur. The sudden 
movement jars the rocks, and this sets up an intense vi- 



197. Seismic sea waves. When an earthquake occurs in 
or near the sea, great sea waves are often produced. The 
water at first recedes from 
the land, and sometimes 
leaves vessels stranded on the 
exposed sea bottom. Then a 
great wave advances, which 
has in some instances swept 
the vessels over [the tops of 
houses and has stranded them 
far inland. At Lisbon, Portu- 
gal, in 1755, some 30,000 
people who had sought safety 
from the earthquake on the 
wharves were drowned by the 
sea wave. 

These waves have been 
called tidal waves, but they 
are obviously not of tidal 
origin. Most seismologists 
now call them tsunamis, a 
Japanese word. Some of them 
are as high as 40 feet, and 
200 miles long, and they 
move with terrific speed up to about 500 miles per hour. 

*198. The seismograph is the instrument used to record earth- 
quake shocks. It operates on the principle of inertia. If one suspends 
a heavy weight on a spring and then suddenly moves the spring 
a little, up and down, the weight hardly moves at all, owing to 
its inertia (Fig. 169). 

Now suppose we arrange a framework attached to bedrock, as 
shown in Fig. 169, and have a suitable drum with a sheet of paper 
on it the drum actuated by a clock. If the bedrock shakes, it will 
move the paper up and down ; and the pencil attached to the weight, 
which remains relatively stationary, will trace the movements of 
the bedrock on the paper. The time will be recorded by the clock. 

FIG. 168. The upper block, sub- 
jected to lateral pressure, as shown in 
the middle block, may break and slip, 
as shown in the lowest block. It is 
the slipping along the fault which 
causes an earthquake. 



Spring Suspension 
for Weight 

Free Hung 

Dead Weight 

(Position Constant) 

Actual seismographs are not so simple as this, but the principle 
is the same. The record of an earthquake is called a seismogram 
(Fig. 170). 

The primary tremor represents a compressional or sound wave 

which travels several miles per sec- 
ond. It is this part of the tremor 
which makes the rumble, noticed 
just before the main shock. 
The secondary travels at about 
half that speed. The secondary 
is a transverse wave like a water 
wave or a wave set up in a 
long rope. It makes no sound, 
but shakes the earth. Both are 
believed to move from the source 
in the interior directly to the in- 

The long waves which repre- 
sent the destructive earthquake 
waves are transverse. We believe 
they travel from the seat of the 
disturbance to the nearest point 
on the surface, and then through 
the crust, around the circumfer- 
ence of the earth. Hence they 

Base Firmly Cemented 
to Bed Rock 

FIG. 169. The Principle of the 

take longer, but they are more destructive, perhaps because the 
surface, being free, can vibrate with greater violence. 

Primary Secondary Long Wave 

FIG. 170. A Seismogram 

199. Precautions against earthquakes. For an earth- 
quake whose center is 1,000 miles away the time interval 
between the primary and the secondary waves will be about 
two minutes; and in that case, one has a warning of two 


minutes before the main shock. This would be sufficient time 
to run to a place of safety if the primary tremor were recog- 

It has been found by careful study, including accurate 
surveys of the fault, that it may be possible to predict 
earthquakes. A careful study of the San Andreas Rift leads 
one geologist to the conclusion that there will be another 
major earthquake in California about 1950. 

Now, while this prediction may be very inaccurate since 
it is impossible to collect all the necessary information by 
surface investigations, it arouses the inhabitants of the 
region to the necessity of taking all possible precautions 
against recurrence of an earthquake. 

It is found that the destructive effects are much worse on 
loose material than on solid bedrock. This was noted in the 
San Francisco earthquake as well as that at Messina. This 
can be illustrated by placing a small object on a table and 
giving the table a sharp rap. While the movement of the 
table is imperceptible, the small loose object will jump into 
the air. 

During the Tokyo earthquake it was noted that buildings 
with steel frames were not badly damaged, while brick 
walls, statuary, and other loose objects were toppled down. 

Dwellings made of wood stand up much better than those 
of stone, because of the greater flexibility of wood. However, 
we must guard against the possibility of fire, which rules 
out wood, and since the water mains are usually broken, 
the destruction by fire is often worse than that caused 
directly by the earthquake. 

The loss of life in an earthquake is due usually to the 
falling of buildings. It is indicated by past events that 
buildings should be placed on bedrock rather than loose, 
unconsolidated mantle rock. They should be of steel, with 
all the trimmings firmly fastened to the framework, so that 
they cannot fall. 

Small buildings in rural districts may be of wood, well 


braced, so that they cannot be shaken apart like a house 
of cards. 

Completion Summary 

Earthquakes occur in the same regions as volcanoes and 

young mountains, not because one of these other, 

but rather because all of them - . 

Major earthquakes - faulting. The rocks, under 

great stress, slip and , - - vibration which is the 


Earthquake shocks - - sea - - tsunamis. 

*The seismograph inertia. The sound wave. The 

secondary , while the long waves, which move from the seat 
of the disturbance to and then travel . 

The primary tremor may occur - - minutes before 

. Destruction of buildings built on - - is much 

worse than - . Loss of life is due chiefly to falling 

. Therefore buildings with - - framework, built 

on solid rock, with - - loose ornaments, are safer in 

earthquake regions. 

In country districts, wooden houses - . 


1. What is an earthquake? 

2. Cite evidence, from the San Francisco earthquake, for our 
present belief that earthquakes are caused by faulting. 

3. What evidence is there that there has been no violent 
earthquake in New England since the Glacial Period? 

4. In what kind of region are earthquakes common? 

5. Where are the earthquake belts? 

6. What is the cause of major earthquakes? 

7. What is a tsunami? 

8. What warning does an earthquake give of the main tremor? 

9. How does the effect of an earthquake on a structure built 
on bedrock compare with the effects on a structure erected on 
loose soil? 


10. What precautions should be taken when erecting buildings 
in regions subject to earthquakes? 

11. Where would there be more danger of earthquakes: along a 
mountainous or along a coastal plain shore? 

^ Optional Exercises 

12. Cite evidence of an earthquake caused by volcanic ex- 

13. What characteristics of most earthquakes were shown at 
the beginning of the Charleston earthquake? 

14. Describe the Messina earthquake. 

15. Why is it commonly believed that earthquakes are caused 
by volcanic action? Is there any connection between the two sets of 
phenomena? Discuss. 

16. Explain the relation between earthquakes, volcanoes, and 
young mountains. 

17. How does the seismograph operate? Explain how inertia is 
used to make the record. 

18. What are the primary, the secondary, and the long waves? 



200. What is a volcano? It is commonly thought that a 
volcano is a burning mountain. A volcano does not begin 
as a mountain, as is shown by Hobbs in describing the birth 
of a volcano on the island of Camiguin, north of Mindanao 
in the Philippines, in 1871. The eruption started with the 
formation of a fissure in a level plain, continued for four 
years, and at that time the height of the cone was 1,900 feet. 
In other words the volcano built its own cone. 

There is no burning taking place in a volcano. There is 
hot material of various kinds expelled, but it is not burning. 
The cloud that overhangs the crater of Vesuvius is chiefly 
steam, not smoke, and the glow sometimes seen is a re- 
flection, from the cloud, of the hot lava in the crater. 

A volcano is an opening in the earth through which lava 
and other heated materials are ejected. This ejection is called 
an eruption. 

Some of these materials pile up about the opening and 
form a cone which may build up higher and higher until it 
forms a mountain. In the top of the cone there is a cup- 
shaped depression called the crater. 

201. Phenomena of eruptions. Eruptions may be classi- 
fied as explosive and quiet, although there are some ex- 

The quiet volcanoes discharge chiefly liquid lava with 
little gas and no solids. This lava is basaltic in composition; 
that is, it contains relatively little silica, melts easily, and 
cools into a very dark rock. The ease with which it melts may 
explain the absence of explosion. 

The explosive type of volcano has highly siliceous lavas 




with a high melting point. They usually contain much gas 
and erupt solid as well as liquid material. All these phe- 
nomena can be explained on the basis of the siliceous lava. 
Since it has a high melting point, it is not so fluid and the 
gases find it difficult to escape. This causes the pressure to 
increase until it bursts a way out and hurls lava into the 
air. With its high melting point, the lava cools at once to 
a solid while hurtling through the air, and falls to the earth 
as solid material. 

The cone of a quiet volcano is not steep, like the explosive 
type, because the molten lava continues to flow after the 
eruption, and tends to seek a level. The cone of the explosive 

Lava Cone 

Quiet Type 

Explosive Type" 

Cinder Cone 

FIG. 171. Two Types of Volcanoes 

volcano, on the other hand, is steep, because much of it is 
built up of solid irregular cinder and ash (Fig. 171). 

Mauna Loa, in Hawaii, is quiet, while Fujiyama, in Japan, 
was explosive (Fig. 172). Vesuvius does not quite belong to 
either of these types but is more explosive than quiet. 

An explosive eruption occurs as follows: A mighty ex- 
plosion blows off the top of the cone, shattering the lava, 
and sends steam, mingled with dust and ashes, high into 
the air, where it spreads out as a peculiar " cauliflower 
cloud." After that, the eruption may proceed quietly since 
the crater is now open, permitting lava and gases to flow 
out. The falling stones and ashes destroy vegetation and 
may even bury whole cities. The rising steam, cooled by 
expansion and by mingling with the cold upper air, is con- 



FIG. 172. Fujiyama, a Volcano Whose Eruptions Were Explosive 

J. S. Diller, U.S.G.S. 

FIG. 173. Vesuvius, near Naples, Italy 


densed and falls as rain accompanied by lightning. The rain 
brings down volcanic dust and ash, and all together form 
immense mud torrents, capable of burying cities, as, for 
example, ancient Herculaneum, in Italy. 

202. Products of volcanic eruptions. It has been estimated 
that some volcanoes discharge about 5 million gallons of 
water per day in the form of steam. 

Other gases include carbon dioxide, hydrogen, hydro- 
chloric acid, hydrogen sulphide, and sulphur dioxide. Mount 
Etna must have discharged enormous amounts of sulphurous 
gases, since we find the porous volcanic rock, scoria, for 
miles around the volcano, filled with pure sulphur. The 
sulphur is extracted by the natives and sold. 

The lava hurled into the air solidifies before it reaches 
the earth; but the expanding gas bubbles give it a spongy 
texture. This is called pumice. If the holes are larger, it is 
called scoria. Other volcanic material has various names, 
depending upon the size of particles: volcanic dust, ashes, 
lapilli, and bombs. 

203. Economic products. A British company purchased 
the cone of Vulcano, a small Mediterranean volcano, be- 
cause of the alum, boracic acid, and sulphur that could be 
obtained from it. Pumice and borax are other valuable 

Traprock, which is volcanic in origin, was used to pave 
the streets of Rome and the famous Appian Way, while 
from the Palisades came much of the material to pave the 
streets of New York City. 

Volcanic dust and ash, when consolidated, form tuff, 
a soft stone, easy to work in the quarry, but hardening in 
air and becoming a very durable building stone, much used 
in Naples and Rome. Some of the oldest sewers in Rome, 
built of tuff 2,500 years ago, are still in good condition. 

Volcanic dust and ash, exposed to rapid weathering, form 
a very fertile soil and it is probably for that reason that the 
farmers on the sides of Vesuvius return to their farms soon 



after an eruption, instead of being frightened away per- 

204. Distribution of volcanoes. Active volcanoes are 
found in the earthquake zones, that is, in the regions of 
young and growing mountains. There are two such belts. 
The better-marked belt surrounds the Pacific Ocean. The 
other belt is an irregular one, passing through the Hawaiian 
Islands and the Mediterranean Sea, and intersecting the 

FIG. 174. Distribution of Volcanoes 

first belt in the East Indies. Three fifths of all active vol- 
canoes are in the Pacific (Fig. 174). 

*205. Life history of a volcanic cone. A volcanic cone passes 
through a cycle of changes, beginning with a period of rapid growth. 
Cinder cones made of loose material are quickly eroded but cones 
that consist, at least in part, of lava, resist erosion and it may be a 
long time before the cone is completely destroyed. 

Fujiyama, the famous Japanese volcano, has a perfect cone 
that shows slight effects of erosion. 

The California cinder cone (Fig. 175), although a mass of loose 
material, shows little effect of erosion. It is a young cone, the 
result of recent volcanic activity, but there is no record of its 



Photograph by M. E. Dittmar, Redding, California 

FIG. 175. A Young Volcanic Cone 

J. 5. Diller, U.S.G.S. 

FIG. 176. Mount Shasta, California, a Volcanic Cone in Late Youth 

O. K. Gilbert, U.S.G.S. 

FIG. 177. San Francisco Mountains of Arizona, a Mature Volcanic Cone 



Mount Shasta in California (Fig. 176) is beginning to show the 
effects of stream and glacial erosion and the San Francisco Moun- 
tains of Arizona are in a more advanced stage (Fig. 177). After the 
cone has been completely eroded the volcanic neck usually remains 
(Fig. 178). Mount Royal, which gives its name to Montreal, is a 
volcanic neck. 

In some of the early periods of the earth's history (page 266) 
volcanoes were very much more numerous than today. In the 
Great Lakes Region there are widespread deposits of volcanic ash 
15 } 000 feet thick and there is evidence of thousands of volcanoes in 

C. B. Hunt, U.S.G.S. 

FIG. 178. A Volcanic Neck 

New England alone. These are detected by the remains of volcanic 
necks still found buried in the sedimentary strata of later periods. 

Crater Lake, Oregon (Fig. 108), illustrates another form 
assumed by some volcanoes in old age. The lake is more 
than five miles in diameter and the walls rise 2,200 feet 
above the water. Such large craters are called calderas, a 
Spanish word meaning cauldron. Calderas are formed by an 
explosion that blows off the entire top of the volcano. 

*206. Etna. The great cone of Etna was known to the Romans 
as the "Forge of Vulcan," the god of fire. The word volcano is 
derived from Vulcan. 

Etna is two miles high and 40 miles in diameter at its base. 
There are some 200 minor cones on its slopes. Its eruptions are 
preceded by earthquakes and loud explosions. Smoke, ashes, and 


cinders are discharged and finally lava flows from the new cone. 
The large proportion of lava accounts for the gentle slope of the 

*207. Vesuvius. The ancients knew Vesuvius as a mountain 
rather than a volcano. At the beginning of the Christian era, its 
crater, then about three miles in diameter, was covered with vege- 
tation. Its slopes were cultivated, towns were located at its base, 
and there was no record of previous activity. 

During the summer of A.D. 79 a series of earthquakes, of in- 
creasing severity, occurred, and a new and strange cloud formed 
above its summit. Explosion after explosion occurred within the 
mountain and the black cloud spread, shutting out the light of 
the sun. 

Tacitus gives us two letters from the younger Pliny, who was 
an eyewitness of this eruption. One of these letters describes the 
experiences of his uncle, the elder Pliny, who lost his life near the 
foot of Vesuvius during the eruption. It seems that his party 
sought shelter from the shower of cinders and stones in a villa 
which " shook from side to side" from frequent earthquakes. 
When the accumulation of stones and ash made it apparent that 
the villa would be buried, the party took to the fields "with 
pillows tied about their heads" to protect them from falling stones. 

The second letter relates the younger Pliny's experiences at 
Misenum, across the Bay of Naples from Vesuvius. He describes 
chariots standing on level ground without horses, which "kept 
running backward and forward" with each earthquake, even 
though blocked by great stones. "Besides this," he continues, "we 
saw the sea sucked down and, as it were, driven back again by the 
earthquake." Across the Bay above Vesuvius "was a dark and 
dreadful cloud, which was broken by zigzag and rapidly vibrating 
flashes of fire, and yawning, showed long shapes of flame. These 
were like lightnings, only of greater extent - . Soon the cloud 
began to descend over the earth and cover the sea - - ashes 
now fell, yet still in small amount. I looked back. A thick mist was 
close at our heels, which followed us, spreading over the country 
like an inundation . Hardly had we sat down, when night 
was upon us not such a night as when there is no moon, and 
clouds cover the sky, but such darkness as one finds in close-shut 
rooms . Little by little it grew light again. We did not think 


it the light of day, but proof that fire was coming nearer. It was 
indeed fire, but it stopped afar off; and again a rain of ashes, 
abundant and heavy; and again we rose and shook them off, else 
we had been covered, and even crushed by the weight . 
Soon the real daylight appeared ; the sun shone out, of a lurid hue, 
to be sure, as in an eclipse. The whole world which met our fright- 
ened eyes was transformed. It was covered with ashes, white as 

Bulwer-Lytton, who lived near Vesuvius for many years, wrote 
the delightful novel, The Last Days of Pompeii, in which he de- 
scribed this eruption of Vesuvius. 

No lava flow accompanied this eruption, but the enormous 
quantity of ash buried Pompeii and, mixed with rain, formed a mud 
stream which overwhelmed Herculaneum. 

Pompeii has since been restored and the ruins furnish us with an 
excellent idea of the life of the times, since the ash, by covering it, 
has protected and preserved buildings, streets, paintings, mosaics, 
tools, and even some of the chemicals used by an apothecary. 

There have been frequent eruptions of Vesuvius since that one; 
those of 1631 and 1906 were especially destructive. In these later 
eruptions, the explosion has been followed by lava flows. 

Unlike Stromboli, which is continuously active but mild, 
Vesuvius has long periods of rest during which the volcano is said 
to be dormant or sleeping. 

*208. Mount Pelee. An eruption of this volcano on May 8, 
1902, destroyed the city of St. Pierre on the island of Martinique 
in the West Indies. Previous to this date it had been dormant for 
50 years, but for days before the eruption it had shown signs of 
activity. Great columns of steam and ash were ejected, boiling 
mud flowed from the sides of the volcano, and repeated explosions 
occurred in its interior. Lightning flashed from the ascending 
cloud, and the frequent earthquakes broke all cables leading to 
the island. 

On the morning of May 8 a dull red reflection was seen on the 
cloud that covered the mountain summit, This became brighter 
and brighter, and soon red hot stones were ejected from the crater 
and bowled down the mountain side, giving off glowing sparks. 
Suddenly a hot blast of gases shot from the crater, and two minutes 
later engulfed the city of St. Pierre, five miles distant, in an 



atmosphere that was fatal to all who breathed it. Thirty thousand 
persons lost their lives. It wiped out all vegetation and all living 
creatures in its path. Buildings of the city and ships in the harbor 
instantly burst into flames. 

There was no lava flow, but the neck was forced upward by the 
pressure from below until it stood 1,200 feet above the crater a 

American Museum of Natural History 

FIG. 179. The Spine of Mount Pelee, a Volcanic Neck 

year after the eruption. A series of explosions finally caused this 
neck to crumble into blocks of stone (Fig. 179). 

*209. Krakatoa. In 1883 the most violent explosive eruption 
of history occurred on the island of Krakatoa in the East Indies. 


The island was 5 miles long and 3 miles wide, with an altitude of 
2,623 feet at its highest point. Nearly the whole of the lower part 
of the island and half of the peak were blown away. Dust was 
thrown into the air to a height of about 20 miles and was carried 
around the earth several times, causing beautiful sunrise and sunset 
effects for many months. The concussion of the explosion broke 
windows in Batavia, 100 miles away, and the report of the ex- 
plosion was heard 2,267 miles away. A mighty seismic wave flooded 
the surrounding coasts, stranding ocean steamers, causing great 
loss of property, and drowning more than 36,000 people. For many 

H. T. Stearns, U.S.G.S. 

FIG. 180. A Lava Flow 

weeks navigation was impeded by floating pumice that covered 
the surface of the sea. 

*210. Hawaiian Islands. Hawaii is one of a group of islands 
with many volcanoes which in the main owe their existence to 
eruptions at the bottom of the ocean. This island is 80 miles long 
arid rises 30,000 feet above the ocean floor. There are four craters 
on the island, of which Mauna Loa is the highest. The eruptions 
of the volcanoes in the Hawaiian Islands are in sharp contrast with 
that of the island of Krakatoa. In these oozing eruptions there 
are no explosions, no showers of dust or ash; no great volume of 
steam is ejected, and earthquakes are rare. The lava flows (Fig. 
180) sometimes continue for months, whereas eruptions of the 
explosive type last but a few days. Before an eruption the lava 
rises quietly in the crater until the great pressure fissures the side 


of the mountain, when a river of molten rock flows to the sea. 
The slopes of the volcano are very gentle, but this must not be 
understood to mean that the cone is small. Mauna Loa is many 
times as large as Vesuvius, and its crater is a typical caldera, 
nearly three miles long, two miles wide, and 1,000 feet deep. 
The Icelandic volcanoes are of this type. 

211. Active volcanoes of North America. Active vol- 
canoes are numerous in Central America and Mexico, and 
some of the Alaskan volcanoes have recently been in erup- 

Mount Katmai. In June, 1912, an eruption of Mount 
Katmai, in Alaska, occurred, that was remarkable for the 
large amount of ash ejected. An area 15 miles wide on the 
south and west sides of the volcano was buried under 50 
feet of ash, houses were damaged 100 miles away, the ash 
was noticeable more than 200 miles away, and total darkness 
prevailed for more than two days. The sound of the explo- 
sions was heard 750 miles down the coast. Fortunately the 
region around this volcano is sparsely inhabited. 

Lassen Peak. In May, 1914, the first volcanic eruption 
in the United States proper to be described by white men 
occurred at Lassen Peak, California. Lassen Peak is in the 
Sacramento Valley, about 210 miles northeast of San Fran- 

The eruption began with geyser like jets or clouds of 
steam which increased steadily until great bursts of smoke, 
rising 2,000 feet, formed a " cauliflower cloud." This was 
followed by pillars of fire visible 100 miles down the Sacra- 
mento Valley. 

The activity of Lassen Peak has continued intermittently 
to the present time. Sometimes the material ejected consists 
chiefly of gases ; again cinders and bombs are hurled high in 
the air; and at other times lava breaks through the sides of 
the cone and runs quietly down the slope. 

The region about the peak is now a national park, con- 
taining many other evidences of the recent volcanic activity 


Diller, U.S.G.S. 

FIG. 181. Lassen Peak, California, an Active Volcano 

of the region, such as a lava flow estimated to have been 
ejected 150 years ago and the large cinder cone 800 feet 
high, believed to have been formed about the year 1700. 

212. Recently extinct volcanoes of the United States. 
Mount Hood. This mountain on the crest of the Cascade 
Range, in Oregon, is noted for its graceful outlines, and for 
the fumaroles* and steaming rifts which still emit sulphurous 
fumes and indicate comparatively recent activity, although 
there has been no eruption within the memory of man. 

Mount Rainier. This stately cone rises from nearly sea 
level to an altitude of 14,500 feet, and so appears much 
higher than most of those that reach a greater altitude. It 
has a bowl-shaped crater, below which, on the sides of the 
mountain, the rims of former craters may be seen. Jets of 
steam and gas still issue from small holes or fumaroles in 
its snow-clad summit, showing that its heat has not en- 
tirely disappeared. 

San Francisco Mountain. This mountain in Arizona is 
much eroded, and no signs of a crater remain; but it is sur- 
rounded by lava flows and beds of cinders, and several 
hundred cinder cones, formed by volcanic eruptions, are 
found in the immediate vicinity. Some of these cones were 

* Fumaroles are openings that emit only hot gases. 


formed so recently that erosion has not modified the original 
form of the cone. 

Mount Taylor. On one of the large mesas or table lands 
of western New Mexico, Mount Taylor rises to an altitude 
of 11,000 feet. The mountain is almost entirely composed of 
lava, and the mesa is covered by a cap of lava. This cone is 
also much eroded. In the lowland about the mesa are many 
volcanic necks, each one a mass of lava which cooled in the 
throat of a volcano that has disappeared. 

Mount Shasta. This extinct volcano of northern Cali- 
fornia is in some respects like Etna. It towers 11,000 feet 
above a base 17 miles in diameter, is snow-clad even in 
summer, and its eruptions were explosive, followed by great 
lava flows. There are two great craters; the younger is near 
the top of one side of the older cone. Some 20 smaller cones 
are found near the base of the mountain, and from one of 
these the lava flow may be followed more than 50 miles. 
The cone is much dissected by glaciers and streams, but it 
is still in its youth (Fig. 176). 

213. Other indications of volcanic activity. The Columbia 
River lava plateau covers a large part of Washington, 
Oregon, and Idaho with successive layers of lava, which in 
places reach a total thickness of 5,000 feet. The section of 
this plateau suggests stratified rock, but the layers repre- 
sent distinct flows of lava, which are sometimes separated 
from the next by layers of soil where the roots and trunks 
of large trees are preserved. This proves that a long interval 
of time elapsed between the flows. Because of the absence of 
cones in this region, it is thought that the lava came through 
fissures. The surface is covered with residual soil of great 
fertility. This plateau is cut by many deep canyons in 
which the structure of the plateau is shown. 

*In Canada we find two million square miles covered by 
granitic gneiss. This represents a surface flow of granite, subse- 
quently metamorphosed into gneiss. The earth's crust must have 
cracked wide open and the lava must have poured out on the 



surface. And we have evidence that the earth's crust again and 
again was broken by the upwelling of enormous masses of molten 
rock. This was in the early periods of the earth's history, the 
Archeozoic Era, when we might expect that the crust was still 
weak. This happened less and less as time went on, but occasionally 
since then, there have been surface flows through fissures like that 
of the Columbia River. 

214. Igneous intrusions. Besides the masses of lava 
that were evidently poured out on the surface of the land, 
there are many others that indicate former volcanic activity, 

-:.V'S..:.*.-\ : i'^ 

FIG. 182. Igneous Intrusions 

although they did not reach the surface of the earth. These 
are called igneous intrusions. 

An igneous intrusion that cuts across strata is called a 
dike. Around a volcanic neck there is usually a system of 
dikes radially arranged. It is by this arrangement that we 
can recognize an old volcanic neck. An igneous intrusion, 
parallel to the pre-existing rocks, is called a sill. If it bows 
up the strata (Fig. 182), it is a laccolith or lake of rock. 

A very large deep-seated igneous intrusion is called a 
batholith. It differs from a laccolith in that the latter rests 
on a definite floor whereas the batholith has no foundation, 
extending presumably down into the earth. 

The Palisades of the Hudson is a sill, several hundred 
feet thick and 30 miles long, intruded between layers of 
sedimentary rocks. The Watchung Mountains of New 
Jersey are surface flows. 


Laccoliths form dome mountains of which the Henry 
Mountains of Utah are a good example. 

There is a great batholith in the Coast Range which is 
exposed, by erosion of the surface covering, for a length of 
1,100 miles and a width of 100 miles. Batholiths, we believe, 
make the cores of mountain ranges. 

*The textures of these igneous rocks show interesting differ- 
ences. A surface flow is made of very tiny crystals since it cooled 
rapidly. A dike and a sill will have larger crystals since they cooled 
between two layers of rock more slowly. If the intrusion is very 
thick, the crystals near the cooling surfaces will be smaller than 
those at the center of the intrusion. Batholiths have the largest 
crystals, because they are the most deep-seated of all intrusions. 

215. Causes of volcanic action. It used to be thought 
that the interior of the earth was molten rock, but we now 
know that it is a solid, although it is very hot hot enough 
to be a liquid if the pressure were not so great. The passage 
of earthquake waves through the interior convinces us that 
it is more rigid than steel. 

The source of the heat is not definitely established. Some 
believe the earth still has its original heat. But it has been 
shown that at the present rate of loss the earth would long 
ago have cooled out. 

Some think the contraction of the earth causes the heat; 
and surely great heat must be generated during movements 
of the earth's crust, when mountains are being formed. 

It is known that granites, in particular, contain radio- 
active minerals which by their atomic disintegration liberate 
great amounts of heat; and the granites make up the crust 
of the earth underlying the continents. 

Active volcanoes are in the same region as young growing 
mountains, and the two are in some way connected with 
each other and with earthquakes. It may be that the cooling 
of the earth causes a shrinkage which results in a collapse of 
the crust. This squeezes the rock underneath and causes it 
to move. Breaking of the crust causes earthquakes and 


squeezing of the interior would cause an eruption wherever 
liquid was formed. 

Kroneis and Crumbein in their very interesting book Down to 
Earth have a very plausible explanation of the cause of volcanic 
eruptions. The great heat produced by radioactive disintegration, 
near the surface, may melt the rock locally in spite of the pressure. 
Hence this liquid rock, exceedingly hot, can now melt some of the 
rock it passes through, increasing its volume until there is actually 
a reservoir of liquid. If the liquid reaches the zone of fractured 
rock at the surface, it will break through in an eruption; if not, we 
have igneous intrusions. 

Completion Summary 

A volcano does not - - mountain. It builds 

There are - types of volcano, - and 

The - type has a - - cone, due to - . 

*An explosive eruption is caused by - . 

Volcanoes discharge much - , - , and ; 

and the - - is often of considerable value. 

There are - - volcanic belts: the chief one , 

while the other . 

*In the early periods of the earth's history, volcanoes . 

is an explosive volcano, as is proved by its 

cone, while is quiet. is an active volcano in 

the United States - - and - - are recently extinct. 

The Columbia River Plateau was formed by . 

*A great area of Canada is covered by granite gneiss. This is 
believed to - . 

Igneous intrusions volcanic surface. A dike 

; a sill ; a laccolith - ; a batholith . 

*The texture of igneous rocks depends . Fine-grained 

while coarse-grained rocks . Most batholiths, 

therefore, grained rocks. 


The earth's interior is - - molten, but it is very hot. 
There are several theories to explain the - - heat. The 

collapse of the crust due to - - may be the cause . 

The disintegration of radioactive material - . 


1. Criticize the statement, " A volcano is a burning mountain, 
belching forth fire, smoke, and lava." 

2. Why does a volcano sometimes seem to be burning? 

3. What two kinds of volcanoes are there? 

4. Explain why one has a steep cone while the other has not. 

5. Why is one type of volcano explosive? 

6. Name a quiet volcano. 

7. Name an explosive volcano. 

8. Describe an explosive eruption. 

9. Name three gases expelled during volcanic eruptions. 

10. How do we know that Mt. Etna has discharged enormous 
quantities of sulphurous gases? 

11. Account for the spongy texture of pumice and scoria. 

12. Name three valuable substances obtained from volcanoes. 

13. Why do people locate farms on the sides of volcanoes? 

14. Locate the two belts of volcanoes. 

15. Where are most of the active volcanoes? 

16. Name a young volcano. 

17. Why are volcanic cones easily eroded? 

18. What part of the volcano usually remains after erosion of 
the cone? 

19. What type of volcano is Etna? 

20. What is a dormant volcano? Mention one. 

21. Mention two active volcanoes in North America. 

22. Name an active volcano in the United States. Where is it? 

23. Name three recently extinct volcanoes in the United States. 
Where are they? 

24. Why does the Columbia River Plateau seem to be built of 
sedimentary rocks? 

25. In what stage of the cycle of erosion is the Columbia River 
Plateau? What is the evidence? 

26. What is a dike? sill? laccolith? batholith? 


27. What is thought to be the cause of volcanic activity? Give 
one explanation. 

28. How can people near a volcano tell that an eruption is 
probably about to take place? 

if Optional Exercises 

29. Using the theory of isostasy, explain the location of the two 
belts of volcanoes. 

30. At what time in geological history were volcanoes very 
numerous? Discuss, showing that it is in agreement with the 
theories of the earth's origin. 

31. Since the volcanoes of past geological periods are com- 
pletely extinct, how do we know of their existence? 

32. What is a caldera? Name one. 

33. In what way has the burial of Pompeii helped us to under- 
stand the life of the ancient Romans? 

34. Describe the eruption of Krakatoa. 

35. Describe an eruption of Mauna Loa. 

36. Cite evidence of volcanic activity in the early history of 
the earth, showing that it has gradually diminished in severity. 

37. Explain the relation between the texture of igneous rocks 
and rate of cooling, mentioning each kind of igneous intrusion. 

38. Explain the theory of volcanism cited from Down to Earth. 



*216. What took place on this earth in the very beginning we 
have no way of telling, because the igneous rocks, which were 
formed when the surface cooled, have left no records which we 
can decipher. We have theories of the earth's origin and of the 
changes that carried it through a gaseous and then a liquid con- 
dition, but not a shred of direct evidence. 

*217. Fossils. Our history begins with the falling of rain, be- 
cause at that time the process of erosion started, sediments were 
carried down to the seas, and some of the remains of plants and 
animals were buried and thereby preserved. We call such speci- 
mens fossils (Latin fossilis, an object dug up). Casts or moulds of 
shells, footprints, or anything that indicates the former existence 
of organisms, we call fossils. 

We believe life started in the water, but if there was any on 
the land, we should not expect to find the traces, because that 
early life was soft-bodied and there was no way it could have been 
preserved on land, exposed to the destructive action of the weather. 

We believe life started in the shallow marine waters on the 
continental shelf, where we find it in profusion today; not too near 
the shore, where wave action keeps churning up the bottom, but 
in the quiet, warm waters whose depth is less than 100 fathoms. 
Here the marine plants grow undisturbed, since sufficient sunlight 
penetrates the water to that depth. Animals, feeding on the plants, 
lived and died in the same zone. The sediments brought down by 
the streams (sand, silt, and mud) were deposited on the continental 
shelf rather near the shore, and gradually covered the remains 
of the organisms which strewed the bottom. If this burial occurred 
soon enough, the remains were protected from further destructive 
action and thereby preserved. 

* This entire chapter is optional. 


Farther out from shore, where sediments do not reach and the 
water is clear, we find abundant vegetation, since more sunlight 
can penetrate the clear water. Here we find very many animals, 
too. The bottom becomes covered with the remains of these plants 
and animals and with the powdery calcium carbonate precipitated 
by calcareous algae, which buries the entire mass. 

In process of time, as we have seen, these sediments (sand, 
mud, and calcium carbonate) are converted into sandstone, shale, 
and limestone, which contain the fossilized remains of organisms 
that existed there in the shallow marine waters. 

At first, the deposits laid down in the waters near the shore are 
very coarse, since the streams flowing down from the high places 
have considerable velocity. As the highlands are worn down, the 
stream velocity decreases and the sediments are finer, until, at 
peneplanation, there are no streams and the waters become clear. 
Hence we can tell by an examination of the sedimentary rocks of 
a past age just where the highlands were at that time, since the 
coarsest sediments, the conglomerates, must have been deposited 
right at their bases. On top of the conglomerates we shall expect 
to find sandstones, then shales, formed from mud, and finally 
limestone covering all, since the highlands were by that time all 
planed down and the waters were clear. 

*218. How we measure geologic time. As the highlands are 
eroded and the sediments piled along the continental shelf, the 
pressure on the underlying crust of the earth causes an isostatic 
adjustment, which slowly pushes up the continent once more; 
erosion begins again with subsequent deposition of conglomerate, 
sandstone, shale, and limestone. We divide geologic history into 
periods, each of which starts with an uplift and ends with penepla- 
nation; or, as we read it in the rocks, a period starts normally with 
a conglomerate and ends with a limestone. 

Our history tells us only what happened on the continental shelf 
and only by inference do we get any history of the continents; 
for example, when a shale is followed by a conglomerate instead of 
a limestone, we must infer that the near-by land was rather sud- 
denly elevated. 

Each time when the sediments of a period are uplifted and 
eroded, we have an erosion interval during which no history is 
written since no sediments are deposited. In other words we have 


gaps in the story. We can identify the erosion interval in the rocks 
by the unconformity between two sediments. Sediments are laid 
down smoothly and horizontally but when they are uplifted and 
erosion sets in they become irregular, and this irregular line be- 
tween two layers we call an unconformity (Fig. 183). 

To figure, in years, just how long ago any geologic event took 
place, we have several methods. The accuracy of one of these 
criteria is beyond dispute. Deposits of clay, laid down in a lake, 
frequently show alternating layers of fine and coarse material; the 
coarser layer was brought in during the spring flow, while the finer 
clays settled during the winter, when the waters were quiet. 

FIG. 183. An Unconformity 

Apparently, the tilted and folded strata below were first eroded to a pene- 
plain and then submerged. The upper, horizontal strata were deposited and 
finally the entire district was uplifted and eroded as we find it today. 

Evidently two varves, as they are called, represent one year. We 
know, for example, that the last glacial period ended 25,000 years 
ago, because there are varved clays containing 25,000 double 
layers from the southern limit of the glacier to its present position 
(Fig. 80); hence it took 25,000 years for the glacier to retreat 
from New England to its present position. 

Another criterion for measuring geologic time is the rate of 
denudation and deposition of sedimentary rocks. By measuring 
the deposits brought down by streams, we find a rate of about one 
foot in 5,000 to 10,000 years over the land. On that basis 50,000 
feet of sediment required, roughly, 500 million years to deposit. 
But this, method is subject to much uncertainty. 

The latest method of determining the lapse of time depends upon 



the radioactive disintegration of uranium and thorium minerals. 

Chemists have found that the time taken for this change is fixed 

and unalterable, being unaffected by heat, pressure, or any other 

force. Hence we may use it as a clock. 

On this basis the oldest rocks so far tested are 1,850,000,000 

years old, and it seems probable that the earth is at least two 

billion years old. 

*219. How we read the pages of geologic history. It is apparent, 

from what we have just seen, that nearly all the evidence we have 

of the events of the past is to be found in sedimentary rocks. The 

succession of events will be seen 
in the relative position of the 
strata, the youngest lying on top 
unless there has been a disturb- 
ance that has caused the normal 
succession to be overturned. The 
age of an igneous intrusion must 
be related to the sedimentary 
rocks which it pierces. It is evi- 
dent that an intrusion must be 
younger than rocks it cuts across 
(Fig. 184). 

Each period is separated from 
the rest by an unconformity 
which marks a gap or erosion 
interval, caused by an upheaval 
which re-elevated the land and 
subjected it to weathering. Such 

FIG. 184. Igneous Intrusions, 
Almost Vertical 

They are younger than the strata 
they cut across. 

a convulsion of nature, changing, as it must have changed, the 
entire environment, brought about great changes in the life of 
the tune. Hence the fossils of two succeeding periods will show 
marked differences. 

Within each period, changes in the elevation of the near-by 
lands are marked by the texture of the rock; fine-grained rocks 
show low elevation and coarse-grained indicate high elevation. At 
the same time the organisms are changing and samples of many of 
these will be left in the sediments. Each page of the history, repre- 
sented by a single layer of rock or a closely related group of layers 
following one another without unconformity, is called a formation. 


Each formation is distinguished by the presence of the same or 
closely related fossils. 

The nature of the fossils in a formation is often an index of the 
climate. For example, we know that coral lives in warm, shallow, 
marine waters; and we must assume that the corals of the past 
did likewise. Therefore, if a coral formation is found in the 
Arctic regions, we infer that the climate there was warm at 
that time. The chapters of geologic history already worked out 
are represented by the geologic table which is shown on pages 
262 to 265. 

It will be noticed that the great divisions or eras of geologic 
history are separated by revolutions. These are widespread crust al 
movements which so changed conditions on earth that the life 
before and after was markedly different. 

Each era is divided into periods, during each of which the sea 
inundated the land, and at the end of each period the land again 
emerged through an uplift, which was not so extensive or long 
continued as a revolution. 

During each period the streams deposited on the submerged 
land a series of sedimentary rocks. Each stratum is called a forma- 
tion when it contains the same or closely related fossils. 

If we compare the rocks to a book, then the eras are chapters, 
the periods are paragraphs, and the formations are sentences. 

*220. The Archeozoic Era. The earliest period in which we find 
any evidence of life at all we call the Archeozoic Era (which means 
the time of very ancient life). It is represented by a few layers of 
sedimentary rocks, but most of the rocks of the period are highly 
metamorphosed and of igneous origin. This agrees with our expec- 
tations, since we believe that life started as one-celled plants and 
animals without any hard parts like shells. Soft-bodied creatures 
would hardly be preserved as fossils after such an enormous lapse 
of time. 

However, we do find great deposits of graphite in Archeozoic 
rocks. Now graphite is carbon, formed from coal or other plant 
material by metamorphosis. There is said to be more carbon in the 
Archeozoic rocks than in those of the Pennsylvanian, which are our 
present source of coal. This would seem to indicate the existence, 
in the Archeozoic Era, of considerable plant life. 

It is claimed that fossil algae and even sponges have been found 













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in rocks of this era, but these claims are not accepted by most 

What sort of picture, then, can we draw of this Archeozoic Era, 
which endured for about 500 million years? In Canada we find two 
million square miles covered by gneiss. This apparently represents 
a surface flow of igneous rock, subsequently metamorphosed into 
gneiss. The earth's crust must have cracked wide open and the 
molten magma of the interior must have been poured out on the 
surface. We have evidence that the crust again and again was 
broken by the upwelling of enormous masses of molten rock. 

Then came long periods of quiet during which the forces of 
erosion were at play, laying down great thicknesses of sediment. 
These sediments are folded, crushed, and injected with igneous 
intrusions, all of which speak of violent disturbances, terrific earth- 
quakes, far surpassing anything man has ever experienced 
mountain formation on a grand scale. 

And when such a period of upheaval was over, quiet again 
reigned over all the earth, quiet unbroken by cry or screech. For 
there were no birds, no insects, no trees or bushes, only bare rocks, 
granites. The scene was quite desolate only here and there a 
little mass of algae clinging to the rocks in the warm waters 
(Fig. 185). 

The close of the Archeozoic Era was in keeping with the rest. 
It was ended by the Laurentian-Algoman Revolution, a period of 
uplift, during which great mountains were raised in Canada, the 
Great Lakes region, and the Adirondacks. This was followed by a 
long period of rest, during which these high mountains were eroded 
down to peneplains. 

*221. The Proterozoic Era. The second great division of history 
is called the Proterozoic Era, or time of " earlier life," referring to 
the fact that geologists at first failed to find any fossils in these 
rocks, older than the Cambrian. Both the Archeozoic and Protero- 
zoic are classed together as pre-Cambrian. 

At the opening of the Proterozoic we find the continent of North 
America marked out as a very definite land mass, considerably 
larger than it is today. Greenland was joined to Scandinavia, and 
the West Indies were attached to Florida and Yucatan (Fig. 186). 

Volcanoes must have been very numerous and very active 
during this time, for we find widespread deposits of volcanic ash, in 






places 15,000 feet thick, but the earth's crust seems to have become 

firmer, for we do not find it cracking wide open as often as it did 

in the Archeozoic. 

The great igneous intrusions of this era brought up with them 

the richest ore deposits ever produced in history. These include 

the iron and copper ores of the 
Lake Superior region, and the 
cobalt, nickel, silver, and gold 
ores of Canada. 

The climate was mild during 
most of the time, but now and 
then it became cold enough to 
allow glaciers to move over the 
land, since we find glacial till 
including faceted boulders. 

The life was still confined to 
the seas simple, soft-bodied 
creatures, for the most part 
algae, sponges, and worms. But 
among them we find siliceous 
sponges and radiolaria, which 
have learned to extract silica 


FIG. 186. North America in the Prot- 
erozoic Era. (After Schuchert) 

The black outline shows the pres- 
ent continent. 

from the waters and to build 
hard structures to protect or 
support their bodies. That there 
was much more life than that, however, is evidenced by the great 
thicknesses of black slate, saturated with carbon formed from 
marine organisms. 

The original atmosphere contained possibly as high as 40% of 
carbon dioxide; but, by this time, much of it had been absorbed 
by the plants which retained the carbon and set free the oxygen. 
We find, here and there, strata of red deposits which we know are 
formed by oxidation of iron compounds; these show that the air 
already contained considerable oxygen. 

The Proterozoic Era closes with the elevation of high mountains 
in the Great Lakes region (Killarney Mountains) as well as in 
Arizona (Grand Canyon) ; hence this period is called the Killarney- 
Grand Canyon Revolution. 

Then follows a period of erosion which we believe was very long, 



because during this time the marine life developed so considerably 
that by the opening of the next period, the Cambrian, we find every 
class of animals except vertebrates. Nowhere have we found any 
direct fossil evidence of this development ; hence it has been called 
the Lipalian (lost) Interval. The Proterozoic Era, including the 
Lipalian, lasted about 500 million years. 

*222. The Paleozoic Era. The curtain now rises on a new scene. 
Instead of the very simple, soft-bodied creatures of the Archeozoic 
and Proterozoic, the seas are teeming with life: thousands of species 
of marine invertebrates (many 
of them in existence today) and 
hordes of each species jostle 
each other in their struggle for 

It was at first thought that 
these fossils represented the 
oldest life of the earth and it 
was consequently called paleo- 
zoic, which means " ancient 
life." The first period of the 
Paleozoic Era is called the Cam- 

During the Cambrian, much 
of the continent was submerged 
and the land was low and un- 
interesting. The climate was 
mild, even warm, but no plants 

FIG. 187. North America in the Cam- 
brian Period. (After Schuchert) 

clung to the bare rocks and no animals were to be seen on the land 
(Fig. 187). 

One of the characteristic animals of the Cambrian, now extinct, 
was the trilobite. Trilobites were numerous and variegated. They 
had an external bony covering like that of the lobster and horse- 
shoe crab. Some of the early ones seem to have been blind; others 
had many eyes that could not be moved, but they had many sets 
of eyes, one for each direction. One species was able to look in 
ninety-eight different directions at the same time. A few of the 
Cambrian animals are shown in Fig. 188. 

In the Ordovician period, following the Cambrian, the greatest 
inundation of history occurred, flooding about 60% of the conti- 



FIG. 188. Cambrian Fossils 
The upper ones are trilobites. 

nent. Most of the rocks of this period are limestones, which are laid 
down in shallow marine waters; and this is evidence of the lowness 
of the land, for if there were highlands near by, the streams must 
have carried sand and mud into the seas to form, ultimately, sand- 


stones and shales. In this period invertebrates are still dominant, 
but fresh-water fishes make their appearance. These are the first 
vertebrates. Trilobites are still numerous but their enemy, the 
cephalopod, an active, predaceous creature, has begun to develop 
rapidly (Fig. 189). 

The Ordovician was brought to a close by an upheaval which 
left the Taconic and Green Mountains of New England; and it was 
followed by the Silurian, a period of widespread submergence in 
the central part of the continent. Now the first land plants make 
their appearance and the first air-breathing animals, the scorpions 
(Fig. 190). 

Corals, which first came in the Ordovician, have developed so 
that they form extensive reefs across entire states. 

The Devonian period presents again a new scene. The land is 
clothed with vegetation, forests of tree-ferns, and several species 
of animals are to be seen (Fig. 191). In the sea there is a great 
increase in the number and variety of fishes (Fig. 192). 

" These, the first strong-jawed tyrants of the sea, came all at 
once, like the rush of the old Norman pirates into the peaceful 
seas of Great Britain. They made a lively time among the sluggish 
beings of that olden sea. Creatures that were able to meet feebler 
enemies were swept away or compelled to undergo great changes, 
and all the life of the oceans seems to have had a spur given it by 
these quicker-formed and quicker-willed animals." 

The Acadian disturbance, which raised mountains in New 
England and near-by Canada, ended the Devonian period. 

During most of the two following periods, the Mississippian 
and Pennsylvanian, sometimes called the Carboniferous or Age of 
Coal, the land was low-lying, warm, and moist. There was great 
abundance of land plants, great swampy forests of scale-trees and 
ferns. Insects were numerous and large. Eight hundred kinds of 
cockroaches (some of them four inches long) have been found in 
the rocks of this age; the Pennsylvanian is sometimes called the 
Age of Cockroaches. The largest known dragonfly, twenty-nine 
inches across the wings, is found in these strata. 

Amphibians were numerous, some of them as large as alligators; 
and, in the Pennsylvanian, the first reptiles roamed the forests. We 
are particularly interested in these because they were the first land 











The forests of this period (Fig. 193) were still quite different 
from ours today. There were none of the modern trees with the 
familiar leaves: the oak, maple, beech, and ash. There were still 
no birds, no mammals, no flowers, no fruits. No sounds broke the 
somber stillness of the forest, save those of the wind, the thunder, 
and the babbling brook. 

In the following period, the Permian, the low-lying lands of the 
coal-making period began to rise. This brought about a cold climate 
and widespread aridity. These rather rapid changes in environment 
seem to have been critical for life, and great changes resulted. The 
draining of the swamps obliged those plants that had been grow- 
ing with their feet in the water to adapt themselves to dry land. 
Most of the trees of the coal-making period disappeared; their 
place was taken by seed plants, better adapted to the changing 
climate. The drying up of the swamps was a challenge to the 
amphibians, which apparently developed into reptiles, animals 
that live entirely on land. Insects became much smaller and de- 
veloped a pupal stage, "to tide them over times of climatic stress." 
But most of the marine invertebrates became extinct, particularly 
the trilobites. 

The drying up of arms of the sea, trapped by the rising land, 
gave us great deposits of salt and gypsum. In fact, most of the 
world's salt deposits are Permian in age. 

The Permian was the last period of the Paleozoic Era, which 
was ended by the great Appalachian Revolution, a period of 
intense mountain making the world over. In the United States, 
the Appalachian Mountains, five miles high, were formed; and 
the seas were driven off the lands, never to return again. 

*223. The Mesozoic Era corresponds to the Middle Ages in 
world history. It is a transition period between the life of ancient 
times and that of modern times. The Appalachian Revolution 
brought with it intensely important physiographic changes. Low- 
lying lands were raised up into high mountainous areas with cor- 
respondingly colder climates, rivers were rejuvenated, and lakes 
were drained. All of these and many more catastrophes wiped out 
entire species of plants and animals and introduced new environ- 
ments, which caused the surviving organisms to undergo vital 

During the Triassic period, which opened the Mesozoic Era, the 




continent of North America looked very much as it does at present, 
except that the Atlantic and Gulf coastal plains were missing. 
The prevalence of aridity is shown by the red color of the sedi- 
mentary rocks, so strikingly shown in the Painted Desert of 
Arizona and in Zion Park, Utah. Salt and gypsum deposits in these 
rocks add further evidence of desert conditions, in marked con- 
trast to the low-lying, moist lands of much of the Paleozoic Era. 

A moderate crustal movement, the Palisade Disturbance, 
brought the Triassic to an end. The name is derived from the 
Palisades on the western bank of the Hudson River, where they 
form a characteristic feature of the landscape. The Palisades owe 
their origin to an intrusion of igneous rock into the sedimentary 
Triassic beds during the Palisade Disturbance. Since that time the 
overlying softer sedimentary rocks have been worn off, leaving 
the resistant igneous rock on the surface. The end of the Triassic 
found the entire continent of North America dry land. 

The Jurassic period, following the Triassic, resembled it in 
many respects. The land was high and dry and the Appalachians 
were eroded down to a peneplain. At the end of the period the 
Nevadian Disturbance lifted up the Sierra Nevada and the coast 
ranges, giving to western United States something of its present 
grandeur. Igneous masses injected into the California rocks dur- 
ing this disturbance brought with them the gold deposits which 
gave us the "Gold Rush of '49." 

The life of the Mesozoic Era is quite different from that of the 
Paleozoic; and while it resembles, or rather foreshadows, the life 
we know, it is still, in the Jurassic, markedly different. 

The forests are made up of cycads and conifers, and while the 
latter begin to resemble modern evergreen trees, the cycads give 
to the scene a fantastic appearance (Fig. 194). There are no trees 
with leaves as we know them: the oak, the maple, the beech, and 
the ash. The animal world is dominated by monstrous reptiles 
not the kinds we know today, but huge, slow-moving creatures. 
"They filled all the rdles now taken by birds and mammals; they 
covered the land with gigantic herbivorous and carnivorous forms, 
they swarmed in the sea, and as literal dragons, they dominated 
the air." 

Brontosaurus attained a length of 65 feet, while diplodocus 
reached 80 feet. Winged reptiles or pterodactyls gave life to the 




air for the first time. Some of them had a 4-foot wing spread. 
Dinosaurs, known as ichthyosaurs and plesiosaurs, dominated the 

The insect world began to take on a modern appearance. 
Dragonflies, beetles, grasshoppers, cockroaches, cicadas, moths, 
flies, ants, and others made the air hum. 

But what is of greater importance to us is the appearance of a 
few mammals. They are small and inconspicuous, none of them 
larger than a modern dog; but already it is evident that they will 
inherit the earth, because of their adaptation to changing environ- 
ments. It is thought by some that the necessity to keep its blood 
warm forced the mammal to move about, and hence arose the 
necessity for adaptation which brought about rapid development 
of new characters suited to the changed environment. This in- 
crease in complexity gave us the highly developed mammals of 
modern times. 

After the Jurassic came the Cretaceous, the chalk period, during 
which there occurred a widespread inundation of the Rocky Moun- 
tain area of North America; but by the close of the Cretaceous 
the land of this region was rising rapidly again, and the continent 
emerged as it is at present. During this submergence much of 
the western coal was laid down and the great mid-continent oil 
field (in Mexico, Texas, and Oklahoma) was formed. 

The pterodactyls or toothed birds developed into the feathered 
variety during this period. Archeopteryx, the first bird, was more 
reptile than bird. It was about the size of a crow (Fig. 195). 

Modern insects, in increasing number, appeared during the 
Cretaceous, and mammals continued their development. Oysters, 
Ibbsters, and other marine forms gave to the seas a slight tinge of 
the modern. 

On the land, magnolia, fig, and poplar trees suddenly appeared 
and made the forests look distinctly modern. The development of 
this group of plants, the angiosperms (covered seeds), is of the 
greatest importance, for they furnish almost all the food of modern 
mammals. The grasses and cereals, the vegetables, fruits, and 
nuts are all angiosperms, and it would not be going too far to say 
that the almost sudden development of birds and mammals 
followed the appearance of flowering plants. The covered seeds 
produced by these plants were well adapted to the seasons of cold 




or drought which must have been frequent in the Jurassic, and by 
the end of the Cretaceous 90% of all plants were angiosperms. 

The Mesozoic Era was brought to a close by the Laramide 
Revolution or "Time of the Great Dying." Not a single dinosaur 
survived this catastrophe and many of the marine invertebrates 
disappeared from the seas. The Rocky Mountains were uplifted, 
with great volcanic activity from Mexico to Alaska, and the 
Appalachians, which had been worn down to a peneplain, were 
re-elevated. The continent now assumed its present form and 
approximately its present topographic features. 

224. The Cenozoic Era. The last period of earth history is called 
the Cenozoic, or period of recent life. The rocks formed during this 
period are chiefly loose and unconsolidated sediments and most 
of them are confined to the continental margins, since seldom did 
the seas transgress the land. The salt domes of the Gulf states are 
associated with Cenozoic sands which contain great sulphur de- 
posits, as well as oil and gas. The Monterey formation of Cali- 
fornia, one of our great oil-bearing rocks, is of Cenozoic age. In the 
Carolinas and Florida these rocks contain the greatest phosphate 
deposits of the world; they are formed from animal remains. 
From about the middle of the era, the Miocene period, intense 
crustal unrest began in the West and continued with increasing 
violence to the end, culminating in the Cascadian Revolution. In 
Washington, Oregon, and Idaho, the Columbia and Snake rivers 
now cut through extensive Cenozoic lava flows which in places 
are 4,000 feet thick. The Sierra Nevada Mountains were uplifted 
over a mile, while the Coast Range, in California, was re-elevated 
and the famous San Andreas Rift developed. It is this break which 
we believe is responsible for many of the earthquakes experienced 
in California in recent times. We have evidence that the uplift 
of the western mountain area is still going on. 

The Cenozoic is called the Age of Mammals, for although we 
find mammals in the Mesozoic, they are few and inconspicuous, 
most of them about the size of a rat. But no sooner has the Lara- 
mide Revolution wiped out the dinosaurs, than mammals begin to 
appear in great number and variety; perhaps because the colder 
climate suited their warm blood or because their food, the angio- 
sperms, covered the earth. 

Early in the era mammals resembling bears, dogs, cats, and 







horses, and the first primates are found. During the Oligocene 
the animals begin to look modern; among them are the horse, 
camel, rhinoceros, peccary, wild dog, and cat. The Miocene is the 
Golden Age of Mammals; herds of horses and camels roamed the 

FIG. 197. Early Types of Men 

grassy plains, pigs six feet high browsed in the woods, and there 
were hornless deer, weasels, martens, and raccoons. Mammals now 
definitely dominate the world. 

The primates have disappeared from North America but tailless 


apes are found in Europe and Asia. The Pliocene is of interest be- 
cause of the appearance of the first manlike ape in Africa, while in 
England we find flints (eoliths) evidently made by man (Fig. 197). 
It is in the Pleistocene that man definitely makes his appearance 
in the warm interglacial periods, about a million and a half years 
ago; he develops rapidly from manlike ape to apelike man, then 
to Heidelberg man, to Neanderthal man, and finally to Cro- 
Magnon man, the beginning of the modern type, about 40,000 
years ago. 

The most important events of the Pleistocene period were the 
series of glacial epochs. There were five of them, and the last one 
ended about 25,000 years ago. 

Accumulating in Canada, the ice, several thousand feet thick, 
moved south in a sheet covering the entire width of the continent. 
It scoured the surface by removing the rock mantle and planing 
the bedrock. Deep grooves or striae bear evidence of the great 
forces at work. Rivers were widened and deepened, and thousands 
of lakes were formed in Canada and northern United States. The 
Great Lakes and the Finger Lakes of New York are of glacial 
origin. The Great Salt Lake is all that is left of the ancient Lake 
Bonneville, which because of the aridity of the climate has evapo- 
rated down to its present size. 

The last glacier seems to have reached the line shown on Fig. 
85, for we find glacial deposits as far south as that; and the land 
north of that line shows all the signs characteristic of glaciation. 
Just why there were five periods of glaciation and whether any 
more are to follow we cannot say for certain. It may be that we 
are living in one of the warm periods between glacial epochs; and 
perhaps there will be other ice sheets advancing from the north, 
forcing man to take refuge in the warmer parts of the earth. 

^Completion Summary 

Evidence of earth history is preserved in rocks. In these 

rocks we find fossils, which are - . 

Geologic time is measured in periods and eras, rather than in 
years. Time in years may sometimes be determined by - , 

by - - denudation and , and by - of uranium 

minerals. The oldest rocks are about billion years old. 


An unconformity represents an interval during which 

no history. 

During the Archeozoic Era there was life in the warm sea, as 
shown by - . The land was - ; volcanic - . 

In the Proterozoic, we find - - fossils. Volcanic activity 

, but the crust , as it did in the Archeozoic. The 

Archeozoic and - - together with the - - lasted about one 
billion years. 

The Paleozoic was a time of great events. The number and 

variety of fossils - . The Cambrian opens with - - life, 

every class of animal except - - being represented. Verte- 
brates appeared in . In the - - the land - - plants, 

and the first . But the land was not clothed in forest until 

the . Then in the and - - we had forests which 

must have been as those of the equatorial regions today. 

Most of the we use was laid down in those periods. These 

forests were quite different from ours, since the trees were chiefly 

, with no or - - or - . The end of the 

Paleozoic Appalachian Mountains and wiped out - 

plants and annuals of . 

The Mesozoic Era was dominated by . The Palisades of 

New York and New Jersey were formed at the end of the - . 

In the Jurassic, huge roamed the earth, swam - and 

even air. But mammals , while a few insects, resem- 
bling , made their appearance. Modern trees first appeared 

in , together with the - - and that we know 

today. The Laramide Revolution ended , and destroyed 

The Cenozoic Era, known as , opened with - in 

great number and variety. In the Oligocene, we have some - 

like those we know today; for example the - - and . 

The first manlike ape , and man-made flints . In 

the , man himself ; first , then , , 

, and finally , resembling modern man. In the Pleis- 
tocene, occurred five epochs of , and it may be that still 

others are to follow. 



1. When does earth history begin? 

2. What is a fossil? 

3. Why are most fossils the remains of marine animals? 

4. How can we tell where a highland was in a past age? 

5. What kind of rock is deposited at the beginning of a 

6. What is the normal succession of rocks in a period? 

7. What inferences can be drawn from the following succes- 
sion of rocks : conglomerate followed by sandstone, with limestone 
on top? 

8. Read the history of the near-by land from the following 
succession of sedimentary rocks: limestone followed by shale, 
then limestone and shale again. 

9. Decipher the meaning of the following strata : conglomerate, 
sandstone, shale, igneous sill, sandstone. 

10. What is an erosion interval? 

11. What relation is there between an erosion interval and an 

12. In what position are sedimentary strata deposited on the 
continental shelf? Show by diagram. 

13. What are varves? How do they help us to determine geo- 
logic time? 

14. What is the rate of deposition of sediments? 

15. What is now considered the most accurate means of de- 
termining the time, in years, of past geologic events? On that basis, 
how old is the oldest rock? 

16. Where are the youngest rocks of a series of strata found? 

17. How is the age of an igneous intrusion related to that of the 
sedimentary rocks it pierces? 

18. How are periods separated from one another? 

19. Why do the fossils of succeeding periods differ? 

20. What are formations? 

21. How do fossils indicate the climate of a past age? 

22. Why are there very few, if any, fossils in the Archeozoic 

23. What indication have we, in Archeozoic rocks, of the exist- 
ence of considerable plant life? 


24. What does the enormous amount of granite gneiss in 
Canada show about the chief events of the Archeozoic? 

25. How do we know that the Archeozoic Era was ended by a 
period of mountain formation followed by a long period of erosion? 

26. What is meant by pre-Cambrian? 

27. What evidence might be found in the rocks of Greenland, 
Scandinavia, and Canada to show that all three were joined in 
the Proterozoic? 

28. What is the evidence of extensive volcanic activity in the 

29. How do we know that the earth's crust did not crack so 
often in the Proterozoic? 

30. What can be inferred from the vast ore deposits laid down 
in the Proterozoic? 

31. Show that the life of the Proterozoic supports the theory of 

32. How do we know there was much more carbon dioxide in 
the early atmosphere than there is today? 

33. What is meant by a revolution, in earth history? 

34. Why do we believe the Lipalian Interval lasted a long time? 

35. What are trilobites? 

36. Describe the land during the Cambrian. 

37. Why are most of the Ordovician rocks limestones? 

38. What vertebrates appeared first? When? 

39. Is there any possible connection between the appearance 
of the first land plants and the first land animals in the Silurian? 

40. What important development in life started in the De- 

41. When did insects become common? 

42. What is meant by the "age of coal"? 

43. How did the Pennsylvanian forest differ from ours? 

44. What does the presence of much salt in Permian deposits 

45. When were the Appalachian Mountains raised? 

46. Why are the animals and plants of the Mesozoic so differ- 
ent from those of the Paleozoic? 

47. When did this continent begin to look as it does today? 

48. Explain how the Palisades were formed and came to their 
present condition. 


49. Why were the Triassic and Jurassic periods generally arid? 

50. When did western United States begin to look moun- 
tainous, somewhat as it does today? 

51. In what way did the forests of the early Mesozoic differ 
from those of today? 

52. What animals dominated the Mesozoic? 

53. What animals of the Jurassic resembled those of today? 

54. When did mammals first appear? 

55. Why were mammals better adapted to the conditions of 
the Mesozoic than reptiles? 

56. Of what economic importance are Cretaceous rocks in 
central United States? 

57. When did the first bird appear? What is it called? 

58. Which of the modern trees appeared first? When? 

59. What relation may exist between the rise of angiosperms 
and of mammals? 

60. Why is the Laramide Revolution called "Time of the Great 
Dying"? ' 

61. When were the Rocky Mountains uplifted? 

62. When were the Appalachian Mountains re-elevated? 

63. Of what economic value are the Cenozoic deposits of the 
Gulf States? Florida? California? 

64. When was the Columbia River Plateau formed? 

65. What is the age of mammals? 

66. When did many of our domestic animals appear? 

67. When does evidence of man appear in England? 

68. When did man appear? 

69. Name some of the types of early man. 

70. Which type of man do we resemble? 

71. About how long ago did Cro-Magnon first appear? 

72. What is the outstanding event of the Pleistocene? 



225. The earth is a ball nearly 25,000 miles around, and 
about 8,000 miles in diameter. It is composed of rock, with 
about three quarters of its surface covered by oceanic waters. 
Both rock and water are enveloped in air. 

The earth has many movements, although it appears to 
us that it is at rest because of the uniformity of these move- 
ments and the fact that we partake of all of them. It some- 
times happens that a train begins to pull out of the station 
very slowly and without the slightest jar. A passenger on 
that train who happens to be looking at a neighboring train 
sometimes imagines that his train is at rest while the other 
one is moving. He discovers his error when he sees posts 
and other objects moving away from him. We are accus- 
tomed to being jounced and jarred by a moving vehicle 
and to seeing objects fly past us. But the earth moves 
smoothly without the slightest jar and although objects do 
move past us, we think they are in motion not we. They 
are everything is in motion. 

We shall study only two of these earth motions: its rota- 
tion on its axis, the time of which we call a day, and its 
revolution about the sun, which marks off the year. 

The earth is only one of a family of rotating and revolving 
spheres controlled by the sun and called the solar system. 
A little study of the earth's relation to these other heavenly 
bodies will make clear many things, particularly with re- 
spect to time and seasons. 

226. Form and size of the earth. The earth's shape is 
almost spherical (Fig. 198). The diameter from pole to pole 
is 27 miles shorter than the diameter at the equator. We 



believe this is due to the effect of the centrifugal force of 
the earth's rotation on its plastic interior (Fig. 199). 

North Pole 

South Pole 

FIG. 198. The Form and Size of the FIG. 199. Centrifugal force causes a 
Earth hoop to bulge at its equator. 

The dotted line is a circle. 


FIG. 200. How Eratosthenes Measured the Circumference of the Earth 

The surface area of the earth is nearly 200 million square 
miles, of which a little more than 50 million square miles 
are land. 

*The first successful attempt to measure the size of the earth 
was made about 200 B.C. by Eratosthenes, an astronomer and 
geographer of Alexandria, in Egypt. He learned that, at Syene, 


the most southern city of ancient Egypt, a vertical pillar cast no 
shadow at noon on June 21. At Alexandria, 5000 stadia directly 
north of Syene, the sun's noon ray, on the same day, made an 
angle of 7.2 with a vertical pillar (Fig. 200). Assuming the earth 
to be a sphere, a line through a vertical pillar, anywhere, passes 
through the center of the earth. 

Since the sun's rays are parallel, we have here a simple problem 
in geometry. The angle at the center of the circle, which subtends 
the arc from Alexandria to Syene, is 7.2 (alternate interior angles 
made by parallel lines). Since the entire circumference is 360, or 
50 X 7.2, it must be 50 times the distance from Alexandria to 

FIG. 201. An Eclipse of the Moon 
The earth's shadow is always the arc of a circle. 

Syene, or 250,000 stadia. We now know that the distance between 
these two places is about 500 miles, giving 25,000 miles as the 

227. Evidence that the earth is spherical. 

1. As ships sail away, their hulls gradually disappear; and 
as they approach, their tops appear first. Before any part 
of the approaching vessel is seen, smoke often appears to 
be coming out of the water; then the smokestack comes in 
view. This shows that the water surface between the observer 
and the ship is actually curved so as to hide the distant ship. 

This curvature is found to be the same in all directions 
on water surfaces, and consequently the earth is a sphere. 

2. During every eclipse of the moon by the earth, that 
portion of the shadow of the earth cast on the moon always 
has a curved edge, apparently the arc of a circle. In Fig. 201 



the moon, moving to the left, is entering the earth's shadow 
at A and leaving it at A'. The curved edge of the earth's 
shadow shows that the form of the earth is spherical, be- 
cause the shadow is always circular. 

3. Circumnavigation proves the earth to be spherical. 
By traveling in one direction, a person always returns to 

FIG. 202. Evidence of the Earth's Curvature 
See paragraph 227, (4). 

the same place; and the distance is always the same, in 
whatever direction he travels. 

This has been accomplished many times by ship and by 

4. On the shores of a calm lake, away from tides and 
swells, the curvature of the earth may be measured directly 


FIG. 203. Effect of the Earth's Curvature on Time 
The parallel rays of the sun strike points farther west at smaller angles. 

by erecting in a straight line three posts, A, B, and C, a mile 
apart (Fig. 202), at the same height above the surface of 
the water. If a telescope is sighted along the top of post A 
to the top of C, the line of sight will run 8 inches below 
the top of post B. This can only be because of the earth's 

5. As a result of the curvature of the earth, places on 
the earth to the east or west have different time. 

In Fig. 203, if at A it is 10 o'clock, then at B, farther west, 



it is earlier, perhaps 9 o'clock, and at (7, 8 o'clock, all, of 
course, at the same instant. If the earth's surface were flat, 
all places would have the same tune. 

*6. The weight of a body at sea level is about the same every- 
where on the earth's surface. According to the Law of Gravitation, 
the weight of a body is a measure of the attraction between the 
earth and the body. A body farther away from the earth's center, 
as on a mountain, weighs less, because the attraction is less when 
the bodies are farther apart. 

Since the weight of a body at sea level is about the same every- 
where on the earth's surface, it must be the same distance from 
the center of the earth. Therefore the earth is a sphere. 

Actually, a body weighs more near the poles than at the equator, 
which proves in another way the flattening at the poles. 

228. The rotation of the earth. The uniform spinning 
motion of the earth on its shortest diameter is called ro- 
tation. The shortest diameter, con- 
necting the poles, is called the axis 
of rotation. The line around the 
earth midway between the poles is 
the equator. 

As late as 1632, the time of Gali- 
leo, there was strong doubt as to 
the daily turning of the earth on 
its axis. The first experimental 
proof that the earth actually ro- 
tates was obtained in 1851, and 
later experiments have proved it 

*In 1851 the physicist Foucault de- 
vised a remarkable proof of the earth's 
rotation by means of a pendulum. 
From the dome of the Pantheon in Paris he hung a heavy iron ball 
by a steel wire more than 200 feet long. The pendulum was started 
swinging back and forth in a plane, and, as time went on, it swung 
over different lines on the floor. Either the pendulum was gradually 
moving around a circle or the floor was turning (Fig. 204). 

FIG. 204. Foucault's Experi- 



Direction O f 

We know that because of inertia the plane of vibration of a 
pendulum never changes. Hence the conclusion must be that the 
floor of the Pantheon, in other words, the earth, is turning about 
an axis. 

We find that the sun, the other planets, and some of the satel- 
lites are all in rotation. One of them rotates in 10J hours, another 
in about 24 hours, like the earth, and some require months. 

229. Effects of rotation. We know now that one complete 
rotation occurs in a day, and that, in consequence, the sun, 
moon, and stars rise in the 
east, pass through the sky, 
and set in the west; or at 
least they seem to, though 
actually we know that their 
apparent movements are due 
to the earth's rotation. 

As the earth receives its 
light from the sun, the side, 
or half turned toward the sun, 

is in light and has day. At 

the same time the opposite 

side is in shadow and has 

night (Fig. 205). As the earth rotates from west to east the 

light moves gradually toward the west ; that is, the sun rises 

in the east and sets in the west. 

230. Star trails. Figure 206 is a sort of moving picture of 
the northern sky at night. If it were a snapshot, each star, 
somewhere on a circle, would make a point of light on the 
picture, but there would be no circles. The photograph was 
taken by pointing a camera toward the northern sky on a 
clear moonless night and exposing a plate for 9 hours. 
During that time the earth, turning around, carried the 
plate along in the arc of a circle while the stars remained 
fixed. Notice that the trails are not complete circles. For ex- 
ample, the trail at the center was made by the Pole Star itself, 
which is near but not exactly at the north pole of the sky. 

FIG. 205. The rotation of the earth 
causes day and night. 


The stars appear to be revolving about this celestial north 
pole, which is the continuation of the earth's axis. 

The North Star was focused at the center of the plate. 
The actual length of any trail will depend upon the distance 
from the North Star, but the number of degrees in the arcs is 

FIG. 206. Star Trails 

the same for all the trails. Since the earth rotates 360 in 24 
hours, in one hour it will move 15 about its axis. Therefore 
each of these trails should be an arc of 9| X 15 or 142.5. 

The stars appear to be moving about the North Star in 
an anticlockwise direction; that is, the stars above the North 
Star appear to move westward, those below eastward. 

This apparent motion of the circumpolar stars is caused 
by the real motion of the earth; hence the apparent anti- 



clockwise direction is due to the turning of the earth in a 
clockwise direction. 

The actual number of miles which a point on the earth's 
surface moves because of rotation depends upon its distance 
from the equator. At the pole this is zero; at the equator, 
25,000 miles in 24 hours. In latitude 40, near New York 
City, it is about 800 miles an hour; at 60, about 500 miles 
an hour. But the angular rate of rotation, 15 per hour, is 
everywhere the same; and this furnishes us the most ac- 
curate timepiece known. All clocks and watches are regu- 
lated by it. 

231. The earth's revolution about the sun. The path of 
the earth about the sun is called its orbit, and the journey 
takes a few minutes less than 

365J days. This period of rev- 
olution determines the length 
of our year. 

The time it takes the other 
planets to go round the sun 
differs greatly. One planet re- 
volves four times while the 
earth revolves once, whereas 
other planets require scores 
of earth years to make one 
complete journey around the 
sun. The rate of the earth's 
revolution is over 66,000 
miles an hour. 

232. Change of season. As 
the eartH moves forward in 
its orbit its axis remains 

tipped, or inclined, in the same direction and always the 
same amount. The inclination of the earth's axis is 23| 
from the perpendicular to the plane of the earth's orbit (Fig. 
207). In other words, the earth's axis is always inclined 23^ 
and therefore it is always parallel to itself. 

FIG. 207. The earth's axis is in- 
clined 23| from the perpendicular 
to the plane of the earth's orbit. 


The causes of the regular change of season may be stated 
as due to: 

1. Inclination of the earth's axis 

2. Its parallelism 

3. Revolution about sun 

If the earth's axis were perpendicular to the plane of the 
earth's orbit, the sun would each day pass through the sky 
in the same path; all our days would be of equal length and 
we should have no change of season. Any place on the earth 
would receive the sun's noon rays at a certain angle which 
would not change from day to day. The sun would rise at 
the same time each day, and set at the same time; and 
weather and climate, which depend so much on the amount 
of heat received from the sun, would be more uniform 
throughout the year, and year after year. 

Or if the axis were inclined, and there were no revolution, 
then, while days and nights would not be 12 hours each, 
they would remain the same for any particular place, the 
amount of heat received from the sun would still be the 
same for each day, and again there would be no change of 
seasons. The change of seasons depends then on both inclina- 
tion and revolution. 

Because of the inclination of the earth's axis, the sun 
passes through the sky at a much higher elevation in summer 
than in winter. This higher elevation of the sun causes longer 
days, and we get the sun's rays at a more nearly vertical 
angle; hence, we have summer. If the inclination of the 
earth's axis was more than 23.5, the change of seasons 
would be more pronounced; that is, our summers would be 
warmer and our winters colder than they are now. 

The regular change of seasons depends upon the earth's 
axis remaining parallel. The length of each season depends 
upon the time of revolution. Should the earth require a 
longer time to go around the sun, the seasons would be 
correspondingly longer. 

The orbit of the earth has the form of an ellipse, but it 



is nearly circular because the two foci of the ellipse are 
near together. The sun is at the north focus, and the earth, 
about January 1, is about 91,500,000 miles from the sun. 
This point on the orbit, called perihelion, is the nearest to 




Aphelion JLr/yjT "~94'soo7oOO mile's 
Summer Solstice 



Autumnal Lquinox.Sept.2Z 

FIG. 208. Four Positions of the Earth in Its Orbit about the Sun 
Corresponding to the Four Seasons 

the sun. On July 1 the earth is farthest from the sun, at 
aphelion, 94,500,000 miles away. 

This change in distance has little effect on the seasons. 
As a matter of fact our northern winter occurs when the 
earth is nearest to the sun, at perihelion, and our summer 
occurs at aphelion. 

In Fig. 208, the earth is shown in four positions, as it 
makes its annual journey around the sun. Each position 



shows the earth at the beginning of a season. On June 21, 
the North Pole, turned toward the sun (Fig. 209), is in the 

FIG. 209. The Earth on June 21 FIG. 210. The Earth on Dec. 21 

middle of a six months' period of sunlight. The South Pole 
at the same time is turned away from the sun, and it is in 

the middle of a six months' 
period of darkness. Summer 
is beginning in the northern 
and winter in the southern 

The daylight circle is just 
touching the polar circles and 
the sun's vertical ray is at 
the Tropic of Cancer. 

On December 21 (Fig. 210), 
the South Pole is tipped to- 
ward the sun and it is in the 
middle of six months of day- 
light. The North Pole is in 

darkness for six months. Summer is beginning in the south- 
ern and winter in the northern hemisphere. 

The daylight circle is just touching the polar circles, and 
the sun's vertical ray is at the Tropic of Capricorn. 

On September 23 (Fig. 211) and on March 21, the sun's 
rays are perpendicular to the earth's axis. Days and nights 

FIG. 211. The Earth on Sep. 23 
and Mar. 21 



13 b. 

12 to, 

are twelve hours each, all over the earth. On March 21, 
spring is beginning in the northern and autumn in the 
southern hemisphere. The daylight circle passes through 
the poles and the sun is vertical at the equator. 

233. Cause of unequal days and nights. Figure 212 shows 
the longest day at different points in the northern hemi- 
sphere. At the equator it is 
12 hours and increases as we 
move north, until at the pole 
it is 6 months. 

The same factors that cause 
change of seasons are respon- 
sible for the changes of the 
length of day and night. 

At the equator the length 
of days and nights is always 
equal. The farther we go from 
the equator, either north or 
south, the greater is the dif- 
ference in length between day and night ; in summer the days 
are long and the nights short, and vice versa in winter. 

The fundamental cause of this inequality of the length 
of days at different times of the year is the inclination of 
the earth's axis. If it were inclined more than 23^, our 
summer days would be longer and our winter days shorter 
than they are now. 

On March 21 and September 23, the sun's vertical ray 
being at the equator, the sun rises due east and sets due 
west, and the days are everywhere equal in length to the 
nights. (See Figs. 213, 214, 215, and 216.) These two dates, 
March 21 and September 23, are called the equinoxes (equal 
nights). On March 21, the vernal or spring equinox, the 
sun is said "to cross the line," as it passes from the southern 
to the northern side of the equator. On September 23, at 
the autumnal equinox, the sun crosses the line again, from 
the northern to the southern side of the equator. 



From the vernal to the autumnal equinox the sun rises north 
of east (Mar. 21 to June 21 and back to Sept. 23 in Fig. 214) 
and sets north of west; and in the northern hemisphere the 
days are longer than the nights, and vice versa in the southern 

FIG. 213. At the equator the 
apparent sky path of the sun is 
always perpendicular to the hori- 
zon, and days and nights are 
equal at all times of the year. 

The observer is at the center 
of the horizon circle. 

FIG. 214. At New York City, 
41 N latitude, the apparent sky 
path of the sun is tipped toward 
the south, giving us long days in 
summer and short days in winter. 

The direction of sunrise and 
sunset may be read from the 

hemisphere. From the autumnal to the vernal equinox, the 
sun rises south of east and sets south of west; and in the 
northern hemisphere the days are shorter than the nights, 
and vice versa in the southern hemisphere. 

The apparent northward journey of the sun ends on 
June 21, called the summer solstice. The word solstice, trans- 
lated literally from the Latin, means "the sun stands." 
The southern journey ends on December 21, called the 
winter solstice, when the sun stands before starting north- 

As shown in Fig. 213, the daily sun paths at the equator 
are always perpendicular to the horizon, and all days and 
nights are equal at all times of the year. The middle sun 
path is actually the same as the sky equator. It shows that 
a vertical sun is over the earth's equator only at the equi- 


noxes. The sun paths at the time of the solstices are 23^ 
from the sky equator. 

At the time of the summer solstice the sun is vertical at 
the Tropic of Cancer; at the winter solstice, at the Tropic 
of Capricorn; each 23 J from the equator. 

In Fig. 214, at New York City, latitude 41 north, the 
paths are tipped toward the south, an angular distance from 
the perpendicular equal to that of the latitude. This is true 
for all latitudes from the equator to the North Pole. South 
of the equator the same relation holds, but the sun paths 
are inclined north. Here the long days in summer and short 
days in winter are shown by relative length of sun paths 
above the horizon on June 21 and December 21. The de- 
parture of sunrise from due east and sunset from due west 
is considerably more than at the equator. 

In Fig. 215, the sun paths are inclined 66.5 from the 
perpendicular, that being the latitude of the Arctic Circle. 
Here the sun path for June 21 just touches the horizon at 
one point, due north; in other words, the sun is above the 
horizon for 24 hours and there is no night. From any position 
on the Arctic Circle on June 21 an observer would have an 
opportunity to see the sun, due north and on the horizon, 
at midnight; or, at least, it is 12 o'clock of what, farther 
south, would be called night. 

At this latitude on December 21 the sun would just ap- 
pear on the horizon at midday and then would set again. 

In the polar regions, however, where periods of light and 
darkness may be many days or weeks, the habit of regu- 
larity of rest and activity is not generally formed. In the 
long summer day, when the sun never sets, the opportunity 
for cultivating fields or harvesting crops must not be missed, 
and the entire family will work 15 hours a day or even 
longer, until they are worn out, before they seek repose. 
One can see boys playing outdoor games at 1 o'clock in the 

The earth's rotation furnishes a simple way to find geo- 



graphical directions. The sun rises approximately in the east 
and sets in the west. That is exactly true only at the equi- 
noxes. The midday sun shows us which way is south, and 
the Pole Star at night marks north for those of us who 
live in the northern hemisphere. (See North Star, Fig., 220, 
next chapter.) 


FIG. 215. At the Arctic Circle, 
66.5 N latitude, the apparent 
sky path of the sun on June 21 
is above the horizon, and on Dec. 
21 it is below the horizon for 
24 hours. 

FIG. 216. At the North Pole, 
the apparent sky paths of the 
sun are nearly horizontal. The 
year is divided into two periods: 
sunlight and darkness. 

*In Fig. 216, at the North Pole, the sun paths are parallel to the 
horizon. The pole is 90 north latitude, and the sun path is in- 
clined 90 from the perpendicular. At the time of the equinoxes 
the sun paths are at the horizon. The sun is said to rise at the vernal 
and to set at the autumnal equinox, causing a period of continuous 
sunlight for six months. During any period of 24 hours, the sun 
apparently moves through the sky approximately at the same 
distance above the horizon. Should the altitude of the sun at inter- 
vals of 12 hours be found to be about the same, it would be proof 
that the observer was approximately at the North Pole. 

As the sun sinks below the horizon, about September 23, not to 
appear again until about March 21, there is a period of six months 
without direct sunlight. This six-month period is not entirely 
devoid of light. For nearly two months there is a gradual fading 
twilight, due to refraction, as the sun continues to sink lower 
below the horizon. 


For about two months all light from the sun is cut off, except 
that reflected by the moon. The dawn begins nearly two months 
before the sun finally appears again at the horizon on March 21. 

Courtesy of Donald MacMUlan 

FIG. 217. The Midnight Sun at Intervals of 20 Minutes 
Its path corresponds to that shown in Fig. 215 for June 21. 

Completion Summary 

The shape of the earth is almost - . The equatorial 
diameter is - - longer than - . About - - of 
the surface is - , the rest - . 

*Eratosthenes first - - about - miles. 

An approaching vessel at sea appears - first, be- 
cause - . 

The shadow of the earth on the moon is , proving 

*If the earth were not a sphere, the weight of a body, which 
is a measure of - , would be - - at different places. 
Foucault, with a long - , proved - . 

The sun, moon, and other heavenly bodies apparently 
rise - and set - - because the earth - . 

The length of the year is caused by the earth's - 


Change of season is due to and . 

- also causes the length of the day . 

At , the earth is 91,500,000 miles from the sun, 

while at - , it is miles from the sun. 

On Sep. 23 and Mar. 21, the sun's rays are , and 

therefore . 

At the equator, the length of the day . Farther 

north, the days in summer, until above the Arctic 

Circle, they may be and at the pole - . The 

sun - continuously for more than 24 hours anywhere 
north of on . 

Sep. 23 and Mar. 21 are called - - because - . 

From Mar. 21 to Sep. 23, the sun - - east and sets 

- west. The apparent northward journey of the sun 

on at the Tropic . This date is called 

. On Dec. 21, the sun southern journey and 

begins - . This date is called . 

At midday, the sun is exactly on line, from which 

we can get direction. 


1. Why is the polar diameter of the earth shorter than the 
equatorial diameter? 

2. What part of the earth's surface is covered by water? 

3. What evidence that the earth is not flat is furnished by 
vessels at sea? 

4. How does a lunar eclipse prove that the earth is a sphere? 

5. How does circumnavigation prove that the earth is 

6. Show by diagram how the earth's curvature might be 
measured directly. 

7. What is meant by rotation of the earth? 

8. What is the axis of the earth? Show by diagram. 

9. Why do the sun, moon, and stars seem to rise in the east 
and set in the west? 

10. Explain the cause of day and night. 

11. How do star trails prove the earth's rotation? 


12. Explain how to tell time by the earth's rotation. 

13. What determines the length of the year? 

14. State, in degrees, the inclination of the earth's axis. 

15. If the earth's axis were perpendicular to its orbit, how 
would that affect the length of the day? 

16. How would a perpendicular axis affect the seasons? 

17. How would a perpendicular axis affect weather and clim- 

18. Explain how change of seasons depends on both inclination 
and revolution. 

19. How would the seasons be affected if the earth revolved 
about the sun in 730 days? 

20. What is the shape of the earth's orbit? 

21. What is perihelion? aphelion? 

22. Why does our winter occur when the earth is nearest the 

23. When does the long polar day begin? 

24. When does summer begin? winter? 

25. Where is the Tropic of Cancer? Capricorn? What relation 
have these to the inclination of the earth's axis? 

26. Why do we start spring on Mar. 21, and autumn on 
Sep. 23? 

27. What locates the Arctic Circle? 

28. When do days and nights have the same length all over the 
earth? What name is given to these days? 

29. Where do we find the greatest variation in the length of 
day? the smallest? 

30. On Dec. 15, about where does the sun rise, in the north 
temperate zone? 

31. What is meant by the summer solstice? the winter solstice? 

32. In about what latitude are the noon rays of the sun vertical 
on Jan. 1? Mar. 1? July 1? Sep. 1? 

33. At the dates mentioned in question 32, which is longer here: 
day or night? 

34. At the above dates does the sun here rise north or south 
of east? 

35. When does the sun shine into windows facing north? 

36. Where can the midnight sun be seen? 

37. Can the day ever be longer than 24 hours? Where? 


if Optional Exercises 

38. Explain, by diagram, Eratosthenes's method of finding the 
circumference of the earth. 

39. How can the approximate shape of the earth be proved by 
gravitational measurements? 

40. Describe Foucault's proof that the earth rotates on an axis. 
Explain the part played by inertia. 

41. How does the rotation of the earth provide us with a means 
of keeping time? 

42. Show that if the inclination of the earth's axis were more 
than 23.5, we should have more extreme seasonal changes. 

43. Explain why the polar day actually lasts more than 6 

44. Explain why day and night are always equal at the 

45. Explain why day and night are everywhere of equal length 
at the equinoxes. 

46. Would there be any daylight at all on Jan. 1 north of 
the Arctic Circle? Explain. 



234. How do we locate places on the earth? The inter- 
section of two lines determines a point, and if a house is 
located by saying it is on the northwest corner of A Street 
and B Avenue, any person familiar with those streets could 
locate the house. 

In locating points on the earth's surface, which is spherical, 
we use a system of two sets of lines at right angles to each 
other, called parallels of latitude and meridians of longitude. 

The equator is the circle drawn around the earth midway 
between the poles. It has -latitude. Other circles drawn 
around the earth parallel to the equator, in other words 
equally distant from the equator at all points, are the par- 
allels of latitude. Each circle gets smaller and smaller until, 
at the North Pole, the circle is a dot. 

Since we are dealing with circles, which are conveniently 
divided into 360, we call the distance from the equator to 
the pole, which is one quarter of a circle, 90. The North 
Pole is therefore at 90 north latitude and the South Pole 
at 90 south latitude. When we draw a map, we locate on 
it as many parallels of latitude as are convenient between 
and 90 (Fig. 218). 

In locating the latitude of a particular place, if it does 
not fall exactly on a degree of latitude, we divide the degree 
into 60 parts, called minutes, and, for still more precise work, 
a minute is divided into 60 seconds. The place may be on a 
parallel of latitude 41 degrees 22 minutes 45 seconds north 
of the equator; that would be designated 41 22' 45" N 
lat. in common practice. 

A degree of latitude is roughly 70 miles (25000 -i- 360). 




Because the equatorial bulge makes the curvature of the 
earth's surface grow gradually less from the equator toward 
the pole, degrees of latitude increase slightly in length toward 
the poles. 

235. Finding latitude by night. The latitude of an ob- 
server in the northern hemisphere may be found on any clear 
night by means of the North Star, Polaris. The number of 
degrees of a heavenly body above the horizon is called its 



U Polaris 

r ft* 


FIG. 218. Parallels of Latitude 

H f 

FIG. 219. Finding Latitude by 
Observation on Polaris 

altitude. At the equator the North Star appears on the hori- 
zon and its altitude is consequently zero. At 40 north of 
the equator the North Star is 40 above the horizon (alti- 
tude 40), and at the North Pole of the earth it is directly 
overhead, or in the zenith; hence its altitude is 90. 

The altitude of the North Star in the northern hemisphere 
equals, therefore, the latitude of the place where the ob- 
servation is made. 

*These relations are shown by simple geometry in Fig. 219. 
An observer at finds the altitude of Polaris, angle HO N f , and that 
is his latitude, since that angle is equal to angle OCE (as HH' is 
perpendicular to CO; and N'O, which is parallel to NS, is perpen- 
dicular to CE). In Fig. 219 the latitude of is 40 N. 



It may be surprising to find all these lines, toward Polaris, 
parallel to each other, until we remember that the star is so far 
away (much farther than the sun) that all lines from it to the 
earth must be practically parallel. , 

Since Polaris is not exactly at the north pole of the sky, but 
describes a small circle about it every day, we must, for precise 
work, make a small correction (Fig. 220). 

J>r"7i T^/ 

/^ \ /' \ Cassiopeia yTs 

'tPcjlarisJ \ r 

/(North Star) f 

\ V 


Alpha ^Xx/ 


^ x'V 


(Big Dipper 

FIG. 220. Rotation of the Heavens about Polaris 

Polaris may be found in the sky, at night, by following 
the pointers, alpha and beta, of the Dipper. It is almost on a 
straight line with the pointers. 



9 ,>70 

FIG. 221. Finding Latitude by the 

*236. Latitude determined by day. Another method of finding 
the latitude of a place is to measure, with the sextant, the eleva- 
tion of the midday sun above the 
horizon. The complement of this 
angle is the latitude of the place 
(Fig. 221). At the time of the 
equinoxes the sun is directly on 
the equator, and the sextant 
angle would be 90. For other 
times the latitude is found in 
a table in the Nautical Almanac 
which gives the sun's declina- 
tion for any time of the year. 

All we need to do is to read the sextant, look up in the Almanac 
the date and the sextant angle, and we shall find the correspond- 
ing latitude. 

237. Longitude. To locate the east and west position 
of a place, circles are drawn through the poles all the way 
round the earth. These circles 
are called meridians (Fig. 
222). They are farthest apart 
at the equator, where the dis- 
tance between meridians is 
about 70 miles (25000 -^ 360). 
At the poles the distance is 
zero. At latitude 40, near 
New York City, the degree 
of longitude, which is -^-Q of 
the parallel of latitude, is a 
little over 50 miles. 

Where shall we start 

longitude? There is no line like the equator and we have had 
to fix arbitrarily some point through which the or prime 
meridian shall pass. This point is fixed at the Royal Observa- 
tory at Greenwich, near London, England. All points east of 
that are said to have east longitude, and points west to have 
west longitude, the maximum being 180 in either direction. 





120 9 






FIG. 222. Meridians of Longitude 


The location of any place on the earth's surface is given 
in latitude and longitude. For example, 40 N and 75 W 
would mean that the place was on the parallel 40 north of 
the equator, at the point where the meridian 75 west of 
Greenwich crosses it. 

238. How longitude is determined. The sun apparently 
circles the earth from east to west in 24 hours, passing over 
all the 360 degrees of longitude. In one hour, therefore, it 
crosses 15 degrees toward the west. If it is 12 o'clock at 
Greenwich it will be 11 o'clock at every place whose longi- 
tude is 15 W. 

To find the longitude of a place, we must know the time 
at Greenwich as well as the local time. Accurate time is 
broadcast by radio from many stations throughout the 
world. From this information, Greenwich tune can be 

Before the days of radio every vessel carried several 
marine chronometers, or accurate clocks, set at Greenwich 
time. An observation on the sun gives local time, since the 
sun crosses the meridian at 12.00 noon. This will be when 
the sun crosses the north-south line, or when it is at maxi- 
mum elevation in the sky. If local time is one hour earlier 
than Greenwich time, the place is at 15 W longitude. 

239. How to find a north-south line. 

1. An observation on Polaris any clear night will give 
the true north. 

2. The gyro-compass points true north. 

3. The magnetic needle, when corrected for declination, 
will give true north; but the declination varies so much 
that this is by no means accurate. 

4. A rough way to find the north-south line is to find 
the direction of the shortest shadow cast by a vertical rod 
on a horizontal surface. 

When the sun is at its highest point in the sky, shadows 
are shortest, and that will be at noon, when the sun crosses 
the meridian, which is a north-south line. 


*240. Navigation. Finding the position of a ship on the open 
sea by observing the sun by day or the stars by night is called 
navigation by observation. When this cannot be done because of 
poor visibility, one of several other methods must be used. 

1. Dead reckoning. By keeping account of the course at sea and 
the distance traveled, the officers can chart the vessel's position 
on a map. 

2. Radio position finder. The apparatus on the vessel measures 
the bearing of any transmitting station or requests a known station 
to send signals. Knowing the positions of two sending stations and 
the directions from which signals are being received, the position 
of the ship can be charted by the intersection of the two direction 

3. Echo sounding. The fathometer, an echo sounding instrument, 
enables the skipper of a vessel to find the depth of water the ship 
is passing over by the time it takes for the sound of a whistle to 
go to the bottom, be reflected, and be received by a microphone 
under the ship. This instrument can also be made to draw the 
profile of the bottom on a strip of paper. 

The Hydrographic Survey has charted the profiles of the 
bottom of the North Atlantic and is charting other important 
ocean lanes. If a skipper has the charts prepared by the Survey 
together with a fathometer, he need only get a profile of the ocean 
floor and compare it with the charts, to discover his position. This 
can be done night or day, in fair or foul weather. 

241. How time is determined. We have determined 
longitude by means of Greenwich time and it is obvious 
that we can find the time by knowing the longitude. It is 
noon at any particular place when the sun crosses the 
meridian. At that same instant it is 1 o'clock P.M. at a 
point 15 of longitude east of the place and 11 o'clock A.M. 
15 west. The time at a place, as determined by observa- 
tion, is called local time or sun time. The apparent motion 
of the sun is faster, when nearer the earth, than when it is 
farther away. Since our solar days are not uniform, we take 
the average length of all the days in the year and call it 



FIG. 223. Sundial 

the mean solar day. Clocks are regulated to keep mean 
solar time, whereas from a sundial the actual solar time can 
be read. 

242. Standard time. If all clocks were to read mean solar 
tune, we should find a different local time at every place 
to which we traveled, east or west. This made little difference 
until the development of rail- 
roads brought confusion and 

the American railways, in 1883, 
adopted a system of standard 
time. Its advantage is that 
neighboring places keep the 
same time, instead of differing 
a few minutes or seconds ac- 
cording to their longitude. 

The standard time belts are 
about 15 in width, and every 

point within the belt has the same or standard time. These 
zones are shown in Fig. 224. This system of standard time 
has spread over the entire world. 

As one travels west, all watches are set back one hour on 
entering each new standard time belt three times from 
the Atlantic coast to the Pacific. 

243. Daylight-saving time. In summer the days become 
longer than in winter and especially in the higher latitudes 
of northern United States. In order to take advantage of 
the sunlight, for health, and to save electricity used for 
lighting, it is customary in many cities to advance the 
clock one hour in spring. A person who ordinarily rises at 
7.00 A.M. will, under daylight saving, rise at 6.00 A.M., 
standard time, but his clock reads 7.00 A.M. In the evening, 
if his work is over at 5.00 P.M., it will really be 4.00 P.M., 
standard time, with the sun still high in the sky. People 
can enjoy several hours of outdoor activity in the sunshine 
and improve their health. Factories that use artificial light 
might save an extra hour of electrical energy. In the autumn, 



when the days get shorter again, there is no daylight saving, 
since it is dark at 7.00 A.M. as well as at 6.00 A.M. At that 
tune the clock is set back on standard time. 

244. The civil day. Our ordinary day, called the civil day, 
begins at midnight and ends on the following midnight. 

FIG. 224. Standard Time Zones of the United States 

Business is generally suspended at midnight, and the change 
of date can be made then with the least confusion. The 
first 12 hours are called A.M., ante meridian, and the second 
period of 12 hours is called P.M., post meridian; 12 M., 
means noon, or sun on the meridian. 

If a person travels westward around the earth, in the 
same direction as the sun, then for each 15 of longitude he 
travels westward he gains an hour, since he must set back 
his watch by one hour. If that person travels westward all 
around the earth, he will gain 24 hours; and having set 
back his watch 24 hours, his day of the week and his date 
are one full day behind what they would be if he had stayed 
at home. In going eastward this confusion is reversed. 





Suppose an airplane were 
capable of traveling as fast as 
the sun and let us assume it 
starts westward at noon on 
Tuesday, April 15, from New 
York. Keeping up with the 
sun, it remains noon April 15 
all the way and it arrives back 
in New York on Tuesday 
noon, April 15. Meanwhile 24 
hours have elapsed and the 
people in New York are call- 
ing it noon of Wednesday, 
April 16. It seems that the 
aviator has lost a day. 

If he had flown east, he 
would have gained a day, ar- 
riving in New York on Thurs- 
day, April 17, whereas it is 
only Wednesday, April 16. 

In traveling around the 
earth westward, then, not 
only must one's watch be set 
back one hour for each stand- 
ard time zone, but his day 
of the week and his date must 
be set ahead one full day. 
But where shall this change 
of date be made? 

To avoid confusion, it has 
been agreed to make the 
change of date at the 180th 
meridian, called the interna- 
tional date line, somewhere in 
the middle of the Pacific 
Ocean (Fig. 225). Since the 180th meridian crosses several 




FIG. 225. The International Date 


islands and land areas, notably New Zealand, the date line 
is shifted so as not to pass through these places. In that way 
no one has to worry about change of date except travelers, 
to whom it does not make any difference. 

If for example, a traveler going from San Francisco to 
Japan reached the international date line on Sunday morn- 
ing, July 16, the day west of the line would be called Monday, 
July 17; and vice versa, if he were going from Japan to 
San Francisco, and arrived at the international date line on 
Sunday, July 16, it would become Saturday, July 15, east 
of the line. 

245. The conventional day. By international agree- 
ment the current day is called the conventional day. It 
begins at the international date line and moves westward 
15 an hour with the sun. In other words, at some places 
on the earth the conventional day may be Wednesday, 
July 11, while at other places it is Thursday, July 12. 

*246. The calendar. The calendar worked out by the Romans 
was based largely on the motions of the moon. As the yearly num- 
ber of the revolutions of the moon varies, the Roman seasons and 
festivals did not keep in place and the calendar fell into a state of 
great confusion. For example, spring festivals would be celebrated 
in the winter, if it were not that every so often days were added 
to the calendar. 

The word calendar is derived from the Latin word Calendae or 
"callings/' because when the officials in charge saw the new moon, 
which was to begin the month, announcements or callings would 
be made from the Capitol regarding the month's calendar. 

The Roman year consisted of 12 months, March being the first, 
and December the tenth. February had 28 days, there were four 
months of 31 days each, and the rest had 29 days. 

*247. The Julian calendar. In 46 B.C. Julius Caesar, with the 
advice of Egyptian astronomers, reformed the Roman calendar. 
He made three consecutive years of 365 days each and the fourth 
of 366 days. The extra day was added to February. The length of 
the Julian year was 365.25 days; and since the true year has 
365.24 days, the Julian year was 0.01 day or 11.2 minutes too long. 


This amounts to a little more than 3 days in 400 years. As a conse- 
quence, the date of the vernal equinox came continually earlier; 
and in 1582 the vernal equinox occurred on March 11. 

*248. The Gregorian calendar. In 1582 Pope Gregory XIII 
directed that ten days be stricken from the calendar, so that the 
spring equinox might occur on March 21. A further reform was 
introduced at this time in order to prevent a recurrence of the 
discrepancy. The Pope decreed that the centurial year should be 
counted as a leap year only when divisible by 400. Thus 1800, 
2100, and so forth are not leap years; but 1600, 2000, and 2400 are 
leap years. The Gregorian calendar is now used in all civilized 

In England it was adopted in 1752. Dates and events which had 
occurred before the Gregorian calendar was adopted are termed 
Old Style (O.S.), and those after the adoption New Style (N.S.). 

Completion Summary 

Parallels of latitude are circles - - equator. There are 
of them, equally spaced from - - pole. The 

equator is at - - degrees of latitude, while - - 90 
latitude. In one degree there are - - minutes and in one 
minute - - seconds of latitude. In miles, a degree is 
about - - and a minute is a little more than - 


Latitude may be found at night by measuring the angle 
of - - above the - . 

*By day, the latitude is equal to the complement of - . 

Longitude is determined by reference to - . The 
prime meridian, through - - is called 0. There are 

- meridians, all passing through - . To determine 
the longitude, it is necessary to know - . 

True north may be determined by - - star, the 

- compass, or by finding - - shadow . 

*To locate a ship's position at sea, an observation on the sun 
or - - may be made, or if these cannot be seen, mariners use 
or . 


It is noon at any place when meridian. 

Standard time belts avoid the confusion of . When 

daylight-saving time is used, clocks are in the spring 

and in the autumn. 

The civil day is the tune between . 

The international date line passes through the 

meridian, except where that meridian . In traveling 

westward, the date is at this line. 

*The old Roman calendar was based on the . Confusion 

resulted because months in the year is not a - num- 
ber. The Julian calendar was based entirely on the sun, the number 
of months being fixed at twelve, because there are about twelve 

in a year. The discrepancy in this calendar is only - 

days in years. This discrepancy was practically eliminated 

in the Gregorian calendar, by introducing years. 


1. What are parallels of latitude? 

2. How many miles are there in one degree of latitude? 

3. How can true north be found at night? 

4. Explain how to find the latitude of a place by an observa- 
tion on Polaris. 

5. How is latitude determined from the sun? 

6. What are meridians of longitude? 

7. What is one degree of longitude at the equator, in miles? 

8. Why does the degree of longitude get shorter as we go 
away from the equator? 

9. Where is the prime meridian? Why? 

10. How can the longitude of a place be found? 

11. When is it noon at a given place? 

12. When it is noon at a certain place, it is 5.00 P.M. at Green- 
wich. What is the longitude of this place? 

13. How can a north-south line be found by means of shadows? 

14. How can the time be determined if the longitude is known? 

15. What is "mean solar time"? 

16. What is standard time? Why is it necessary? 

17. How many standard time belts are there in the United 
States? Name at least one large city in each time belt. 


18. Explain the advantage of daylight-saving time. 

19. Explain the need for an international date line. 

20. If it is Thursday just east of the international date line, 
what day is it west of the line? 

21. When it is noon at New York, what time is it at Chicago? 
at San Francisco? at London? 

22. What is meant by the conventional day? 

^Optional Exercises 

23. Explain why the gyro-compass points true north. 

24. Explain how to determine the position of a ship by dead 

25. Explain the use of the radio position finder. 

26. How can the fathometer be used to find the position of a 

27. Explain the difficulties with the Roman calendar. 

28. How did the Julian calendar remedy the defects of the old 
Roman calendar? 

29. Explain how the Gregorian calendar met most of the dis- 
crepancies of the old calendar. 

30. What is meant by O.S. and N.S.? 



249. Distance and size. The moon's average distance 
from the earth is about 240,000 miles. The actual distance 
during a single month varies about 30,000 miles, causing a 
corresponding variation in its apparent size. 

The diameter of the moon is 2,163 miles or about one 
quarter of the earth's diameter. The earth has about 50 
times the volume of the moon. 

250. Motion of the moon. The apparent motion of the 
moon and stars, by night, and the sun, by day, is due to 
the earth's rotation from west to east. There is a real east- 
ward motion of the moon, as may be seen by noting from 
night to night the position of the moon among the stars. 

*Since the moon makes one complete revolution about the earth 
in 27J days, the eastward motion is about 13 a day (360 * 27) ; 
and as the sun also appears to move eastward among the stars 
about 1 a day, the eastward daily gain of the moon is about 12. 
That is to say, the moon rises about 50 minutes later each day 
(12 -^ 360 X 24 hours). 

*251. The moon has no atmosphere. The absence of an atmos- 
phere is shown by the fact that when the moon hides a star, the 
star disappears suddenly and not gradually, as it would if its light 
passed through an atmosphere. 

There seem to be no effects of erosion on the moon, which also 
goes to show that there is no atmosphere. If the moon ever had an 
atmosphere at any stage of its development, it has lost it. 

Neither is there any water, for if there were, it would evaporate 
during the long hot day and form an atmosphere of cloud and 

*252. Surface of the moon. Moonlight is but reflected sunlight. 
The surface markings of the moon are known to be due to uneven- 



ness. The visible surface has an area about the size of South 
America and nearly one half of this is covered with dark gray 
patches which were once supposed to be seas. The rest of the 
surface consists of mountains, so-called volcanoes, craters, and 
ringed valleys (Fig. 226). Some of the mountain chains have 
peaks nearly four miles high. 

FIG. 226. The Moon's Surface 

253. Same face is always toward the earth. Since the 
same side of the moon is always turned toward the earth, it 
follows that the period of rotation of the moon on its axis 
and its period of revolution about the earth are the same, 
about 27 J days; but owing to an apparent oscillation, we see, 
throughout the month, about six tenths of the lunar surface. 


We know nothing, therefore, about the other four tenths of 
the surface of the moon. 

The side of the moon that is toward the sun is always 
brightly illuminated, and the side turned away from the 
sun is in darkness. As the moon makes its way eastward 
around the earth, different portions of the illuminated surface 
are seen. This causes the phases of the moon. 

FIG. 227. The Phases of the Moon 

The same half of the moon is always illuminated by the sun, but we on the 
earth can see only the part of this illuminated surface between the dotted lines. 
These parts, shown next to each position, are called the phases of the moon. 

254. New moon. When the moon and the sun are on 
the same side of the earth, the dark side of the moon is 
turned toward the earth, and we have new moon (Fig. 227). 
New moon occurs, strictly speaking, when none of the 
bright surface is visible. Popularly, the moon is said to be 
new when seen as a very thin crescent. A day or two later, 
when the moon has moved a little eastward of the sun, we 
may see in the early evening, in the western sky, a small 
portion of the illuminated half in the form of a crescent, 
convex westward, or toward the sun, with the horns turned 
eastward, or away from the sun. 


255. First quarter. A week after new moon, half of the 
illuminated hemisphere may be seen. The moon has now 
reached first quarter, and its shape is that of a half circle 
convex to the right. A line connecting it with the earth is 
at right angles to a line connecting the sun and the earth. 

As the moon passes beyond the first quarter, the boundary 
line between the light and the dark area begins to be con- 
vex eastward and the illuminated portion continues to grow 

256. Full moon. When the moon and the sun are on 
opposite sides of the earth, the whole lighted half of the 
moon is turned toward the earth, and we have full moon, 
about a week after the first quarter. The line dividing the 
light and the dark areas, after full moon, changes from the 
left side to the right side of the moon's disk. 

257. Third quarter. The moon reaches the last or third 
quarter about a week after full moon. In this phase the half 
circle is convex toward the left. After third quarter, the 
moon being west of the sun, the crescent curves to the left 
or toward the sun, and the horns point to the right, away 
from the sun. 

258. Waxing and waning. In its revolution from new 
to full moon, the visible illuminated area increases and the 
moon is said to wax. 

From full to new moon, the illuminated area decreases 
and the moon is said to wane. 

259. Earthshine. The dark portion of the moon is some- 
times lighted by sunlight reflected from the earth, called 
earthshine. This occurs at the young and old crescent phases, 
and makes the entire disk of the moon faintly visible. 

260. Eclipses. All the planets and their satellites are 
opaque bodies and cast long, cone-shaped shadows, away 
from the sun (Fig. 228). The moon is eclipsed when it 
passes into the earth's shadow. The sun is eclipsed when 
the moon passes between it and the earth. During a lunar 
eclipse the moon is really darkened, light from the sun 


being cut off by the earth. During a solar eclipse the sun is 
only apparently darkened; the moon cuts off light that 
would otherwise reach the earth. In reality it is the earth, 
rather than the sun, that is eclipsed. 

261. Lunar eclipses. In Fig. 228 the moon is passing 
through the earth's shadow and is totally eclipsed. The 

FIG. 228. Eclipses of the Sun and Moon 

moon's disk is usually visible at this time, because of sunlight 
refracted into the earth's shadow by our atmosphere. This 
gives to the moon, during a total eclipse, a dull, copper- 
colored appearance. 

When the moon passes slightly above or below the center 
of the earth's shadow, and only a part of the moon's disk 
enters the shadow, a partial lunar eclipse occurs. 

The moon in its monthly revolution about the earth 
usually escapes the earth's shadow entirely and it is there- 
fore not eclipsed. 

262. Solar eclipse. The moon, being an opaque body, 
casts a shadow; and when the moon is between the sun and 
the earth, this shadow may be long enough to reach the 
earth and cause a total solar eclipse for those who are in the 
path of the shadow (Fig. 228). For an observer so located, 
the moon appears to move across the face of the sun and to 
cut off the light from the sun (Fig. 229). 

The eclipse begins when the black body of the moon 
appears to cut a notch from the edge of the sun. As the moon 
moves over its face, the sun appears as a diminishing cres- 


cent; and as the moon moves off, an increasing crescent is 
seen with the horns turned in the opposite direction. 

Near the beginning of totality, the pearly white halo of 
the corona of the sun flashes out and this is visible during 
totality, which is never more than about eight minutes. 
The time of the entire eclipse is about one hour. 

FIG. 229. Solar Eclipse 
The moon is moving to the right. 

Should any of the brighter planets happen to be near the 
sun during totality, they can generally be recognized be- 
cause the sky is almost as dark as night. Usually a few of 
the brightest stars also appear. 

Several streamers of light, equal in length to the diameter 
of the sun, and longer, may be seen extending out from the 
corona. Near the sun, red flames known as prominences 
appear. As the path of the moon's shadow on the earth is 
always less than 170 miles wide, it is an event of a lifetime 
for a person to see a total eclipse of the sun without making 
a journey to distant places. 

In consequence of the eastward motion of the moon 
about the earth, the shadow moves eastward and with 
great velocity. 

The total eclipse of the sun is of much scientific interest 
to astronomers, because it enables them by means of the 
spectroscope to discover new elements in the sun's corona, 
because they can then obtain more accurate data concern- 
ing the moon's motion, and can study light and shadow 


An observer in the penumbra (the outer part of the 
shadow from which the light is not entirely cut off) of the 
moon's shadow would see only a part of the sun's disk. That 
is called a partial eclipse. 

*Sometimes the moon's shadow is not long enough to reach the 
earth, and the moon passes centrally across the sun's face, leaving 
a ring of the sun exposed. Such an eclipse is said to be annular. 
The moon appears as a black spot covering the central portion of 
the sun's disk, surrounded by a ring of light. 

*263. Number of eclipses in a year. There are always at least 
two eclipses of the sun in a year, and there may be as many as 
four. The largest number of lunar eclipses in a year is three. As 
every eclipse of the moon is visible at one time from all points on 
half the earth, and eclipses of the sun can be seen from a narrow 
area only, many more lunar eclipses are visible at a given place. 
London during twelve centuries was privileged to see only two 
total eclipses of the sun. 

Completion Summary 

We always see - - moon, because the moon revolves 
in the same time it takes to - on its axis. 

When both moon and - are on the same side of the 
earth, we have - .A week afterward, when half of the 
surface of the moon turned toward us is , we have 

. The illuminated surface increases in area until fully 

of the moon is seen. This phase is called . 

From then on, less and less of the moon is seen each night, 
until we get - . The moon is said to wax and wane as 
the illuminated portion - . 

An eclipse of the moon is caused when the earth 

sun. An eclipse of the sun - - moon - sun. When 

the moon hides only part of the sun, the sun is said to be 

in . 


1. What is the approximate distance of the moon from the 
earth? How many times as far away is the sun? 


2. What is the approximate diameter of the moon? 

3. Why does the moon seem to be moving toward the west? 

4. What is the period of revolution of the moon? 

5. Why do we always see the same side of the moon? 

6. What is meant by the new moon? How long is it visible? 

7. What is meant by first quarter? 

8. What conditions account for the full moon? 

9. What are the phases of the moon? 

10. What is meant by third quarter? 

11. What is the meaning of the expression "the moon waxes 
and wanes"? 

12. Account for earthshine. 

13. What causes a solar eclipse? Why can it not be seen all 
over the earth? 

14. What is meant by a partial eclipse of the sun? 

if Optional Exercises 

15. Explain why the moon rises 50 minutes later each day. 

16. What evidence convinces us that the moon has no atmos- 

17. Explain the difference between a phase of the moon and a 
lunar eclipse. 

18. What is an annular eclipse? 

19. Why are more total eclipses of the moon than of the sun 
seen at any place? 

20. The time from full moon to full moon, called a lunar month, 
is 29.5 days, while the actual time of revolution of the moon about 
the earth is 27.3 days. To what is this difference due? 



264. Solar system defined. The sun together with the 
bodies revolving about it is called the solar system. The 
members of the solar system are the sun, the planets and 
their satellites, some comets, and meteors. 

The sun, near the center of the system, is a very large, hot, 
luminous body, giving heat and light to the other members 
of the solar system. Its gravitational attraction controls 
their motions. 

The planets, nine in number, upon one of which we live, 
revolve about the sun in elliptic orbits, in different periods 
of tune, and at different distances from the sun (Fig. 1). 

Planets are distinguished from stars by their changing 
position among the stars, and by their visible disk when seen 
through a telescope. Stars seem to twinkle; planets do not. 

Stars keep their relative position in the sky and through 
a telescope appear as points of light. The following table 
gives some information about the planets which is meant to 
be merely comparative and should not be memorized. 




















































Reliable information about Pluto, the furthermost and 
probably the smallest of the planets, is not yet available. 

All except two of the planets have satellites revolving about 
them. The satellites are very unevenly distributed among 
six of the planets as seen in the table. Our moon is a satellite. 

*The planetoids (planetlike bodies), about a thousand in num- 
ber, are small bodies, the largest being about 500 miles in diameter, 
that revolve about the sun between the orbits of Mars and Jupiter. 
The planets between the planetoids and the sun are much denser 
than the others. 

Comets are bodies that are temporarily visible, of large dimen- 
sions and small mass, unstable in form, usually with long tails and 
with uncertain orbits. They are believed to be entirely gaseous. 

FIG. 230. A Comet 

Some comets revolve about the sun in closed orbits, have fairly 
definite periods of revolution, and are consequently members of 
the solar system (Fig. 230). Other comets, with open orbits, enter 
and leave our solar system without seeming to be members of it. 

Meteors are comparatively small masses of stone or metal 
(Fig. 231) that enter the earth's atmosphere from space, and, as 
they do so, they glow because they get hot from the friction of the 
atmosphere. Millions of meteors fall on the earth and those which 
become visible are called shooting stars. 

*265. Size of the solar system. An idea of the size of the solar 
system may perhaps be gained from Sir Wm. Herschel's apt 
illustration : 

Choose any well-leveled field. On it place a globe two feet in 
diameter. This will represent the sun. Mercury will be represented 


FIG. 231. A Meteor 
The hammer shows the comparative size of the meteor. 

by a grain of mustard seed on the circumference of a circle 164 feet 
in diameter, for its orbit; Venus, a pea, on a circle of 284 feet in 
diameter; the earth, also a pea on a circle of 430 feet; Mars, a 
rather large pin's head on a circle 654 feet; the asteroids, grains of 
sand in orbits 1,000 to 2,000 feet; Jupiter, a moderate-sized orange 
in a circle of nearly half a mile across ; Saturn, a small orange on a 
circle of four fifths of a mile ; Uranus, a full-sized cherry or a small 
plum upon the circumference of a circle more than a mile and a 
half; and finally Neptune, a good-sized plum, on a circle about two 
miles and a half in diameter. 

266. Space outside the solar system. When we look 
about us in space we find we are rather isolated, the nearest 
star being 25 million million miles away. The stars, which 
are very numerous, are suns like ours, but whether each 
one is the center of a system of planets like ours we do not 
know; but modern astronomers are inclined to believe that 
our solar system is a freak and does not often occur in the 
universe. When speaking of the stars we usually refer to 


those found in the Milky Way, the galaxy to which we 
belong. It has been estimated that there are about 3,000 
million stars in our own galactic system. 

The spiral nebulae (Fig. 232) are believed to be galaxies 
like ours. In fact, stars can be seen in some of them. 

FIG. 232. Spiral Nebula 

The nearest galaxy to ours is one million light years distant. 
Light travels 186,000 miles per second. How far would it 
travel in a year? That distance is a light year. It is about six 
million million miles. Hence the nearest galaxy is a million 
times as far as that. 

267. The sun. Our sun is a rather small star; Antares 
has a diameter 400 times that of the sun. But the sun is the 
largest body near us. It has a diameter of 864,000 miles, 



about 100 times that of the earth, and it has about one 
quarter the density of the earth. 

The visible surface of the sun is called the photosphere. 
It is cloudlike in appearance and gives forth most of the 
light and heat which the sun radiates. 

268. Elements in the sun. By means of an instrument 
called the spectroscope it is possible to analyze the sun, 

Photograph by Mt. Wilson Observatory 

FIG. 233. Sunspots 

and we find that the same elements which make up the 
earth, oxygen, hydrogen, carbon, iron, and others, also make 
up the sun; 66 of them appear in the sun's atmosphere. 

*269. Sunspots. Dark spots of irregular outline, called sun- 
spots, often many thousands of miles in diameter, mar at times the 
brightness of the photosphere (Fig. 233). These sunspots are be- 
lieved to be connected with the hidden circulation in the great body 
of the sun below the photosphere, and are dark only by com- 
parison with it. 


Observers of sunspots soon found that the sun turns on its axis 
from west to east. 

The earth's magnetism is disturbed during a period of sunspot 
activity. A large number of sunspots appear and a greater develop- 
ment of solar prominences occurs most frequently at these times. 
The period of maximum disturbance occurs on an average about 
every 11 years. 

As the sun rotates on its axis in about 26 days, no spot would 
remain continuously visible for more than 13 days or half the 
period of rotation. Some spots last, however, only a few days; 
others persist for months. They are now believed to be solar 
storms of great violence. 

*270. The chromosphere. Outside the photosphere is a deep 
envelope of gas, mostly hydrogen and helium, called the chromo- 
sphere or color sphere. Portions of the hydrogen are thrown up in 
huge tonguelike flames, called prominences. 

When the moon comes between the earth and the sun, the light 
from the photosphere is cut off and the sun is said to be eclipsed. 
During a solar eclipse the chromosphere can be seen as a brilliant 
scarlet ring, with the prominences shooting out thousands of 

*271. The corona. The outermost portion of the sun is the 
corona (crown), a halo of pearly light extending out many thou- 
sands of miles, with streamers reaching out millions of miles. 
(See frontispiece.) It is believed that the light of the corona is 
due to the reflection from dust particles, liquid globules, and small 
masses of gas. 

*272. The sun's heat. The temperature of the sun near its 
surface is at least 10,000 F. and it is radiating energy at such a 
rate that if the source of the heat were due to ordinary burning, 
even if the sun were made entirely of coal, oil, and other combusti- 
bles, it would have burned out in 5,000 years. But we have evidence 
that its temperature has not even diminished noticeably within 
geologic history. What then is the source of its heat? The latest 
theory, held by Eddington and Jeans, is that, in the sun, atomic 
disintegration and creation may be going on all the time. If simple 
atoms were to be built up into complex ones, a certain amount of 
the mass would be converted into energy and set free. 

For example, if hydrogen were to be converted into helium, 


4.032 pounds of hydrogen would yield 4.000 pounds of helium and 
0.032 pounds would be transformed into energy. And so enormous 
would be the energy set free that if this change went on every 
second, it would be equivalent to the generation of 1,800 billion 

If that is what is going on in the sun, then its matter is being 
consumed, but at such a slow rate that it can continue to radiate, 
at its present rate, for 10 million million years. 

*273. Mercury. So far as is known at present, Mercury is the 
smallest planet, the nearest to the sun, and the swiftest in its 
movements about the sun. It can be seen only in the direction of 
the sun, during early twilight or late dawn. 

Mercury has a thin atmosphere, if any. It has surface markings 
of permanent streaks and a known rotation period equal in length 
to its year of 88 days. Since the periods of rotation and revolution 
are the same in length, Mercury always turns the same side to the 
sun. This side has perpetual daylight and summer, while the other 
side is always cold and in darkness. 

274. Venus. Venus shines in the sky with peculiar brightness. 
It is only a little smaller than the earth. The period of rotation is 
255 days and also equal to the period of revolution. 

Mercury and Venus pass between the earth and the sun and 
therefore present all the phases similar to those of the moon. The 
passages are called transits and occur at irregular and relatively 
long intervals of time. During these passages Venus and Mercury 
look like small, round, black spots passing across the sun. 

275. The earth. Although we know that the earth is a 
planet moving about the sun like the other planets, the 
earth seems to us to be the center about which other heavenly 
bodies move. The earth has the general form of the other 
planets, that of a spheroid. It is the third in distance from 
the sun, and the largest of the four smaller planets whose 
orbits lie within those of the planetoids. The earth makes 
365J rotations during one revolution. 

276. Mars. Mars appears in the sky, shining with a 
steady, pale-red light. Though having only a little more than 
half the diameter, Mars resembles the earth in more re- 


spects than any of the other planets. Its period of rotation 
is 24 hours 37 minutes, or a little more than our day. The 
inclination of its axis is about 24, ours is 23.5. Therefore, 
except for its greater distance from the sun, the days and 
change of seasons resemble those on the earth. 

Surface markings on Mars indicate to some astronomers 
snow fields and canals. There seems to be little doubt about 
the white polar caps that appear and disappear according 
to the season. It is not certain however that they are fields 
of snow. 

Although we have as yet no evidence for a positive state- 
ment concerning inhabitants on Mars, it may be said that, 
if any of the planets, other than earth, is inhabited, it is 
probably Mars. 

*277. Jupiter. Jupiter is the largest of all the planets, and with 
the exception of Venus, often the brightest in the sky. Surface 
markings on Jupiter are described as parallel belts and spots. 
Because of the lack of permanency of the markings, they are 
thought to be due to a deep atmosphere surrounding the planet. 
From observations of the spots it has been found that Jupiter has 
a rotation period of about 10 hours, the shortest of any of the 

The circumference is about 11 times that of the earth. At its 
equator it is rotating about 30,000 miles an hour, about 30 times 
as fast as the earth. 

The outer five planets, Jupiter, Saturn, Uranus, Neptune, and 
Pluto, are supposed to be of a higher temperature, of less density, 
and in not so advanced a stage of development as the other four 

278. Saturn. Saturn is distinguished from all other planets by 
three thin, flat, meteoric rings about 100 miles thick (easily visible 
through a small telescope), which surround it in the plane of its 
equator. The rings are together about 40,000 miles wide and the 
inner edge less than 6,000 miles from the planet (Fig. 234). 

At distances ranging from 100,000 miles to nearly 8 million miles 
from Saturn, are 10 satellites, more than have yet been discovered 
belonging to any other planet of the solar system. 



FIG. 234. Saturn 

The surface markings on Saturn are not seen nearly so well as 
those on Jupiter, because Saturn is nearly twice as far from us. 
There are bright and dark belts, and at times faint spots. 

Saturn rotates on its axis in about 10 hours. Because Saturn has 
a density less than water, it is believed to be largely in a vaporous 
condition. It may be seen shining in the sky with a steady yellowish 
light, with about the same degree of brightness as the brightest star. 

*279. Uranus, Neptune, and Pluto. Uranus was discovered in 
1781, Neptune in 1846, and Pluto in 1930. All the other planets 
were known to the ancients. 

Uranus is a very faint object in the sky; Neptune and Pluto are 
invisible to the naked eye. Pluto has the longest period of revolu- 
tion, equal to 300 earth years. 

It may be inferred that the physical condition of these planets 
is much the same as that of Jupiter and Saturn. The rotation period 
of Uranus, as indicated by surface markings, is between 10 and 
12 hours. 

Since these planets are so far from the sun, they receive very 
little heat per unit area compared with the earth. 

280. Characteristics of a planet. 

1. The planets move in the same direction about the 
sun, from west to east. The sun rotates in the same direction, 
which, as viewed from above the North Pole of the earth, is 
counterclockwise . 


2. The paths, or orbits, of all the planets are ellipses, 
with the sun at one of the foci. 

3. The planets are nonluminous, like the earth; the light 
coming from them is reflected sunlight. 

4. Most of the planets are known to rotate in the same 
direction as the earth, from west to east. 

5. All the planets, as well as the sun, are composed of 
the same elements. 

*281. The satellites. Previous to 1610 the only known satellite 
was our moon. In that year Galileo first pointed his telescope to 
the sky and saw four large moons of Jupiter. 

Our moon is more than 2,000 miles in diameter, but not so large 
as three of the 8 moons of Jupiter and one of the 10 moons of 
Saturn. The largest satellite of Jupiter is 3,558 miles in diameter, 
considerably larger than the planet Mercury. The smallest satel- 
lites known are the two belonging to the planet Mars, both of which 
are probably less than 10 miles in diameter. One of the two is only 
5,800 miles distant from Mars and makes a revolution in less than 
8 hours, one third of the time it takes Mars to rotate. 

The earth's satellite, the moon, is about 240,000 miles distant 
from the earth and makes a complete revolution in about 27J 

The most distant satellite of Saturn takes considerably more 
than an earth year to make one revolution. 

Our moon is larger compared to the earth than any other satel- 
lite compared to its planet. Mercury and Venus have no satellites, 
Uranus has four, and Neptune one. 

*282. Planetoids. The planetoids, sometimes called asteroids, 
move about the sun just as the planets do. They are so small that 
they are invisible to the naked eye. Jeffreys thinks that they may 
have originated from the explosion of a planet. 

Not until the beginning of the nineteenth century were any of 
these bodies discovered, but a formula for the distances of the 
planets from the sun called for a planet between Mars and Jupiter. 
Long and careful search led to the discovery of Ceres, 485 miles in 
diameter. There are several whose diameters are more than 100 
miles, but the majority are much smaller, ranging down to 10 
miles in diameter. New ones are being found every year. 


Over 1,300 planetoids have been found, all together having about 
one five hundredth the mass of the earth. 

Each one has an orbit and an inclined axis like the planets. 

*283. Comets. Comets are in strong contrast with planets in 
appearance and physical condition. Most of them enter the solar 
system with orbits in the form of open curves, make one turn about 
the sun, and pass away, probably forever. 

Of the few comets that belong permanently to the solar system, 
all have definite periods of revolution about the sun, varying from 
3.3 years (Encke's comet) to about 76 years (Halley's comet). 
Halley's comet last appeared during May, 1910. 

The typical comet is largely self-luminous and is composed of a 
head and a tail. In the center of the head is a bright, starlike nucleus 
surrounded by faintly luminous matter, called the coma. The tail 
acts as your shadow does when you walk around a lamp. It always 
points away from the light. Some astronomers maintain that it is 
the pressure of sunlight that drives the gaseous particles from the 
nucleus, and thus forms the comet's tail. 

The head may have a diameter greater than that of the sun, 
with a nucleus as large as the earth, and the tail equal in length 
to the distance of the earth from the sun. The amount of matter 
in a comet is very small, in most cases less than one millionth of 
that of the earth. 

The orbits of the planets are slightly elliptical, and all are ap- 
proximately in one plane ; those of the comets are greatly elongated 
and lie in every possible position. With the unaided eye it is a rare 
sight to see a comet. 

Halley's comet has been pursuing its fixed orbit about the sun 
since the dawn of history and undoubtedly long before. The ac- 
counts of many of its earlier appearances seem to indicate that it 
has often been a conspicuous object in the sky. The last appear- 
ance, during May, 1910, was disappointing. This tends to show 
that the great comet has for ages been slowly disintegrating. 

Under the most favorable conditions the nucleus of Halley's 
comet was brighter than stars of the first magnitude, the coma was 
a faint light, and the tail was a band of light about 8 wide at its 
widest place and 120 long. Stars were plainly visible through the 
comet's tail. It is believed that the earth passed through the tail 
on May 18, 1910. At that time there were no unusual manifesta- 


tions seen, such as the falling of an unusual number of meteors, a 
glow of the sky, or the appearance of deadly gases, all of which had 
been predicted. 

*284. Meteors. The earth in its path about the sun encounters 
daily many millions of small bodies which enter its atmosphere 
from outside space. On a clear, moonless night, one may see several 
an hour. They often appear at altitudes of 100 miles, move many 
miles a second, give out light and heat, and are usually consumed 
before they reach the surface of the earth. These bodies are called 

The appearance of a large number of meteors, usually in August 
and November, is known as a meteoric shower. Sometimes bodies 
weighing from a few pounds up to several tons fall to the earth's 
surface, unconsumed. Such bodies are known as meteorites. Some 
are composed of nearly pure iron, with a little nickel. Most 
meteorites are composed of stone, often with traces of iron in them. 
About thirty of the different elements found in the earth have been 
found in meteorites. 

285. Nebular hypothesis of Laplace. Many hypotheses 
concerning the origin of the solar system have been pro- 
posed, but the one that has exercised the greatest influence 
upon thinking people of the past century is the nebular 
hypothesis, as formulated by Laplace. This hypothesis main- 
tains : 

1. That the matter of the solar system was once a highly 
heated mass of gas called a nebula. 

2. That the form was a vast spheroid extending beyond 
the orbit of the farthest planet. 

3. That the nebula was in process of cooling, and the 
cooling caused shrinkage. An effect of shrinkage was to 
increase the rate of rotation, and this increased the equa- 
torial bulge. 

4. That when the rotation increased to a certain speed, 
the centrifugal force at the equator of the spheroid equaled 
the attraction due to gravitation. Upon further cooling and 
contraction, the equatorial portion separated from the great 
rotating mass, forming a ring resembling the rings of Saturn. 


5. That as the cooling and contraction of the spheroid 
continued, additional rings were separated. The first ring 
gave rise to the outermost planet, and the later ones to the 
other planets in turn. 

6. That the central body was the sun. 

7. That each ring parted at its weakest point as cooling 
caused contraction, and the matter was ultimately col- 
lected into a planet, which was hot and gaseous. 

8. That the cooling of the planet caused further con- 
traction, which in turn increased the rate of rotation and 
consequently the amount of bulging. Some of the planets 
followed the example of the parent nebula and formed rings 
which became satellites. 

9. That as the cooling and shrinking went on, the gases 
changed to a liquid and then to a solid state. In the case of 
the earth, the volume decreased from a rotating mass of 
gas extending to the orbit of the moon, to its present 

10. That the more volatile material of the earth remained 
in a gaseous state and formed our atmosphere, originally 
much deeper and of a higher temperature than now. As 
the atmosphere cooled, the water vapor condensed and 
formed clouds. As cooling continued, rain fell and the oceans 

*286. How well does the nebular hypothesis explain the facts? 
The nebular hypothesis explains the following facts rather well: 

1. All the planets are composed of the same elements because 
they are all derived from a common nebula. 

2. They revolve about the sun in the same plane, in the same 
direction, and in nearly circular orbits. 

If they were split off from the sun as circular rings which were 
revolving with the sun, they would naturally continue to revolve 
on that same circle in the same direction and in the same plane, 
because by the law of inertia they would change only when acted 
on by an outside force, and there is none. 

3. They are at different distances from the sun because, as each 


ring was split off, the parent sun became smaller and the next ring 
was consequently smaller. 

4. The sun is hot because it was originally hot. 

5. The interior of the earth is hot because it was originally 
hotter and has cooled only on the surface. 

6. The earth has an atmosphere, which is naturally what is left- 
after all the other substances had cooled and changed to solids or 

The nebular hypothesis does not explain the following facts 

1. The sun is rotating, but rather slowly. 

According to the hypothesis it should be rotating rapidly. 

2. The sun has no equatorial bulge. 

Since the sun is still gaseous, at least on the outside, it should be 
showing a bulge at the equator from which a new ring is about to 
split off. 

3. All satellites do not revolve in the same direction and in the 
same plane. 

There are many more objections which need not be mentioned 
here, but they are so fundamental that it is evident that a solar 
system could never be evolved by such a process as that described 
in the nebular hypothesis. 

287. The planetesimal hypothesis. In 1900 Chamber- 
lin and Moulton, two American scientists, enunciated the 
planetesimal hypothesis, which attempted to overcome the 
objections to the nebular hypothesis. 

The planetesimal hypothesis starts with a mass of hot 
gas, like the nebular hypothesis. A star of large magnitude 
passed near by, and, by gravitation, caused two tidal bulges, 
like the two high tides on the earth, 180 apart. The spiral 
nebula, thus formed, revolved as it was drawn out of its 
original position, and the star passed on its 'way, leaving 
the nebula, with two arms curved about the nucleus or 
sun, to cool rapidly (by expansion) into solid particles. 

The formation of planets then took place as follows: 

1. We started with a cold nebula, spiral in form, which is 
the most common type now seen. 


2. The spiral nebula consisted of a central portion or 
nucleus, which became our sun, with two arms starting 
from opposite sides and curved spirally about the nucleus 
or center. 

3. A significant feature of the spiral nebula was the 
presence of numerous nebulous knots in the arms. These 
knots were the denser portions of the nebula, and the nuclei 
of future planets and satellites. 

4. The knots or nuclei were surrounded by a nebulous 
haze, which was composed not only of gaseous particles 
but also of innumerable solid or liquid particles. These 
particles revolved about the center of the nebula like little 
planets. They are called planetesimals. The nuclei grew and 
became planets and satellites by attracting other planetesi- 
mals. The earth and moon were two companion nuclei of 
unequal size. 

5. The earth developed from a knot in the arm of a 
spiral nebula by the capture of outside planetesimals. The 
increasing gravitational compression of the interior produced 
the internal heat of the earth. 

6. Gases were held in the solid planetesimals as they 
are held in meteorites that now fall to the earth. As the 
growing earth became heated by internal compression, the 
gases were given forth gradually, thus forming an atmos- 
phere about the earth. Until the earth had attained a mass 
greater than that of the moon (^ of the earth), its gravity 
was probably insufficient to enable it to hold the gases of 
an atmosphere such as we now know. The gases now issuing 
from volcanoes were occluded in the original planetesimals 
which formed the earth. 

7. When the earth had reached such size that water 
vapor was held in the atmosphere in sufficient quantity to 
reach the saturation point, the water vapor began to con- 
dense, and rain fell on the earth. The water ran down from 
the higher to the lower places, where it collected and ulti- 
mately formed the oceans. 


288. The two hypotheses contrasted. 


1. Nebula, hot and large, formed 1. Nebula, cold, formed two arms 
rings around central mass or sun. around central mass or sun. 

2. Rings became planets. 2. Nuclei or knots became planets 

and satellites. 

3. Smaller rings separated from 3. Smaller knots were captured by 
planets and became satellites. larger knots and became satel- 

4. Planets and satellites originally 4. Planets and satellites originally 
hot and large, gradually cooling cold and small, gradually heat- 
and growing smaller. ing and growing larger. 

5. Outermost planet, Pluto, formed 5. Planets and satellites formed at 
first and others at later periods. same time. 

6. Earth always had an atmos- 6. Earth, when small, without an 
phere. atmosphere. 

289. Objections to the planetesimal hypothesis. 

1. We have evidence that the earth has passed through 
a liquid condition, because of the arrangement of its mass 
into a dense core surrounded by less and less dense matter. 

2. Some of the planets seem still to be gaseous and there- 
fore hot. 

3. If the earth were a small solid to begin with, and ero- 
sion started to take place at once, the products of the erosion 
of more than half the earth would produce much more sedi- 
mentary material than we have. There would be, for ex- 
ample, much more salt than there is. The salt in the oceans 
corresponds rather well with erosion of a thin crust. 

There are many more objections, chiefly astronomical, 
and some of these have been overcome by the new tidal 
theory of Jeans and Jeffreys. 

290. Tidal disruption theory of Jeans and Jeffreys. 

1. We start, as in the planetesimal theory, with a tidal 
disruption of the hot gas by a passing star. 

2. The passing of the star left our sun with two revolv- 
ing arms or filaments of hot gas, like a spiral nebula. 

3. The cooling of each filament broke it up into 5 masses 
which began to contract toward nuclei, forming 5 planets 


from one filament and 5 from the other; but one of them, 
the planetoids, remained separate, having no definite nu- 

4. The outer planets would be less dense than those nearer 
the sun, and this agrees with the facts. 

5. Those nearer the sun would lose much of their outer, 
less dense material to the sun by attraction, making them 
denser. This also agrees with the facts. 

6. The satellites were formed when the star ceased its 
attraction and the planets were snapped back by the at- 
traction of the sun. As each one passed the sun, a tidal 
disruption occurred and formed satellites. 

7. The gaseous planets and their satellites continued to 
revolve, as they did after the star passed away. 

8. Gradual cooling formed liquids and finally solids, 
while gases remained outside as an atmosphere. 

This theory seems to agree with more of the facts than 
either of the others. 

Completion Summary 

The solar system consists of the sun and which 

about the sun in orbits. Our sun is a luminous 

body like the . It is a member of the galaxy called 

. The nearest galaxy to ours is one million 


The sun is composed as the earth. 

The surface photosphere. Outside of that 

chromosphere. The outermost portion or crown is called 

*The heat of the sun cannot be due to , or it would have 

in 5,000 years. The latest theory accounts for the enormous 

heat as due to building up of complex from simple - , with 
transformation of some of the mass into - . 

Many of the characteristics of the planets a com- 
mon origin. 


A comet seems to have an orbit, but most of them . 

Halley's comet seems to have an orbit around . 

*Meteors are - , which enter the - . The planetesimal 
hypothesis would call them - . 

The nebular hypothesis attempts to explain the origin 

of - . It all started as a hot , extending from the 

sun out beyond the orbit of the planet . It cooled 

and - , causing it to increase its rotation. Centrifugal 

force caused the central to bulge, and finally to throw 

off a - . Nine rings in all were thrown off, which 
formed - . 

The planetesimal hypothesis starts with nebula. 

The passing of a star near by caused . These two arms 

broke up - . The planetesimals collected about - 

to form - . The planets continued to grow by 


The tidal disruption theory of Jeans and Jeffreys starts 

like - , but instead of cold planetesimals, , the 

two arms or filaments of the nebula remained . 


1. What is meant by the solar system? 

2. Name the planets of the solar system. 

3. What is the difference between a planet and a star? 

4. What is a satellite? 

5. What is a galaxy? What is our galaxy called? 

6. What is a spiral nebula? 

7. What is a light year? 

8. How far away is the nearest neighboring galaxy? 

9. About what is the diameter of the sun? the earth? 

10. How does the composition of the sun compare with that of 
the earth? 

11. What is the shape of a spheroid? 

12. In what respects does Mars resemble the earth? 

13. In what direction do all the planets revolve? 

14. What is the shape of the orbit of each planet? 


15. What is the direction of rotation of each planet? 

16. How do the planets compare with each other in composi- 

17. In what respects does the planetesimal hypothesis differ 
from the nebular hypothesis? 

18. What are the main features of the tidal disruption theory 
of Jeans and Jeffreys? 

19. Name one fact explained by the tidal disruption theory 
which could not be satisfactorily explained by either of the other 

^ Optional Exercises 

20. Where are the planetoids? What is supposed to be their 

21. What is a comet? 

22. What is a meteor? 

23. How do we determine what elements are present in the sun? 

24. What are sunspots? How did they help us to discover the 
sun's rotation? 

25. What is the photosphere? the chromosphere? 

26. What is the corona? 

27. Explain the source of the sun's enormous energy. 

28. If the theory is correct, how long can the sun continue to 
radiate energy at its present rate? 

29. Which planet has no change of season? Why? 

30. Which of the planets show phases like the moon? 

31. Which of the other planets is most likely to be inhabited? 
Why do we think so? 

32. Which is the largest planet? What is its period of rotation? 

33. What distinguishes Saturn from the other planets? 

34. Which planet has the longest year? 

35. Name a planetoid. How large is it? 

36. In what ways do the planetoids resemble planets? 

37. In what way does a comet differ from a planet? Name a 

38. How does the composition of a meteor compare with that 
of the earth? 

39. State one fact not satisfactorily explained by the nebular 



291. No part of his environment is of more immediate 
concern to man than the air he breathes. If it is pure, he is 
strong; vitiate it and he sickens. Withdraw it for but a few 
minutes and he dies. 

No other part of his environment has had so great an 
influence in helping or retarding him in his struggle for 
existence, or in his effort to improve his condition. How he 
dresses, what he produces, and what he eats are matters 
chiefly of weather and climate. Too great heat and too great 
cold, both of which depend so much on the air, are alike 
deadening to his striving for better things. 

The savages of equatorial Africa and the Eskimos of 
the frozen North are both low in the scale of civilization; the 
former, because the enervating climate kills ambition; the 
latter, because providing for his mere physical needs ex- 
hausts his energies and leaves no time or desire for the 
cultivation of higher things. Both must adapt themselves to 
their climatic environment; neither can change it. 

The sun and the atmosphere are the chief factors in 
weather and climate. 

The atmosphere is the outer or gaseous envelope of the 
earth. The air fills all mines, caves, and underground passages. 
As ground air, it penetrates the soil and, in solution, it is 
carried to the greatest depths of rivers, lakes, and seas. 

292. How high is the atmosphere? We know from ob- 
servations made by aviators and by means of trial balloons 
that the density of the air decreases with distance from the 
earth; that is, it is rarer, higher up. But how high does the 
atmosphere extend above the earth? At even two miles above 



the earth breathing becomes difficult, and aviators who go 
higher are supplied with tanks of oxygen. The only direct 
evidence we have regarding the upper reaches of the at- 
mosphere is based upon the glowing of meteors which fall 
on the earth. When they enter the earth's atmosphere, air 
friction finally makes them hot enough to glow, so that 
they can be seen; and by trigonometric methods, their height 
can be calculated. The highest observed was at about 
200 miles. 

But that is not to say that the atmosphere is only 200 miles 
high. For the meteors must travel far in the rarer atmos- 
phere beyond 200 miles before they become red hot. And 
so we can say the atmosphere is much more than 200 miles 
high. Theoretically, it gets rarer and rarer and extends in- 
definitely out into space. 

All we can say with assurance is that we live at the 
bottom of an ocean of air, which extends far beyond 200 miles 
and becomes less and less dense. 

293. Properties of air. Pure air is an invisible mixture of 
gases; colorless, odorless, and tasteless; perfectly elastic and 
easily compressed. It is very mobile, and, like all matter, 
it has weight: about one twelfth of a pound per cubic foot. 
When cooled to a low temperature and compressed, air 
changes to a liquid. 

Compressed air is used to run the drills in deep mines 
because it furnishes ventilation as well as power. 

The inertia of the air causes resistance to motion, retard- 
ing the speed of the runner, the automobile, and the air- 
plane. On the other hand, it makes possible the flight of 
birds and of aircraft. 

Moving air produces pressure on surfaces, which is made 
use of in driving ships and windmills, and which has to be 
reckoned with in designing tall buildings. 

The buoyancy of air aids in the distribution of seeds and 
the sailing of balloons. Just as a piece of wood rises in water 
because it weighs less than its own volume of water, so also 


a balloon rises in air because it weighs less than its own 
volume of air; and it will continue to rise until it reaches a 
level where it weighs the same as its own volume of air. 

294. Composition of air. The lower air is essentially a 
mixture of nitrogen, oxygen, carbon dioxide, water vapor, 
dust, and several rare gases, among which are argon, neon, 
and helium. The approximate percentages of the gases are 
practically the same everywhere, while the water vapor 
varies very much: from 0.2% up to 2.5%. 



Nitrogen 78 

Oxygen 21 
Argon and other rare gases 1 

Carbon dioxide 0.03 

That the air is a uniform mixture is probably due to air 
movements; otherwise we should expect the carbon dioxide, 
which is densest, to settle down and form a layer over the 
earth, as it sometimes does in wells. 

Carbon dioxide, since it is one of the gases expelled by 
volcanoes, is most abundant near active volcanoes. Being 
likewise a product of combustion and decomposition of or- 
ganic matter, it is more abundant in cities than in the open 
country, and more abundant in winter than in summer. Its 
use by growing plants also decreases its summer percentage. 

Water vapor, the most variable component of air and one of 
the most important, is more abundant over the sea than over 
the land. Warm air holds much more water vapor than cold 
air; and therefore equatorial air contains much more water 
vapor than polar ah-; air at sea level much more than higher 
up. When the air ascends it loses its moisture as clouds, 
rain, snow, etc., and above a few miles there is practically 
no moisture in the air. 

Dust is of two kinds, organic and inorganic. Organic dust 
includes microscopic animals and plants, pollen, and fibers 
of cloth and wood. Inorganic dust consists chiefly of smoke 


particles, powdered rock (soil or volcanic ash), and bits of 
metal. Dust is more abundant over the land than over the 
sea and is confined to the lower air. There is usually more 
dust in the city than in the country, and more in dry than 
in rainy weather. Dust storms frequently occur in the cen- 
tral regions of the United States during periods of drought. 

Mountain health resorts are sought partly because of the 
greater dryness of the air and partly because of its freedom 
from dust and disease germs that constitute part of the air 
at lower levels. But the process of air-conditioning, which 
regulates the proper composition of the air, makes traveling 
unnecessary for that purpose alone. 

The invigorating quality of air at sea and after a thun- 
derstorm is due to ozone, a condensed form of oxygen. The 
percentage of ozone increases with altitude. 

The composition of air changes materially as we rise. The 
dust and water vapor decrease rapidly above a few thousand 
feet and the oxygen increases slightly. Carbon dioxide de- 
creases; and hydrogen, which at sea level is less than 0.01%, 
is supposed to rise to 95% at a height of 60 miles. 

295. Functions of the air. Although the most important 
uses of the air are those of its individual components, yet 
the air as a whole has important functions. These are: 

1. Its buoyancy helps birds and aircraft. 

2. Sound is transmitted by air. 

3. Ships and windmills are driven. 

4. Seeds, pollen, microbes are carried. 

5. Rain is distributed over land. 

6. Waves and currents are produced in the sea. 

7. Tornadoes and hurricanes are air in violent motion. 

8. Wind erosion in arid regions 

a. Wears down rock. 

b. Carries away topsoil dust storms. 

c. Deposits dunes and loess. 

9. Aids in cultivation of crops. (Soil without air is not 


296. Functions of oxygen. The oxygen of the air is the 
supporter of the chemical action called combustion. Things 
burn in air because they combine with oxygen. Oxygen also 
combines with substances without burning. This is slow oxida- 
tion. In the rusting of metals, oxygen combines with them, pro- 
ducing heat, but the heat is carried away so fast, by the 
metal, that the metal never gets hot enough to give out light. 
In the animal, oxygen is essential to life. It combines with 
carbon and hydrogen of the animal's tissues to produce heat. 

The readiness with which oxygen unites with most other 
substances makes it active in promoting the disintegration 
of rocks and minerals. It is an important agent in the de- 
composition of dead animal and plant material. 

Oxygen is slightly soluble in water ; and it is this dissolved 
oxygen which is used by marine animals. 

297. Functions of carbon dioxide. The carbon dioxide of 
the air, though of no direct use to animals, is essential to 
the growth of plants. Through the action of sunlight in the 
presence of chlorophyll, the green coloring matter of plants, 
carbon is taken from the carbon dioxide and united with 
other elements obtained from water and other materials ab- 
sorbed through the roots. The plant in this way makes 
its own woody tissue, starch, sugar, proteins, and other 
compounds which it needs in its growth. The oxygen of 
the carbon dioxide is set free and returned to the air. 

Carbon dioxide intercepts long heat waves and prevents 
radiation of heat from the earth. 

When plants decay, or are burned, the carbon stored up 
in their tissues is returned to the air again as carbon dioxide. 

*We have reason to believe that at the formation of the earth's 
atmosphere, early in its history, the carbon dioxide was a major 
constituent of the air. Certainly none of the coal and graphite now 
stored in the rock strata was there at that time, but must have 
been in the air as carbon dioxide; and an estimate, based on the 
amount of coal and graphite in the earth, tells us that there was 
about 40% carbon dioxide in the air at some past time. 


Growth of plants on an enormous scale in the early periods of 
the earth's history (see page 271) extracted most of the carbon 
dioxide from the air, and formed thick and extensive deposits of 
coal, some of which have been changed to graphite. The amount of 
vegetation on the earth today is considerably less than it was in 
some of the past periods. It may be that we have now attained a 
balance: the plants extract carbon dioxide from the air, while the 
animals, together with decaying vegetation and combustion, return 
that same amount of carbon dioxide to the air. Besides its effects 
on living things, carbon dioxide has very important effects on 
rocks, as we have seen. It helps water to dissolve limestone, form- 
ing sinks and caverns; and from the solution, limestone is re- 
deposited as stalactites and other forms. From this solution also, 
marine animals precipitate the limestone to form their shells and 
other hard parts. Carbon dioxide helps in the attack on minerals 
containing iron, giving rocks their rusty appearance. 

298. Functions of nitrogen. Nitrogen is a necessary ele- 
ment of the food of all plants, since it is a constituent of 
all protoplasm, present in the nucleus of every living cell. 
If the soil is lacking in this element, no plant will thrive. 
But nitrogen is not taken in by the plant directly from 
the air; it can make no use whatever of that free nitrogen. 
It needs nitrogen compounds, soluble in water, which are 
absorbed through the roots. When lightning passes through 
air, it unites some of the nitrogen and oxygen into a com- 
pound, which is carried down by the rain. Decaying plants 
and animals give off nitrogen compounds and hence are use- 
ful as fertilizers. 

The class of plants called legumes peas, clover, beans, 
alfalfa, etc. are nitrogen gatherers. Colonies of bacteria, 
on their roots, seem to be able to take nitrogen from the 
air and change it into compounds, so that these plants are 
useful in cultivating other crops. 

Man has learned to make, from air, artificial nitrogen 
compounds to supplement those in nature; and by this 
means he has increased many times the yield of the soil. 

299. Functions of water vapor. The water vapor of the 


air is the source of clouds, fogs, rain, snow, and several 
other forms of precipitation. 

Moist air is less dense than dry air. Like carbon dioxide, 
it absorbs heat rays, and when condensed as clouds it is 
even more effective. 

When it is precipitated as rain, it supplies water for 
growing plants; and as snow it retards radiation and pro- 
tects crops from the intense cold of winter. This function 
of snow is very important in the wheat-growing regions of 
the northwest. 

300. Functions of dust. Dust contains so many things, 
different in different regions, that only a few of its effects 
can be mentioned. The air carries pollen from one plant to 
another, and thereby assists in reproduction of these plants. 
Fermentation and putrefaction are brought about by con- 
stituents of dust, and the germs of many diseases are carried 
through the air. 

Every dust particle in the air is a nucleus about which 
water vapor may condense; consequently, dust in the air 
promotes cloud formation and rainfall. 

Dust scatters the short blue rays of light, making the sky 
blue; and it is only when the air is laden with dust that we 
get beautiful sunrise and sunset effects. 

*301. Origin of the atmosphere. According to the planetesimal 
hypothesis of the origin of the solar system, this planet earth had 
no atmosphere when it was first formed, because it was so small 
that its gravitational influence on gases was too weak to hold 
them, and they were drawn away by other larger heavenly bodies. 
But as the earth grew by attracting other planetesimals, the gases 
expelled from the interior by volcanic emissions were held and 
ultimately formed the present atmosphere. 

According to the nebular hypothesis of Laplace as well as the 
modern Jeans hypothesis, the present atmosphere is a remnant of 
a former much denser atmosphere. Before the earth was formed 
as a solid, it existed as a mixture of gases. As cooling took place, 
one by one the denser materials have liquefied and solidified ; first 
the rocks and the metals, then the liquids like water, each substance 


separating out as the mass cooled below its boiling point. The 
substances left in the air today will turn to liquids if the earth is 
cooled sufficiently. We should then have no water but ice; and 
lakes and seas would be made of liquid air. 

Completion Summary 

The atmosphere is at least - - high. 

Air in motion is called - . It causes erosion 

and dust - . 

The important substances in the air, from the standpoint 
of the physiographer, are , - , - , and . 

Oxygen is essential to - - on the earth, and attacks, 

chemically, - , so that it is an important agent 

of rocks. 

Carbon dioxide is a very important substance for . 

*In the early history of the earth, we believe - 40% 
carbon dioxide, which by growth of plants on an enormous scale 
, leaving the air as it is today, with - - 1% carbon 

Nitrogen is essential for - , and every must 

have - - nitrogen. 

The - - of the air is responsible for most of the changes 
- weather. 

Dust - - clouds and rain. 

According to the planetesimal hypothesis, the earth at 
first - - atmosphere. Our present atmosphere is an ac- 
cumulation . 

*According to the nebular and tidal disruption theories, our 
atmosphere remnant , due to gradual . 


1. How high is the air, according to observations on meteors? 
Why is this not conclusive? 

2. How can it be shown that air has weight? 

3. Compare buoyancy in air and in water by an example. 


4. Which constituent of air varies very much? 

5. What is the approximate ratio, in whole numbers, of nitro- 
gen to oxygen in the air? 

6. Where does the per cent of carbon dioxide increase? 

7. Of what value to plants is carbon dioxide? 

8. Why does the per cent of water vapor in air vary so much? 

9. Name five substances present in dust. 

10. In what respects is country air purer than city air? 

11. Which constituents of air decrease with elevation? 

12. Why do we ventilate our homes? 

13. What effect has oxygen in the animal's life? 

14. What is the effect of oxygen on metals? 

15. How does oxygen affect rocks? 

16. How does carbon dioxide indirectly affect the lives of 

17. What is the effect of carbon dioxide on rocks? 

18. What rocks owe their origin to carbon dioxide? 

19. What effect has carbon dioxide on marine animals? 

20. Why is nitrogen vitally necessary to plants and animals? 

21. In what form is nitrogen used by plants? In what form is it 

22. Why is water vapor the most important constituent of the 
air, in changes of weather? 

23. What constituents of the air have an effect on radiation 
of heat from the earth? What is this effect? 

24. What effect has dust on weather? 

if Optional Exercises 

25. Why can we not find the height of the atmosphere from the 
pressure and density, as we can with water? 

26. Why does a balloon rise in air? 

27. Explain photosynthesis, mentioning carbon dioxide, water, 
starch, and oxygen. 

28. Why do we believe there was 40% carbon dioxide in the air 
at some past stage of earth history? 

29. What is supposed to be the origin of the atmosphere, ac- 
cording to the planetesimal hypothesis? the nebular hypothesis? 
the tidal disruption theory? 



302. Variations of temperature. The sun is the chief 
source of our heat. The days are warmer than the nights, 
and it is warmer when the days are longer. As we move 
toward the equator, it becomes warmer because the sun's 
rays are more nearly vertical; the sun is more nearly over- 
head. The coldest season is that in which the nights are 
longer than the days and when the sun hangs low in the sky. 

There are other sources of heat besides the sun. One of 
them is the heat of the earth, which warms up deep mines 
and is important in volcanoes and hot springs. 

The surface of the land varies in temperature from day 
to night and from summer to winter; but if we descend 
below the surface, the variation is less and less, and we 
finally reach a level where the temperature from summer to 
winter, during night and day, is about the same. If we 
descend farther than this level, it gets warmer and warmer. 
From these observations we conclude that the interior of 
the earth is intensely hot. 

On the other hand, if we ascend in the air, it grows colder, 
and at the elevation of only a few miles (beyond the snow 
line on mountains) freezing temperatures, even in summer, 
are reached. It has been determined by sounding balloons, 
sent up with automatic recording instruments but without 
men, that the temperature decreases up to about 6 miles, 
after which it remains the same, - 67 F. This zone of con- 
stant temperature is the stratosphere. 

303. Insolation. The radiant energy that comes to us 
from the sun is called insolation. It travels at the enormously 
rapid rate of 186,000 miles per second. It includes visible 

358 . 


light waves as well as infrared, or long waves, and ultra- 
violet, or chemical rays, which are short. Infrared radiation 
is the chief source of our heat. Ultraviolet rays produce 
sunburn, cause plants to grow, affect photographic film, 
and bring about other chemical effects. Heat is energy of 
molecular motion, and a body that is heated increases its 
molecular motion. 

Heat passes from a hot to a colder body, that is, from a 
body of higher to one of lower temperature. The sun, being 
much hotter than the earth, radiates heat energy to us. 
The distance between us is, however, so great (93 million 
miles) that we intercept only a very small fraction of the 
sun's radiation. But still this energy amounts to enough to 
yield 1J horsepower for each square yard of the earth's 
surface, if it were absorbed at 100% efficiency. It is upon 
this minute part of the total solar energy that all our life 
and activities depend. 

304. What happens to insolation? When insolation is 
received, it is disposed of in three ways: by reflection, by 
transmission, and by absorption. It is only absorbed in- 
solation that makes a body hotter. 

Some substances are good reflectors, some are good trans- 
mitters, and some are good absorbers. In general, gases 
are the best transmitters, liquids the best reflectors, and 
solids the best absorbers. This is shown in the table below: 






Very little 
Very much 


Very little 
Very much 
Very little 

The absorptive power of a body may be materially 
modified by a change of color or of surface. Dark-colored 
and irregular surfaces absorb better, while brightly colored 
and smooth surfaces reflect better. By increasing the reflect- 
ing power of a body we decrease its absorption. 



A good absorber is a good radiator of heat ; hence increas- 
ing its reflecting power diminishes its radiation. 

305. Transfer of heat. There are three modes of heat 
transfer: conduction, convection, and radiation. 

In solids, conduction is the chief means of transferring 
heat. Heat is molecular motion. When a solid is heated at 
one end, the motion of the molecules increases, they bom- 
bard neighboring molecules and give up some of their motion 
to them; in this way, the increased molecular motion - 

heat is carried from one end to 
the other. This is conduction. Since 
increased heat means increased 
molecular motion, heat causes sub- 
stances to expand. 

Metals are the best conductors; 
most solids are better than liquids; 
and liquids are better than gases. 

Convection of heat can take 
place only in liquids and gases. 
When a vessel of water is heated 
(Fig. 235), the molecules at the 
bottom of the vessel move more 
vigorously. The liquid expands and 
becomes less dense, so that an equal 
volume of colder liquid weighs more than the warm liquid. 
The colder, heavier liquid drops down, therefore, and pushes 
the warmer liquid up. In this way the colder liquid becomes 
warm in its turn, until the entire contents are warmed. The 
continuous movement of the warm and cold portions of the 
liquid creates a convection current. It is apparent that con- 
vection cannot take place in solids. Convection in air is 
analogous to convection in liquids. 

Radiation is quite different from convection and conduc- 
tion. Whereas these can take place only in a material me- 
dium, radiation can pass through space itself, entirely devoid 
of matter. Conduction and convection are slow, but radiation 

FIG. 235. Convection in a 


is instantaneous. The difference can be shown by comparing 
the heating effect of an open fire with that of a steam 
radiator. One is instantly warmed by radiation from the 
open fire, whereas the steam-heating system may take an 
hour or more to warm a room by convection currents in 
the air. (The steam radiator does not do much radiating.) 

A body radiates heat to every other body which is cooler 
than itself, whether it is in contact with it or not, but conduc- 
tion and convection can take place only between bodies 
which are in contact. 

306. How the atmosphere is heated. Dense air absorbs 
more heat then rare air. Carbon dioxide, water vapor, and 
dust absorb better than air. 

When insolation passes through air, very little is absorbed 
by the upper layers of air. As it penetrates farther and 
farther into the denser, dustier, and moister air, more and 
more is absorbed and the air is more and more heated. The 
air is heated most at the bottom, not only because of the 
increased absorbing power of the lower layers, but also be- 
cause of their contact with the warmer land and water sur- 

Another very important aid in the heating of the air near 
the earth is its convectional mixing. The heated lower air 
expands and the cooler, heavier air above sinks and takes 
its place. In this way the entire mass of air would ultimately 
be warmed. 

The convectional rise of heated air may be observed above 
a heated stove or a bonfire. Rooms are ventilated by admit- 
ting cool, fresh air at the bottom and permitting the escape 
of the heated air above. 

*307. The stratosphere. This process of convection takes place 
only in the lower layers of air, up to about 6 miles. It has been 
learned by sounding balloons that the temperature decreases with 
altitude up to 6 miles, after which it remains the same, 67 F. 
If there is no difference in temperature, there can be no convection 
except that due to density. It follows then, that, while the air 



below 6 miles has its oxygen, nitrogen, etc., all mixed up by con- 
vection, the air above that point is stratified; that is, there is a 
layer of carbon dioxide, the densest gas, a layer of oxygen, one of 
nitrogen, then helium, and hydrogen at the top, since hydrogen is 
the least dense of all gases. 

This portion of the atmosphere is called the stratosphere because 
the gases are supposed to be stratified. A recent balloon ascent 
into the stratosphere has failed to confirm this stratification, but 


Miles of 
Altitude Barometric 

Gases in Layers 

FIG. 236. The Atmosphere 

the volume of air brought down was very little, and new tests are 
being made to determine this point. 

But whether or not the gases of the stratosphere are stratified, 
the evidence that the temperature is constant is definitely estab- 
lished. Hence this part of the atmosphere is called, by Humphreys, 
the isothermal layer, which means the layer of constant tempera- 

We shall have very little to do with the stratosphere in our 
study of winds, clouds, and weather, since there is quiet in the 
stratosphere. All the movement takes place in the region below 
the stratosphere, called the troposphere (Fig. 236). 

There is practically no water vapor in the stratosphere because 


of the very low temperature, and hence no clouds. The absence of 
water vapor has an important effect on the amount of ozone, and 
this, in turn, has important effects on the amount of insolation 
received by the earth. Ozone is a form of oxygen. When ultra- 
violet light passes through oxygen, ozone is formed; but it is 
changed back to oxygen again by water vapor. In the stratosphere, 
where water vapor is practically nonexistent, there is more ozone. 
This ozone in the stratosphere has two effects on insolation. It 
protects the earth from excessive ultraviolet radiation and acts 
as a blanket, together with the carbon dioxide, water vapor, and 
dust of the troposphere, to prevent earth radiation the so-called 
" greenhouse effect." 

Ozone is believed to have another effect. It has long been 
noticed that during times of maximum solar radiation, when 
sunspots face the earth, the earth receives less heat from the sun; 
in other words, the hotter the sun, the cooler the earth. 

This may be explained as follows. At sunspot minima there is 
more ultraviolet radiation. This produces more ozone in the 
stratosphere, which exerts its blanket effect on the earth to 
intercept radiation. Ozone is seven times more effective in absorb- 
ing earth radiation than solar radiation. 

308. How air is cooled. At night, when insolation ceases, 
the conditions are reversed. Radiation from the earth cools 
it off. Radiation also takes place during the day; but at that 
time the air gains more than it loses, while at night it loses 

Since a good absorber is a good radiator, the lower por- 
tions of the air are cooled most when insolation ceases. The 
rare upper portion is little affected, because there will be no 
convection when the lower layer of air is cooled, since the 
cool air is heavier. On this account the lower air warms up 
faster than it cools down. 

The coldest hours of the day are from 4 to 6 A.M., and 
the warmest from 1 to 3 P.M., depending on the season. Thus 
it takes from 7 to 9 hours for the air to warm up, from 15 
to 17 hours to cool down. 

Water vapor, carbon dioxide, ozone, and dust retard the 


earth's radiation. A clear night in winter is cold because of 
the absence of clouds. 

In Florida and California, smudges are used to prevent 
freezing of the buds on fruit trees at night. Smudges are 
small fires that cover the orchard with a blanket of smoke. 
The dust and carbon dioxide exert their blanketing effect, 
and frost is often avoided. 

309. Thermometers are instruments for measuring the 
temperature. The ordinary thermometer is a tube of mer- 

FIG. 237. A Metallic Thermometer FIG. 238. A Thermograph 

cury, which expands when it is heated. Alcohol is used in 
cold climates, because mercury freezes there. 

Some thermometers use the expansion of metals, like 
brass, to measure temperatures. A coil of the metal unwinds 
as it is heated, and moves a pointer on a dial (Fig. 237). 
The thermograph, an instrument which automatically records 
temperatures, is constructed on this principle (Fig. 238). 
The thermograph gives a daily and hourly record of tempera- 

In the United States and other English-speaking coun- 
tries, the Fahrenheit thermometer is in common use. The 
peoples of most European countries, and scientists the 
world over, use the centigrade thermometer. Both ther- 
mometers are alike in construction but differ in the scale of 
degrees used (Fig. 239). 



In making either thermometer, the tube is filled with 
mercury and sealed. The boiling point is marked by placing 
the thermometer bulb in the steam from boiling water. 
Then the freezing point is determined by 
placing the bulb in melting, cracked ice. 
For a centigrade thermometer, these 
points are marked 100 and 0, respec- 
tively; and for a Fahrenheit thermometer, 
the same points are marked 212 and 32. 

310. Distribution of insolation. The 
amount of insolation received by a given 
area of land or water in a given time 
depends mainly upon the following 
factors : 

1. Solar radiation, intensity, and dura- 
tion (-no 


2. Distance from the sun 37-^ H98.6 C 







-212 Boiling 







68 Ordinary 
so Temperature 



-32 Freezing 



Distance from the sun 

Angle of insolation 

Absorption by air 

311. Intensity of solar radiation. The 
earth receives a very tiny fraction of the 
sun's radiation, but even that small 
amount is sufficient, if completely trans- 
formed, to yield 1.5 horsepower per square 
yard of surface, which, from our point of 
view, is stupendous. 

Whether the sun is radiating heat at a 
uniform rate from year to year we are not 
certain, because observations are too few 
to draw a valid conclusion. But it is 
known that there is a 2% variation due to 
sunspots. It is found that the average tem- 
perature of the earth is higher when sunspots are at a mini- 
mum, although the sun at such times is radiating with greater 
intensity. This has already been explained in paragraph 

FIG. 239. A Ther- 
mometer with Centi- 
grade and Fahrenheit 


The chief variation in solar intensity is in the ultraviolet 
rays; these are at times twice as intense as at others. 

Because of the inclination of the earth's axis, the period 
of insolation, or number of hours of sunshine, is not the 
same for all places; and it also varies from day to day at 
any particular place. No place, besides the equator, always 
has 12 hours of daylight and 12 hours of darkness. In other 
places that will be true at the equinoxes, twice a year. At 
other times the period of daylight will be longer than 12 
hours when the sun is on the same side of the equator as 
the observer, and shorter than 12 hours when on the oppo- 
site side of the equator. 

In the polar circles the period of continuous sunlight is as 
much as 24 hours or more in midsummer, while in midwinter 
there is none. 

Not considering other factors, the amount of insolation 
increases with the number of hours of sunlight. In Norway, 
within the Arctic Circle plants grow at abnormal speeds, 
and some of them to abnormal size during the short sum- 
mer, because the period of insolation is at times as much as 
24 hours or more. 

312. Effect of distance from the sun on insolation. The 
earth is about three million miles nearer the sun at peri- 
helion, about January 1, than at aphelion, about July 1. In 
consequence, a place receiving vertical insolation around 
January 1 receives almost 7% more insolation than one 
receiving vertical insolation around July 1. This makes a 
difference in temperature of about 7F. 

Since the earth is nearer the sun during the winter of the 
northern hemisphere, our winter is warmer and our summer 
cooler than they would be, otherwise. 

313. Effect of angle of insolation. Increasing obliquity, 
or slant of the sun's rays, is proportional to decrease of 
insolation; and actually, the insolation absorbed decreases 
faster than the angle. It is found that while water reflects 
only 2% of vertical insolation, it reflects about 65% when 


the sun is only 10 above the horizon. On this account, the 
early morning and late afternoon rays, as well as the rays 
received in regions far from the equator, have little effect 
in increasing temperatures. For this reason alone, the poles 
could never be warm. 

When we consider also the fact that in the polar regions 
the lands and frozen seas are for much of the year covered 
with snow and ice, both very poor heat absorbers, and that 
the heat they receive must first be used to melt the snow and 
ice, before they can be warmed up, we can better under- 
stand the low temperatures which prevail there. When ice 
is melting, it remains at the same temperature throughout 
the process, even though it absorbs a great amount of heat. 

The northern hemisphere, where there is much more land 
area, is warmer in summer and colder in winter than the 
southern hemisphere, because of the greater area of water 
there. This is due to the fact that the land is a better ab- 
sorber and also a better radiator than water; and further- 
more it takes more heat to warm up water than land, weight 
for weight. 

The direction and character of winds and ocean currents 
and the differences in the elevation of land are likewise 
important factors in the distribution of heat over the earth. 

All things combine to give the regions along the equator 
the greatest total amount of heat; and the change in heat 
during the year is least there. 

314. Absorption of insolation by air. We have been con- 
sidering, thus far, the amount of insolation received from 
the sun; but what really counts is not that, but the amount 
absorbed by our air. 

Much of it is reflected by clouds and earth, and some is 
scattered by dust and other constituents of air. It is esti- 
mated that about half the sun's insolation is lost in these ways. 

Dust scatters some rays, carbon dioxide, ozone, and water 
vapor absorb heat rays, while oxygen, when dry, takes up 
ultraviolet rays. 


315. Shifting of the heat equator. The zone of greatest heat 
near the equator is known as the doldrum belt, or simply 
the doldrums. The irregular line in this belt, passing through 
places with the highest temperatures, is called the heat 
equator (Fig. 240). Since the sun's vertical rays shift during 
the year, so also the doldrums and the heat equator shift 
(Fig. 241). 

The temperature of a place continues to increase as long 
as more heat is received than is lost by radiation. The cool- 
ing down begins ordinarily an hour or two past noon, al- 
though most heat is received at noon; and the highest 
temperature of the year occurs usually some weeks after 
the longest day, although most heat is received on that day. 
The heat equator, therefore, does not attain its extreme 
position, north and south, at the times when the sun is 
at its most northerly and most southerly positions, but 
weeks after. Places in between these extreme positions, 
having vertical insolation twice annually, have two maxi- 
mum and two minimum temperatures during the year and 
experience their highest maximum temperature, shortly 
after vertical insolation, upon the sun's return toward the 

Being a better absorber and better radiator than water, 
land has a higher temperature in summer and a lower 
temperature in winter than the sea near it. This excessive 
warming and cooling is most pronounced in its effects in the 
northern hemisphere, where the great land areas are; and it 
is also more pronounced over the relatively narrow Atlantic 
than over the broader Pacific (Figs. 240 and 241). 

316. Average position of the heat equator. The heat 
equator shifts farther and remains for a longer time north 
of the geographical equator than it does south of it. This is, 
in part, because the sun is seven days longer north of the 
equator than south of it, and also because of the shape of 
the continents and oceans. Owing to the positions and out- 
lines of continents, more of the warm ocean currents are 






turned into the northern oceans than into the southern and 
these make the northern hemisphere somewhat warmer. 

Moreover, the Pacific Ocean, being almost closed on the 
north, shuts out the cold polar currents and remains warmer 
than the Atlantic, which is open at the north. 

317. Isotherms. Lines drawn through places having the 
same temperature at the same time are called isotherms. 
They may represent the temperature at a particular time 
or the average for any given period a week, a month, or 
the entire year. 

Such lines, while very irregular, have in general an east- 
west direction (Figs. 240 and 241). This is what we should ex- 
pect, since the number of hours of sunlight as well as the angle 
of insolation depend upon the distance from the equator, and 
therefore these lines should be parallel to the equator. 

Isotherms are continuous lines which for a limited area 
may appear on the map as closed curves. From the defini- 
tion, two isotherms cannot intersect. The heat equator is not 
an isotherm, though it extends around the earth in the same 
general direction as isotherms. It may even cross isotherms. 

*318. Temperature gradient. If we pass from one isotherm to 
the next of higher or lower temperature, we must pass through all 
the intermediate temperatures. There might be many routes by 
which one could go from a place on one isotherm to a place on the 
other; but the shortest route would give the greatest rate of change 
of temperature. This shortest route is the direction of the tempera- 
ture gradient. 

Temperature gradient may be defined as the rate of change of 
temperature between two places. The closer together the isotherms, 
the more rapid the change of temperature, or, as we say, the steeper 
the gradient, while widely separated isotherms indicate gentle gra- 

Completion Summary 

The temperature of the air - - as we ascend. At an 

elevation of about - - miles, the temperature . 

This is called layer or - . 


The heat received from the sun is called . Land 

warms up - , while water - , and air - . 

Heat is transferred from a hotter to - - by three 

methods: , - , and - . Solids heat; 

in liquids and gases heat is transferred by ; all bodies 

- heat to bodies. 

The air is heated most, near - ; this air and 

rises, and the cooler air and is warmed, in turn. 

This is a current. 

*The atmosphere up to - - miles is called the - ; 
above that, it is called - . Here - - convection, because 
there is no - , and it is believed the different gases have 
separated , with a layer of - at the top. 

There are no clouds in , and hence ultraviolet radia- 
tion transforms oxygen into . This protects the earth from 

and prevents earth heat from . 

At night, the earth - - heat. This loss is 

modified by the presence in the air of , , 

, and - . 

On the Fahrenheit thermometer, the boiling point of 
water is - and the - - is 32. On the centigrade 

thermometer, the boiling point is and - 0. 

Ordinary room temperature is - C. or F. 

The amount of insolation received at any place depends 
on the number sunshine and the angle . 

More heat is absorbed than by . On that 

account, the northern hemisphere - than the southern. 
The heat equator, or line of - temperature, shifts be- 
cause - . It remains - for a longer time. 

Isotherms are lines on the - passing through . 

*The temperature gradient is represented by the line 

that can be drawn between two . 



1. How does the temperature change as we descend into a 

2. What evidence have we that the interior of the earth is 
very hot? 

3. How does the temperature change as we ascend in the air? 

4. What happens to the temperature beyond an elevation of 
seven miles? What do we call this part of the air? 

5. If the interior of the earth were the chief source of heat, 
what part of the earth's surface would be the hottest? 

6. How do water and land compare in heat absorption? 

7. Which substances conduct heat best? 

8. What is the chief method of heat transfer in liquids? in 

9. By which method is heat transferred from the sun to the 

10. Where is the atmosphere heated most? Why? 

11. What is a convection current? How do we use it in venti- 
lation? in heating liquids? 

12. Why does the lower air warm up faster than it cools off? 

13. What are the coldest and the warmest hours of the day? 

14. What constituents of the air retard radiation from the 

15. Why is it colder on a clear night, in winter, than on a 
cloudy night? 

16. Explain the use of smudges in Florida orchards. 

17. What is a thermograph? What is the actuating device? 

18. What are the boiling and freezing points on the centigrade 
and Fahrenheit thermometers? 

19. What are, approximately, the ordinary room temperatures 
on each scale? 

20. What is the variation in solar radiation? To what is it due? 

21. What kind of solar radiation shows the greatest variation? 

22. Upon what does the amount of sunlight received at any 
place depend? 

23. Explain the rapid growth of vegetation, in Norway, during 
the short summer. 

24. Why do some places north of the equator receive more 


insolation than places at the same south latitude? How much 

25. What effect has the angle of insolation on the heating 
power of the sun's rays? 

26. State several reasons for the low temperatures of the polar 

27. Why is the northern hemisphere colder in winter and hotter 
in summer than the southern hemisphere? 

28. In what part of the earth is the variation of temperature 
least? Why? 

29. Where is the heat equator? Why does it shift? 

30. Why does the heat equator remain in the northern hemi- 
sphere longer than in the southern? 

31. What are isotherms? 

32. Why cannot two isotherms intersect? 

if Optional Exercises 

33. The higher we ascend in the air, the more intense the inso- 
lation, yet the colder it is. Explain. 

34. Why are the gases in the isothermal layer thought to be 

35. Why are there no clouds in the stratosphere? 

36. Why are all winds confined to the troposphere? 

37. Explain the greenhouse effect of ozone. Why is this more 
important in the stratosphere? 

38. Why is the earth cooler when sunspot activity is greatest? 

39. Explain how oxygen absorbs ultraviolet rays. 

40. What is meant by the temperature gradient? 



319. Air has weight. Many errors are made in regard to 
air, because it is an invisible mixture of gases. An " empty 
bottle' 7 is really a bottle full of air. "A balloon rises because 
hydrogen weighs less than nothing" should be "a balloon 
rises because hydrogen weighs less than its own volume of 
air." Air has weight. This can be shown by weighing an 

FIG. 242. The lamp weighs more after it is punctured. 

evacuated electric lamp, then puncturing it and reweighing. 
It weighs more when the air is permitted to enter (Fig. 242). 

Air weighs 1.3 grams per liter, or about 1 pound for 12 
cubic feet. This is called the density of air. The air in a room 
14 feet long, 12 feet wide, and 10 feet high weighs about 
140 pounds. 

320. Air pressure. We live at the bottom of a sea of air 
and, since air has weight, it presses down upon everything 




in it. The air pressure amounts 
at sea level to 14.7 pounds on 
every square inch of surface. 
This is equal to several tons 
for every human being. How 
do we sustain it? The heart 
pumps the blood to every part 
of the body with a pressure 
slightly greater than air pres- 
sure. In fact, blood pressure 
is sometimes so much greater than air pressure, on high 
mountains, that a blood vessel is ruptured. In other words, the 

FIG. 243. Effect of Air Pressure on 

a Tin Can from Which the Air Has 

Been Exhausted 


FIG. 244. A Homemade 
Mercury Barometer 

FIG. 245. The mercury column is 
lower here than in Fig. 244. Why? 

air pressure decreases as we go up, so that at an elevation of 6 
miles, it is only 4 pounds per square inch, and at 10 miles it 



is 2 pounds per square inch. If the air is removed from a tin 
can, it can be crushed by the external air pressure (Fig. 243). 

321. The barometer. If we fill a glass tube, 34 inches 
long, with mercury and invert it in a vessel containing 
mercury, the mercury does not fall down, but 

remains at a height of about 30 inches (Fig. 
244). Referring to Fig. 245 , if some of the air 
is sucked out of the bottle, the mercury falls. 
If more air is blown in, the mercury rises. This 
can be explained as follows: The air itself 
holds up a mercury column 30 inches high, 
because the air over a square inch weighs as 
much as a column of mercury 30 inches high, 
over a square inch. Each of them weighs 14-7 
pounds. Remove some of the air and the pres- 
sure, being less than 14.7 pounds per square 
inch, cannot hold the mercury up 30 inches 
high. Blowing into the bottle increases the air 
pressure and forces the mercury up more than 
30 inches. 

We may say then that the mercury column 
balances the air pressure; hence we use it to 
measure air pressure, and call it a mercury 

The glass tube is usually surrounded by a 
metal tube for protection and with some de- 
vice for measuring the height of the mercury 
column (Fig. 246). 

322. The aneroid barometer. The mercury 
barometer is very inconvenient to carry up a 
mountain or in an airplane. For such purposes 
we use the aneroid barometer. Aneroid means 
without liquid. 

In Fig. 243 it was shown that a tin can could be crushed 
by removing the air from the can. Now suppose we remove 
only a little of the air. In that case the walls of the can will 

FIG. 246. A Mer- 
cury Barometer 



be forced in only a little (Fig. 247). If then the air be per- 
mitted to return, the can returns to its normal shape and 
size. Again, if air is blown into the can, its sides bulge out- 
ward. This is the principle of the aneroid barometer. A small 
flat metal box is partially exhausted and sealed. One sur- 
face of the box is connected to a spring which moves a 
pointer across the scale (Fig. 248). 

323. Variation in barometer readings. At sea level the 
average reading of the barometer is 30 inches. As the instru- 
ment is carried up through the air, the " barometer drops" 


FIG. 247. Principle of the 
Aneroid Barometer 

FIG. 248. An Aneroid Barometer 

about one inch per thousand feet, because it is the air above 
which causes the pressure. It does not continue to drop at 
the same rate, however. For example, here are actual figures : 
at the 910-foot level the barometer dropped 1 inch; at the 
1850-foot level, it dropped 2 inches. This is because the 
density of air decreases with altitude; the air itself is af- 
fected by the pressure and one cubic foot of air at sea level 
weighs more than anywhere above sea level. 

The barometer reading is never stationary, even at sea 
level. It changes from day to day and from hour to hour. 
We can understand these changes if we imagine the air as 
continually in motion (Fig. 249). Like the sea, there are 
huge waves on the surface of the sea of air, and the pressure 




of Air 

FIG. 249 

under the crest of a wave is greater than it is under the 
trough. Furthermore the sea of air, like the ocean of water, 
is subject to tides. 

Again, moist air is less dense than dry air; hence a column 
of moist air will weigh less and the barometer will be low. 
Cold air is more dense, giving us a higher barometer in 
winter. If a continuous record of air pressures is desired, an 
instrument called the baro- 
graph is used. The barograph 
is an aneroid barometer with 
a pen attached to the pointer 
(Fig. 250). The record is at- 
tached to the drum which is 
turned by a clock, giving 
weekly records of barometer 

FIG. 250. A Barograph 

324. Isobars. If a stationary barometer is read from hour 
to hour, it will be noted that its readings change continually. 
This seems to be due to a series of surges or waves in the 
upper atmosphere. These are called lows and highs. Lows 
are sometimes called cyclones, and highs, anticyclones. 

If our barometer registers a low, it will be found that as 
we move away from our position in any direction, the 
barometer will be higher, and vice versa for a high. We may 
draw lines through places having the same barometer read- 


Monday / Tuesday , Wednesday , Thursday , Friday , Saturday , Sunday 

' i i i i I t I I i ' ' i I I I I I I I I I I I I I I I I I I I I I! i II 1! ! , , ;: I I I 

FIG. 251. A Barograph Record 

ing. Such lines are called isobars (Fig. 252). About strongly 
developed lows and highs the isobars are closed curves and 
approximately parallel. 

The isobars about a high may be likened to the contours 
of a hill on a topographic map ; those about a low are similar 

FIG. 252. Isobars about a Low and a High 

to the contours of a depression, the low being an atmos- 
pheric basin. 

325. Pressure gradient. Just as the temperature gradient 
is the shortest distance from one isotherm to the next, so 
we may get the pressure gradient at any place by taking the 
shortest distance between the isobars at that place. 

The pressure gradient is the rate of change of pressure be- 
tween two places; and, like the temperature gradient, it is 
in the direction of the most rapid change. Crowded isobars, 
therefore, mean steep pressure gradients. We shall see that 
the direction and strength of the wind are closely related 
to the pressure gradient. 





326. Pressure belts. The distribution of pressure over 
the earth depends upon the temperature; high temperature 
causes low pressure. The equatorial belt is therefore a re- 
gion of low pressure. This region is known as the doldrums. 
North and south of the doldrums, in the two temperate 
zones of the earth, there are regions or belts of high pressure 
called the horse latitudes. These lie around latitude 30 N 
and 30 S. 

Toward the poles the pressure decreases. As a result of 
this arrangement of pressures, the isobars of the world have 
a general east-west trend, and shift with the heat equator. 

327. Uses of the barometer. The barometer is used by 
mountain climbers and aviators to determine altitudes; but 
the most important use of the barometer is in forecasting 

When there is a difference of pressure between two places, 
there will be a movement of air, a wind, from the place of 
high to that of low pressure; and the steeper the pressure gra- 
dient, the greater the velocity of the wind. 

In order to forecast the movements of air, it is necessary 
to have barometer readings from many places, and the 
government maintains meteorological stations at numerous 
points all over the country. 

Completion Summary 

Air weight, and because of that pressure. 

The pressure of the atmosphere at is equal to . 

At higher altitudes is less. Air pressure 


The mercury column at sea level inches. The 

aneroid barometer has no , and it is therefore more 

than the barometer. 

A steep pressure gradient is shown, on a map, . 

is a region of low pressure. It is near the . 

The horse latitudes are in the zones. They are 

regions of pressure. 


A barometer is one of the instruments weather. 

Winds - - region of low pressure. 


1. What is usually meant by an empty bottle? 

2. What is meant by the density of air? 

3. State the air pressure, in pounds per square inch, at sea 

4. How do we know that blood pressure is greater than air 

5. Explain how a tin can is crushed by removing air from the 

6. What is a barometer? 

7. Explain why the mercury column drops when the air pres- 
sure diminishes. 

8. What is an aneroid barometer? Why is it more convenient 
than a mercury barometer? 

9. Why does the barometer drop as we ascend a mountain? 

10. Why is the barometer low in moist weather? 

11. Why is the barometer often lower in summer than in 

12. What is a cyclone? an anticyclone? 

13. What are isobars? 

14. How do we find the pressure gradient? 

15. What is a barograph? 

16. Why is the equatorial belt a region of low pressure? 

17. What are the pressure belts? 

18. What causes a wind? 

^Optional Exercises 

19. Show by the use of a funnel, a bottle, and some water that a 
bottle of air is not empty. 

20. How much is the air pressure on a human being? What 
force would the air exert on a person whose surface area is 10 
square feet? 

21. Explain how a barograph works. 

22. How can the barometer be used to determine altitude? 

23. Explain the use of the barometer in weather forecasting. 



328. How are winds started? The atmosphere is heated 
most over the equator, and therefore it expands most there. 
Referring to Fig. 254, the expanding air produces a ridge 

Warm Region 

30 in. 

Barometric Pressure before Winds 
30 in. 

30 in. 

30.5 in. 

Barometric Pressure after Winds 
29.5 in. 

FIG. 254. Cause of Winds 

30.5 in, 

over the equator, like a huge wave, and this ridge tends 
to flatten out, by a movement of air in the upper regions 
of the atmosphere, in directions away from the heated 
region, that is, toward the poles. This adds air to these 




cooler regions and increases their barometric pressure, while 
it decreases the pressure over the equator. The difference in 
pressure, or pressure gradient, between the cooler and 
warmer regions causes a movement of air along the earth 
toward the equator. Considering only this one factor, the 
circulation of air on the earth, 
like the convection currents 
in a room, are shown in Fig. 

Winds are horizontal move- 
ments of the atmosphere close to 
the earth. We distinguish 
between winds and vertical 
movements of the air, or 
movements in the upper lay- 
ers of the troposphere, both FIG. 255. Circulation of Air in the 

Earth's Atmosphere As It Would Be 

if It Were Caused by Convection 


of which are called currents. 

329. Terrestrial winds. The 
upward movement of the air 

in the equatorial regions makes it a belt of equatorial calms 
or light winds, called the doldrums. The winds which blow 
from the horse latitudes, or belts of high pressure, toward 
the equator are the trade winds. Beyond the trade- wind belt 
in each hemisphere, we have a belt of prevailing westerlies 
(Fig. 256). 

This atmospheric circulation for other planets of the solar 
system must be the same as it is for the earth, since the con- 
ditions that bring about our winds are the same on all the 
planets. On this account those winds are sometimes called 
planetary winds. 

330. Deflection of winds due to earth's rotation. If the 
earth did not rotate, the wind system would be very simple, 
as already shown. But the rotation causes deflection of the 
trade winds, so that, instead of coming from the north, they 
seem to come from the northeast, and, in the southern hemi- 
sphere, from the southeast. 



Let us try to explain this. These winds arise from the 
movement of air along the earth toward the equator, so 
that they ought to be north and south winds. The air, as 
well as the earth, is in rotation around the earth's axis. At 
the equator the circumference of the earth is about 25,000 






FIG. 256. Terrestrial Wind Belts 

miles; and since this rotates once every 24 hours, points on 
the equator, as well as the air, are moving about 1,000 miles 
per hour. At the poles this motion is zero ; and at intermedi- 
ate points it is less than 1,000 miles per hour. 

Let us follow a trade wind in the northern hemisphere 
from the horse latitude to the equator. Let us assume that 
the earth and the air move 500 miles per hour. As the air 
moves south it continues also to rotate 500 miles per hour, 
from west to east. As it moves south it passes over land which 
is moving more than 500 miles per hour, so that the wind falls 
behind or the earth gets ahead. The result is, as shown in 
Fig. 257, that a wind starting south from A to B on the earth's 
surface will take the apparent direction A'B', because B 
has moved ahead faster than the air. The trade wind, then, 



instead of being a north wind, becomes a northeast wind. In 
the southern hemisphere it is a southeast wind. 

This is often stated as follows : in the northern hemisphere 
a wind will be deflected to the right of its straight course, and 
in the southern hemisphere, to the 
left. This is known as FerrePs 

In the northern hemisphere, 
then, following the trade wind in 
its path toward the south we find 
that it is deflected to the right or 
west, and hence seems to come 
from the northeast, which gives it 
the name: northeast trades. The 
planetary winds are shown in 
Fig. 256. 

The prevailing westerlies which, 
in the northern hemisphere, are 
southwesterlies, get their direction 
as follows. From the horse latitude 
or region of high pressure, the air 
moves toward the equator and 
poles. The air moving south be- 
comes the trade winds, as we have 
seen. That which moves north to- 
ward the pole is rotating from west to east, and as it moves 
north it moves over land which is not rotating as fast as it 
is. It therefore gets ahead of the earth, more to the east, 
and seems to be coming from the southwest. It has been 
turned to the right of its straight course. Hence they are 
called the prevailing southwesterlies in the northern hemi- 
sphere and the prevailing northwesterlies in the southern. 

331. Description of the wind belts. The wind belts depend 
upon the pressure belts; and these in turn are determined by 
the distribution of temperature. Since the temperature belts 
shift, in like manner the pressure and wind belts shift. 

FIG. 257. Ferrel's Law 


The doldrums are named because of the light winds and 
calms that characterize this belt. It is a belt of high tempera- 
ture and consequently low pressure. The winds move 
obliquely in toward the doldrums from north and south. 
Movement of air in the doldrum belt is chiefly upward. 
Hence it is a belt of calms. The trade winds blow obliquely 
in from north and south toward the doldrums. They derive 
their name from the fact that sailing vessels in bygone days 
made use of the regularity of these winds, which blow con- 
tinuously and always in the same direction. Trade fol- 
lowed these winds. 

In the northern hemisphere they are called the northeast 
trades and in the southern, the southeast trades. They have 
their origin in the high-pressure belts of the horse latitudes 
and move toward the low-pressure belt of the doldrums. 

Instead of following the steepest gradient directly north 
and south, as they would do if there were no rotation, they 
are deflected, in accordance with FerrePs Law, to the right, 
in the northern hemisphere, and to the left, in the southern. 

The horse latitudes are belts of high pressure between the 
trade- wind belts and the westerlies. It is there that the air, 
which has risen over the equator, starts to fall toward earth. 
The horse latitudes are belts of high pressure and air moves 
away from them toward the equator and also toward the 
poles. Since the air movement is chiefly downward, the 
winds along the earth are relatively calm, irregular in direc- 
tion, and unsteady in duration. 

The prevailing westerlies flow away from the horse lati- 
tudes toward the poles but are deflected to the east by the 
rotation of the earth. Hence they seem to come from the 
west and they are called westerlies. They are not so constant 
in their direction nor in their duration as the trades. 

*As the prevailing westerlies approach the poles, a circumpolar 
whirl develops. They spiral around the North Pole in a counter- 
clockwise direction and about the South Pole in a clockwise direc- 

WINDS 389 

As a result of this whirl it is believed the polar regions are areas 
of low pressure. 

Because of excessive cooling of the northern continents in winter, 
the North Atlantic and North Pacific oceans are warmer than the 
continents and are therefore centers of low pressure. About these 
centers the winds spiral, like the circumpolar whirl, and from these 
secondary centers winter cyclones are projected. Those from the Pa- 
cific often move southeastward into Canada and the United States. 

332. Shifting of the wind belts. The pressure belts and 
wind belts follow the shifting of the heat equator. As a result, 
places near the borders of the various wind belts lie some- 
times in one belt and sometimes in another. Southern 
Florida, southern California, and northern Mexico are in the 
horse latitudes in winter and in the northeast trades in sum- 
mer; and the Panama Canal Zone is in the doldrums in 
summer and in the northeast trades in winter. The Amazon 
Valley, usually in the doldrums, is sometimes swept by the 

333. Land and sea breezes. Since land heats up faster 
than water, an area of low pressure is developed over the 
land during the daytime, and a wind will blow from the sea 
to the land (Fig. 258). This is 

a sea breeze. In the night time f f A t f * 

this condition is reversed and Sea Breeze 

a land breeze blows from the 
land to the sea. 

Advantage is taken of this FIG. 258 

change of breeze by fishing 

and pleasure craft that depend upon sails, to carry them 
away from land at night and bring them back during the 
day. Similar land and lake breezes are felt along the shores 
of our great lakes and inland seas. 

If the land should remain colder than the water through- 
out the 24 hours, there would be a continuous land breeze. 
This often happens in winter, especially when the land is 
covered with snow. 


On the other hand, in midsummer it sometimes happens 
that the land does not cool down, during the night, below 
the temperature of the neighboring body of water. Then the 
sea breeze continues throughout the night. 

334. Mountain and valley breezes. Winds similar to land 
and sea breezes develop on mountains, particularly at night. 
The mountain mass cools rapidly at night, and the air in 
contact with it becomes cool and flows down the slope into 
the valley. This is a mountain breeze. 

In the daytime the reverse holds good. The mountain is 
heated first and a convection current starts up the slope. 
But this current is not so noticeable as the mountain breeze, 
which is aided by gravity. 

335. Continental air drifts. Land and sea breezes on a 
continental scale are called continental winds. They are 
usually so easily obscured by the prevailing winds as to be 
scarcely noticeable except by their effects on the prevailing 

Thus, on our western coast the westerlies are weakened 
in winter because the land is colder than the water, causing 
a land breeze, which moves toward the west and hence 
weakens the westerlies, which move toward the east. In 
summer the continental wind strengthens the prevailing wester- 

On our eastern coast these conditions are reversed, and 
the winds are strengthened in winter and weakened in summer. 

336. Monsoons. Some continental winds are strong 
enough to reverse the prevailing winds at some season of the 
year. Such winds are called monsoons. While many regions 
have monsoon winds, they are best developed over the 
northern Indian Ocean and the adjacent lands to the north 
and east. 

During the winter the winds move from the cold moun- 
tainous area out over the Indian Ocean, strengthening the 
prevailing northeast trades (Fig. 259). This is the winter 
monsoon. During the summer, on the other hand, the sea 



breeze off the Indian Ocean reverses the prevailing northeast 
trades into a southwest wind. This is the summer monsoon 
(Fig. 260). 
All continents show some tendency to develop monsoons 



FIG. 259. Winter Monsoon over the FIG. 260. Summer Monsoon over 
Indian Ocean, Showing Isobars the Indian Ocean, Showing Isobars 

China for example; but in most cases the effect on the 
prevailing winds is not sufficient to reverse their direction. 

337. Cyclonic winds. In temperate latitudes, by far the 
most important winds are those which are not regular, and 

FIG. 261. Isobars about a High and a Low in the Northern Hemisphere 

The arrows show the direction of the wind from high- to low-pressure 
areas. This direction is clockwise for the high and counterclockwise for the low. 

which are caused by the irregular distribution of pressure 
in lows and highs (Fig. 261). These are known as cyclonic 
winds. More particularly, the low-pressure area develops a 
cyclone; the high, an anticyclone. 


WINDS 393 

The wind in a cyclone, or low, moves obliquely in toward 
the center of the low, spiralling about the center in a counter- 
clockwise direction in the northern hemisphere. In anticy- 
clones, or highs, the winds move spirally out from the center 
in a clockwise direction. 

*Although their origin is not well understood, these cyclonic 
whirls are now believed to be the result of friction between the 

Cold N 

/ ^;oia -D 1N r* 

Meeting of A / ^own-currents B f / C R ltof 

:old polar air //// /// 7 /'//<_ factlon: 

cold polir air //// /// / / 

and warm /// * L//I \ \\ 1 r ^ A cycione 

equatorial x U ^K I T rMA^X with counter 

inrrPTit in the /y *- . ) / \ \ LOW v N clockwise 

current in the // x . , ) I ] ^^ L ^VV v " clockwise 

upper y // fj /// / , \C^ y i \ spiral 

atmosphere ////^Warm // / 1 ~^/ / I T ascending 

/ / Up-currents ' c J^ / / I 

Development of Anticyclone between Two Cyclones 
FIG. 263. The Bjerknes Theory of the Origin of Cyclones 

great masses of cold air moving from the poles toward the equator, 
and the warm air moving toward the poles. Somewhere these must 
pass each other and as they do, the whirls are started. 

Referring to Fig. 263A, the cold polar air is deflected to the west 
as it moves south, while the warm equatorial current, moving north 
in the upper atmosphere, is deflected to the east as shown, accord- 
ing to FerrePs Law. Somewhere in the upper atmosphere these 
must come into contact and the friction between the two, on their 
edges, causes the whirl shown in B. The warm air ascends inward 
and we have a cyclone, as shown in C. 

Between two lows there must be a high-pressure area developed, 
as shown in the lower part of Fig. 263, with the down moving 
cooler air, or high, at the center. 

These anticyclones may originate in the higher currents and 
sink to the bottom of the air fully developed. 



338. Movements of cyclones. Four distinct movements 
of air in lows must be noted. 

1. Oblique movement toward center 

2. Upward movement near center 

3. Spiral outflow above 

4. Eastward movement of the low itself 

Most cyclones follow the general direction of the wester- 
lies. In the United States there are three general paths. 

FIG. 264. Paths of Lows in the United States 

Those originating in the northwest move southeast until 
they reach the Mississippi and then move northeast (Fig. 
264). Those having their origin in the southwest move sys- 
tematically northeastward across the continent. 

Tropical cyclones having their origin in the region of 
the West Indies move first northwestward until they reach 
the horse latitudes or region of high-pressure calms, then 
northeastward in the westerlies. They usually cross the high- 
pressure belt near the land, where the high pressures are not 
so well developed. 

All cyclones sooner or later conform to the course of the 
prevailing westerlies. 

WINDS 395 

339. Velocity of cyclonic winds. The velocity of cyclonic 
winds averages about 10 miles per hour, being a little more 
in winter than in summer. In the higher layers of air it may 
be as high as 45 miles per hour. The velocity increases near 
the center of the low; and if a strong spiral movement is 
developed, the velocity is greatly increased. 

In the anticyclone, the wind velocity is least at the center. 
Consequently the strength of the wind increases with the 
approach of a low and decreases with the approach of a 

When the low-pressure area is very small, often less than 
100 feet, the spiralling winds may attain a velocity as great 
as 100 miles per hour and become destructive. Such wind 
storms, with velocities more than 100 miles per hour, are 
known as tornadoes. Similar but larger storms in the West 
Indies are called hurricanes, and in the East Indies, typhoons. 

Hurricanes often attain a velocity of 60 miles an hour, 
with diameters of 100 miles or more. The smaller the area 
of the storm, the stronger the wind, because the pressure 
gradient is steeper. 

The term cyclone is often incorrectly used for especially 
strong winds; but in reality it may be a gentle breeze of 
four miles per hour, or a driving gale of 40 miles per hour. 
A cyclone is simply a low-pressure area, and the winds cir- 
culating around it may have any velocity. 

*Cyclones of wide extent, within the tropics, sometimes have 
an area of clear skies within the whirl of destructive winds. This 
area, called the eye of the storm, may have as much as one tenth of 
the diameter of the cyclonic area. Vessels passing through the eye 
of the storm experience equally strong winds in the front and in 
the rear of the cyclone, though from opposite directions. 

340. Thunderstorms and tornadoes. In summer, after 
a day or so of excessive heat, the rapid convectional ascent 
of the air about a low may set up locally a more limited, 
though more intense, cyclonic whirl. The rapid condensa- 
tion of vapor in the rising and cooling air may give rise to, 



or be accompanied by, brilliant displays of lightning and 
heavy thunder. Such storms are known as thunderstorms. 
Torrential downpour of rain may follow quickly after the 
most brilliant discharges of lightning, but it is a notable 
fact that the lightning flashes become rapidly less frequent 
after the rain begins to fall. 

Thunderstorms are usually summer and daytime phe- 
nomena and they seldom occur in winter or at night. They 

Direction of Storm ^^I^5"^g^-^Gtaw Owribw 


,Q \ ^Mrs^i 

\*f %^ Ctfffiu.lus , Cloud - >^ .^X 

^> V * ***^ *^<^^**tt 

FIG. 265. A Thunderstorm in Front of a Low 

are much more common in front of lows than behind them. 
In the United States they occur most frequently in the 
southeastern quadrant of the low-pressure area (Fig. 265). 

If the local whirl thus developed is destructive in violence, 
it is called a tornado. The destructive path of a tornado is 
rarely a mile in width and usually but a few miles in length; 
more commonly it is but a hundred yards in width (Fig. 
266). Within that narrow path, the violence of the wind is 
such that few structures above ground are strong enough 
to withstand it. In those states in the Middle West where 
tornadoes are most frequent, underground structures called 
" cyclone cellars' 7 are built. These seem to offer the greatest 
security from danger. 



Wide World 

FIG. 266. Photograph of a Tornado near Oklahoma City, Oklahoma 


Underwood & Underwood 
FIG. 267. What the Same Tornado Did in Texas 

*The destructive effect of the tornado on buildings seems to be 
the result of the exceedingly low pressure at the center of the cy- 
clone. The air inside the house is at normal atmospheric pressure; 
and this is so much greater than the low pressure in the tornado 
that it pushes the roof and walls out (Fig. 267). 


If we assume the pressure in the whirling air to be only 1 Ib. 
per square inch less than that in the house, and the house to be a 
box 20 ft. X 30 ft. X 25 ft., a force of about half a million pounds 
outward would be developed. 

*341. Shifting of winds. When a place lies near the path of a 
low or high, the winds at that place will shift in a systematic way 
as the barometric disturbance passes. In the westerlies of the 
northern hemisphere, where cyclones and anticyclones follow 
each other across the continent from west to east, the shifting of 
the winds is shown in Fig. 268. 

If a low passes directly over a place, the winds will start as an 
east wind as the low approaches and change suddenly to a west 
wind as the low passes off to the east. It is just the reverse for a high. 

*342. Special winds. In every part of the world, winds of 
special character and of exceptional occurrence are known, and 
local names given to them. 

Among warm winds may be mentioned: 

1. The hot wave. This blows from the west or southwest over 
western central United States, sometimes continuing for days and 
withering all vegetation. 

2. The sirocco. A south wind from the Sahara Desert, felt as far 
as the north shore of the Mediterranean Sea. It is usually dry and 

3. The simoon. An intensely hot, dry, sand-laden wind of the 
Arabian Desert. It is probably a convectional whirlwind, similar 
to the dust-laden whirlwinds of all dry, hot climates. It lasts 
usually less than 10 minutes, and often forms sand spouts. 

4. The chinook. An American wind, which moves down the 
slopes of mountains toward a low-pressure area at their base. 
Though starting as a cold wind, it gets warmer by compression 
in its descent, and if the mountain is high, it may reach the base 
as a warm or even a hot wind. 

In all cases, because of its dryness, it evaporates or melts the 
snow fields over which it blows and often causes destructive 
avalanches by melting the snow on the steep slopes. 

It is of frequent occurrence along the eastern bases of the Sierra 
Nevada and Rocky Mountains. Many valleys here are kept prac- 
tically free of snow, and their temperatures are so mild as to make 


Path of Low North of A 



^ -^ 

Shifting of Winds at A 

A. When the low is at 1, the wind blows from the southeast toward A. 
As the low moves east, the winds shift, as shown, to the southwest. 

Shifting of Winds at B 

Path of Low South of B 
B. In this case, the wind starts from the northeast and shifts to the northwest. 

Path of High North of C 

C. By means of a curved arrow, show how the winds shift in this case. 

Path of High South of D 
D. By means of a curved arrow, show how the winds shift in this case. 

FIG. 268. Shifting of Winds in Cyclonic Storms 



shelter for stock, in winter, unnecessary and to permit grazing 
throughout the year. 

The chinook is scarcely noticeable in summer. 
5. The foehn. This is the European chinook, common on the 
northern slopes of the Alps, where in the north-south valleys it 
hastens the ripening of grapes in the fall, and in winter rapidly 
melts the snows in its path. This has earned for it the name snow- 

To the class of cold winds belong: 

1. The norther of southwestern United States. It is the cold 
inflow of winds from the north at the rear of the winter cyclone. A 
fall of temperature of 50 in two hours has been noted. These winds 

often cause great suffering, loss 
of livestock, and even loss of 
human life. 

2. The blizzard. The Ameri- 
can name for a cold wind of high 
velocity, accompanied by snow. 
Winds of 50 miles an hour have 
been noted in blizzards with tem- 
peratures below zero. The bliz- 
zard begins with a cyclonic storm 
during which snow falls. This is 
followed by an anticyclone which 
brings rapidly falling tempera- 
tures from the northwest. It 
may not actually be snowing at 
the time, but the wind blows up 

the snow from the ground, producing what seems to be a blinding 

3. The bora. A cold wind in the territories of Istria and Dal- 

4. The mistral comes from the northwest and cools the coast of 
southern France, particularly around Marseilles. 

343. Velocity of winds. Wind velocity is measured by an 
instrument called the anemometer (Fig. 269). 

Approximate estimates of wind velocity may be made by 
means of the following table : 

FIG. 269. An Anemometer 






Light breeze 
Fresh wind 
Brisk to strong 
High wind 

Flags limp; leaves unmoved. 
Moves leaves on trees. 
Moves branches of trees; blows up dust. 
Sways branches of trees; makes whitecaps. 
Sways trees; moves twigs on ground. 
Breaks branches; dangerous sailing. 
Destroys houses ; uproots trees. 


Completion Summary 

Winds move from 


earth. Movements of air 


pressure areas, - 

are called currents. 

The doldrum belt is a region of - - winds. The winds 
that blow from the region of high pressure or - , to- 
ward - are called - . Between these and the poles 
are - . Instead of moving north and south, these 

winds are turned 

in the 



in the 

southern hemisphere. This is known as 

*In winter the land masses in the northern hemisphere are 

, and the oceans - - cyclones, which move into 

the United States. 

The apparent movement of the sun, north and south of 
the equator, causes a corresponding movement of - 
equator, which changes somewhat belts. These 

changes affect the - in the United States. 

A sea breeze is developed when the land and the 

air pressure, there, - - than that over the water. This 

is usually - - at night, starting breeze. For the 

same reason, - - and valley breezes are started. 

On a continental scale, these breezes become 
if they are strong enough - 


prevailing winds, 

In the temperate zone, the most important winds are 
. The spiral movement of air, in a low, is turned 


, in a direction. In a high, called an , 

the winds move in direction. 

*0ne theory of cyclones explains them as follows: friction be- 
tween and starts the whirl. 

A succession of highs and lows moves across the United 
States from to . 

Cyclonic winds velocity; but it is always 

at the center. Anticyclones have - - at the center. 
When velocities are high, they are called - or . 

In summer a cyclonic wind may develop into a , 

in which is accompanied by and . 

*The destructive effect of tornadoes on buildings is due to 
pressure . The difference of pressure great 

force which pushes 

When a low or a high passes near a place, there is a regular 
shifting of , in every case due to movement of the air from 
pressure to - . 

A hot wave, in the United States, blows - and - - for 

The chinook is a - - breeze which becomes - - as it 
descends and has a beneficial effect on the climate of the valley. 

A norther is - - wind, met in winter in United 


A blizzard high wind with . 


1. Show by diagram the relation between barometric pressure 
and temperature. 

2. What is a wind? 

3. How is a wind started? 

4. How do air currents differ from winds? In what way are 
they alike? 

5. Where are the doldrums? Why are they situated there? 

6. Where are the horse latitudes? 

7. What are the trade winds? Explain how they arise. 

8. Where are the prevailing westerlies? 

9. Why are these winds called planetary winds? 

WINDS 403 

10. Explain the direction of the northeast trade winds. 

11. Explain FerrePs Law. 

12. Explain the direction of the prevailing southwesterlies. 

13. Explain the connection between high temperature and low 

14. Explain the calm winds of the doldrum belt. 

15. Why are the trade winds so called? 

16. Explain the direction of the prevailing westerlies. 

17. Why do the wind belts shift? What effect has this on the 
United States? 

18. What is a sea breeze? a land breeze? What causes them? 

19. Why does a sea breeze sometimes continue to blow through- 
out day and night? 

20. How does a mountain breeze develop? Why is it stronger 
than a valley breeze? 

21. What are continental winds? 

22. What effects have the continental winds of the United 
States on the prevailing westerlies? 

23. What are monsoons? Where are they well developed? 

24. Do we have monsoons here? Explain. 

25. How do the winds move in a cyclone? an anticyclone? 

26. What is the general direction of movement of cyclones in 
the United States? Why? 

27. What is the average velocity of a cyclonic wind? In what 
part of the low is the velocity greatest? 

28. Why does the wind velocity decrease with the approach of 
a high? 

29. What are tornadoes? Why are they destructive? 

30. What are hurricanes? 

31. What are typhoons? 

32. Why is the wind stronger when the area is smaller? 

33. What is a thunderstorm? 

34. What are cyclone cellars? 

35. What is an anemometer? 

if Optional Exercises 

36. Explain the irregularity of winds in the horse latitudes. 

37. Why are summer days more apt to be windy than summer 
nights? than winter days? 


38. Why are winter winds stronger than summer winds? 

39. What is the source of winter cyclones in the United States? 

40. Explain how cyclones develop their counterclockwise ro- 

41. What is the source of tropical cyclones? What path do they 

42. Why do thunderstorms rarely occur at night or in winter? 

43. How do the winds shift as a low approaches and passes 
north of a given place? 

44. How do the winds shift as a low passes south of a given 

45. How do the winds shift as a high passes north of a given 

46. How do the winds shift as a high passes south of a given 

47. What is a hot wave? a blizzard? 

48. What is a norther? the chinook? 




344. Humidity. There is always a certain amount of 
water vapor in the air, which we call humidity. When it is 
small, we say the air is dry; when there is much humidity, 
we say the air is moist or muggy or close, particularly in 
summer. The water vapor manifests itself in various ways: 
by collecting as dew or frost on grass, by fog, clouds, rain, 
and snow. Very many of the changes involved in weather 
are a result of humidity. 

The amount of water vapor per cubic foot of air is called 
the absolute humidity. This might be 10 grains per cubic 
foot. In that case, the water in a moderate-sized room would 
weigh about 3 pounds. 

If more moisture is evaporated, the absolute humidity 
will increase, but finally the air will take no more; it has 
enough, it is satisfied or saturated, as we say. 

Suppose, again, the absolute humidity is 10 grains per 
cubic foot and that the same air could hold at saturation, 20 
grains per cubic foot. It is therefore half, or 50%, saturated. 
Usually we express this by saying: the relative humidity is 
50%. For human comfort, about 50% relative humidity 
at 68 F. is best. Plants thrive at about 75% relative hu- 
midity. We call the air dry when it has 25% or less relative 
humidity, and moist above 75%. 

If the temperature is raised, the air can hold more mois- 
ture. If, then, the absolute humidity at 68 F. is 3 grams 
per cubic foot, and the air at that temperature could hold 
6 grains per cubic foot, it has 50% relative humidity. If the 
temperature of that same air is raised to 86 F., it could hold 




12 grains per cubic foot, but only holds 3, and its relative 
humidity, therefore, has dropped to 25%. 

Raising the temperature, then, decreases the relative 
humidity (not the absolute humidity), and lowering the 
temperature increases the relative humidity. 

345. Dew point. We have just seen, in the last paragraph, 
that cooling the air will increase its relative humidity. Could 
the relative humidity be increased to 100% 
by cooling? The best way to answer that 
would be by a simple experiment (Fig. 270). 
Place some water in a metal container, add 
small pieces of ice, and stir gently with a 
thermometer. As soon as dew forms on the 
outside of the metal, read the thermometer. 
This is the dew point. It is 40 F. in the figure. 
What has happened in this ex- 
periment? The air contained mois- 
ture ; and as the temperature of the 
metal was lowered, it cooled the 
air immediately about it, increas- 
ing its relative humidity. When 
the relative humidity reached 
100%, the air could hold no more 
moisture. From that point any 
further cooling would squeeze moisture out of the air, since 
the air could hold no more. This is the dew point. 

The dew point is that temperature to which the air must be 
cooled in order to cause moisture to settle out. If the entire air 
were cooled to the dew point, it would rain. 

If the dew point is below 32 F., then we have/rosZ instead of 
dew; and if the entire air is cooled to that point, we get snow. 
The dew on objects out of doors, at night, is formed be- 
cause they radiate heat and drop in temperature. At the 
dew point, water or dew is precipitated on them. It does 
not rain, because the temperature of the entire air, while it 
may drop at night, does not fall below the dew point. 

FIG. 270. Finding the Dew 


346. Evaporation and condensation. Evaporation takes 
place from all moist surfaces and is increased by heat. Even 
ice evaporates, but much more slowly than water. It re- 
quires much heat to cause water to evaporate, and there- 
fore, in evaporating, water absorbs heat from the surface. 
Swimmers sometimes say that it is warmer in the water than 
in the air. This is true when a wind, especially a dry wind, 
is blowing, because the evaporation caused by the dry wind 
takes much heat from the wet skin. Moving air causes 
more rapid evaporation than still air, because the air above 
the wet surface may become saturated and prevent further 
evaporation; but when this saturated air is removed, more 
water can evaporate. Evaporation is more rapid when the 
relative humidity is low. 

All animals and plants evaporate water. It is estimated 
that a tree gives off 500 pounds of water on a hot day. 

All the moisture of the air comes ultimately from the 
oceans, to which it again returns, so that there is a great 
cycle of water in nature. 

Condensation is the opposite of evaporation. It is in- 
creased by lowering the temperature. When the air is satu- 
rated, 100% relative humidity, any further lowering of the 
temperature causes condensation to a liquid or solid; liquid 
if the air is above 32 F., and solid if it is below. 

The cooling which causes condensation may result 

1. Loss of heat by radiation from air to land or water. In 
the lower air, fogs are caused in this way. 

2. Contact of air with cold surfaces. Dew and frost are 
formed in this way. 

3. Mixing of cold and warm currents of air. Clouds, fogs, 
or rain may be caused in this way. 

4. Cooling by expansion. This is true of ascending air 
currents and is the chief cause of cloud formation and rain. 

We all know that cold contracts and heat expands. It 
takes heat to expand air, just as it does to evaporate water, 



FIG. 271. A Hair 

since that is a kind of expansion, too. And if the air is forced 
to expand without adding heat, its temperature will drop. 

When air currents are rising, then, 
they are expanding because of lower 
pressure aloft, and therefore cooling; 
and when the dew point 
is reached, condensa- 
tion takes place. 

When air descends, it 
is compressed and be- 
comes warmer; hence 
its relative humidity 

The doldrum belt, 
the region of lows, and 
the windward slopes of 
mountains, where air currents are rising, are 
apt to be rainy, while the region of highs and 
the leeward slopes of mountains, where air 
currents are descending, have clear skies. 

347. How to find relative humidity. 
Women often complain that their hair 
straightens out in moist air. This principle 
is made use of in the hair hygrometer, an in- 
strument in which a hair increases its length 
when the air is moist. The dial of the hygrom- 
eter reads the per cent of relative humidity 
(Fig. 271). 

Another method is by use of the sling 
psychrometer (Fig. 272). This consists essen- 
tially of two thermometers, mounted so that 
they can be swung around. One thermometer 
has a piece of wet cloth around its bulb, while 
the other is dry. Since evaporation takes up heat, the wet 
thermometer will register a lower temperature than the dry 
one; and if it is moved rapidly by swinging it, the evaporation 

FIG. 272. Sling 


will be rapid. If the air is saturated, 100% relative humidity, 
there will be no evaporation and both thermometers will read 
alike. If the air is very dry, low relative humidity, there will 
be rapid evaporation, and the wet bulb thermometer will 
drop many degrees below the dry. The dry thermometer re- 
mains the same in every case it merely registers the tem- 
perature of the air. A large difference between the two 
thermometers, then, means low relative humidity, and a 
small difference means high relative humidity. 

By means of a table furnished with the instrument, the 
relative humidity at any temperature can be determined by 
the difference in the readings of the two thermometers. 

348. Distribution of water vapor. Since most evaporation 
takes place at sea level, the lower air is of higher absolute 
humidity; that is, it contains more moisture. The moist air 
from the sea areas is distributed by winds and currents 
throughout the lower air or troposphere. Water vapor, and 
therefore clouds, are practically confined to the lower six 
miles of air: the troposphere. 

Although there is more water vapor in the lower air, the 
relative humidity is higher at greater altitudes, because of 
the cooling of the air as it rises and expands. It is this 
change, then, which causes condensation in the upper air, 
resulting in clouds and rain. 

Relative humidity varies with change of place and also in 
the same place, from hour to hour. It is highest at the 
coolest part of the day, usually in the morning, and de- 
creases as the day advances and the air warms up. It is 
lowest at the warmest hour of the day, from 1.00 to 3.00 
P.M., after which it slowly rises. 

349. Dew and frost. When air is cooled below the dew 
point by contact with a cold object, condensation takes place 
upon the cold object. Dew is formed, if the dew point is 
above 32 F., and frost if below 32 F. Dew and frost form; 
they do not fall. Frost is not frozen dew. Frozen dew becomes 
transparent beads of ice. 


Anything that checks the cooling of the ground and lower 
ah* tends to prevent the formation of dew and frost. The 
ground beneath trees and shrubs is often protected from 
dew, although it forms freely on exposed ground. A cloudy 
sky checks radiation and prevents the excessive cooling 
that forms dew and frost. Likewise wind, by carrying away 
the saturated air, prevents the deposition of dew or frost, 
but the cooling is even greater. It is rare to have dew or 
frost on cloudy or windy nights. 

Dew is usually formed on grass, because the grass is giving 
off much moisture and the relative humidity near the grass 
is very high. 

Fruit trees are often protected from frost by smudges. 
The smoke thus produced hangs over the orchard, checks 
radiation as a blanket would, and often prevents frost. 

When condensation begins, heat is set free, which checks 
further cooling; so if saturation occurs much above freezing, 
that is, when the relative humidity is high, frost is unlikely. 
Housewives often protect their flowers from frost on cold 
nights by exposing shallow pans of water in the room, in 
order to increase the relative humidity. As the room cools, 
condensation takes place and the heat set free may keep the 
temperature above freezing. 

This principle has been applied on a large scale for the 
protection of orchards, when there is no wind. By spraying 
water into the air, in and about the orchard, the relative 
humidity is increased and the dew point raised. 

350. Fog. When a volume of air is cooled below the dew 
point, condensation takes place on particles of dust, forming 
tiny droplets of water or crystals of ice. Without dust, con- 
densation will not take place. 

When droplets of water are so small that they float, we 
have a cloud. If it is low enough to reach the land or water, 
it is called fog; that is, a fog is a cloud at the surface of the 

London fogs, sometimes called "pea soup," are caused by 


condensation of moisture from humid air on particles of 
carbon thrown into the air by the use of soft coal. These 
fogs are sometimes so thick that traffic must be stopped. 

Fogs usually result from warm, humid air passing over 
cold surfaces. In winter, winds blowing from the sea are 
likely to produce fogs. 

Fogs are more frequent in valleys than on mountains. The 
cooler, heavier air accumulates in the valley, where there 
is more moisture, and condensation sets in. 

The fogs of the Grand Banks, off the east coast of New- 
foundland, are known to all navigators of the North At- 
lantic. They are caused by easterly winds, moisture-laden, 
passing over the cold Labrador current. Another cause of 
much of the Newfoundland fog is the ice brought down from 
the Arctic by the Labrador current. These icebergs are apt 
to be centers of dense fogs, especially in summer. These fogs 
seriously delay and endanger ships passing through them; the 
ships must reduce speed and often completely stop, with 
continuous warning signals. They also interfere with trans- 
atlantic airplane travel. 

Traffic in many harbors is frequently interrupted by 
dense fogs. Infrared radiation penetrates fog and cloud, and 
airplane photographs of the earth can be taken by the use 
of infrared film. A device for transforming infrared into 
visible light has just been invented, and this will enable the 
skipper of a vessel to see through the fog. 

351. Clouds are fogs formed high in the air, but this 
never happens above 6 miles, because of the absence of 
water in the stratosphere. As the warm current of air rises 
from the earth, it expands and cools, and an elevation is 
reached where the dew point is passed and condensation 
begins. The dew point at high elevations is often below 
freezing, so that clouds often contain ice or snow crystals. 

Clouds are classified chiefly by their form. The thin, 
feathery white clouds high in the air, frequently seen on 
fair days, are cirrus clouds. Cirrus means curls or ringlets. 



FIG. 273. Cirrus Clouds 

U. S. Weather Bureau 

These filmy clouds are composed of slender crystals of ice 
called spicules. Their average summer altitude is about 6 
miles and they generally move eastward (in the westerlies) 
at the rate of 60 miles an hour. In winter they move east- 
ward at a height of 5 miles, at about 100 miles an hour. 

Cirrus clouds are often forerunners of storms. They are 
due to upward air currents in a cyclone extending nearly to 
the stratosphere. As the warm air rises, it expands and cools 
below the dew point, which at that high elevation is about 
40 F. Rapid crystallization from the rarefied air produces 
the tiny crystals of the cirrus cloud and accounts for their 
being feathery and often transparent. The winds at the 
high elevation help to draw the cloud out into long wisps. 

Cumulus clouds are massive piles of clouds with an even 
base, resembling piled-up fleeces of wool or volumes of 
condensing steam from a locomotive. They are the result of 
rising currents of air, and are therefore storm clouds. These 
clouds, which sometimes are called thunderheads, are land 
clouds, formed by day, and "are more commonly in motion 
than at rest. 

The average summer height of cumulus clouds is about a 
mile and a half, and in winter they are about a mile high. 



FIG. 274. Cumulus Clouds 

U. S. Weather Bureau 

U. S. Weather Bureau 

FIG. 275. Nimbus Clouds 

Their velocities are from 6 to 9 miles an hour. Their even 
bases are from a quarter to a half mile above the earth. 

Nimbus is the name given to any cloud from which rain 
or snow is falling or may be expected to fall. They are of 
variable height, usually less than a mile, sometimes only a 
few hundred feet above the surface (Fig. 275). Nimbus clouds 
are of an even grayish tint, sometimes completely overcast- 
ing the skies for hours and even for days. 



FIG. 276. Snow Crystals 

U. S. Weather Bureau 

Stratus is any cloud, about half a mile high, spread out in 
parallel layers. It is a night cloud, common over the sea 
and in valleys. It includes fogs. 

Many clouds which do not assume quite one shape or the 
other of those mentioned are given combination names, like 
cirro-stratus, alto-cumulus, and cirro-cumulus. 

352. Rain and snow. We have seen how cloud is formed 
below the dew point, and we know that from the ordinary 
cloud we do not get rain. In order to get rain, the cloud 
particles must grow to such size (called drizzle size) that 
they will fall in air, and ordinary droplets do not unite on 
collision, but rebound like rubber balls. 

When vertical convection is going on, cloud particles are 
first formed about dust and other nuclei. This process soon 
removes the dust and the ascending air is clean, so that no 
further condensation can take place on such nuclei. There 
remains nothing but a few smaller cloud particles, to rise 
with the moist air; and further condensation must take 
place on these, until raindrops are formed. Condensation on 
fewer particles produces larger drops, heavy enough to fall. 


This process is called precipitation. If it occurs above 32 F., 
we have rain; if below 32 F., we get snow. 

Snow is not frozen rain. The water vapor passes at once 
from vapor to the solid condition. Rain and snow bear the 
same relation to each other as dew and frost. Snowflakes 
are crystallized water vapor, built upon patterns resembling 
beautiful six-rayed stars. Above a certain elevation, called 
the snow line, there is always snow on the ground. In the tem- 
perate zone the snow line is about two miles high. In the 
polar regions it is at sea level. 

353. Hail and sleet. If raindrops in their fall pass through 
a layer of air sufficiently cold, they are frozen into bits of 
ice called sleet. As the conditions described are most likely 
to occur in winter, when the land is colder than the air above 
it, sleet is a winter phenomenon. Sleet is frozen rain. 

In summer, especially on a hot afternoon and near the 
center of a cyclonic storm, large pellets of ice called hail 
often fall. Upon examination, hailstones prove to be made 
of concentric layers of ice. This structure, together with 
their often great size, suggests that they are frozen rain- 
drops, enlarged by successive condensations and freezings 
upon the surfaces. This occurs as follows: In the rising 
current of air some of the droplets may be carried up to 
an elevation where the temperature is below freezing. The 
raindrop is frozen. In the irregular, whirling current, some 
of these will be thrown to the side, where convection is not 
strong, and they fall into the lower layers, where condensa- 
tion on their surfaces wets them. Carried up again by strong 
air currents, the water is frozen and the process repeated 
until their size is so great that they can no longer be kept 
from falling. The velocity of the upward current for one-inch 
hailstones is about 50 miles per hour, so that such storms 
are very dangerous for aircraft (Fig. 277). 

Hailstorms are sometimes very destructive. Their paths 
are only a few miles in width and fortunately not of great 
length; for often, growing crops, orchards, and even forests 



U. S. Weather Survey 

FIG. 277. Hailstones from Emporia, Kansas 

are destroyed. Leaves, bark, and branches are stripped 
from trees; young animals are killed, and windows and roofs 
broken by the hailstones, which have been known to be 
larger than baseballs and even large oranges. 

354. Sheet ice. Sometimes rain falls in winter, but im- 
mediately, upon touching the ground or trees, it is frozen. 
This occurs when the lower air is just above the freezing 
point, while the ground and all solid objects near it, being 
better radiators, have cooled below freezing. This is sheet 
ice. It is popularly though erroneously called sleet. 

Sometimes the weight of ice on twigs and boughs is suf- 
ficient to break the branches. Telephone and telegraph 
wires are often broken down by sheet ice. 


Completion Summary 

Absolute humidity is measured in per . 

Relative humidity is % when the air is saturated; is 

% when the air is half saturated; and is % 

when the air contains one quarter of the moisture it could 

contain at . If the temperature is raised without 

adding more moisture, the relative humidity . If the 

temperature is lowered, the relative humidity until 

it reaches %, after which takes place. This 

temperature is called . If the dew point is below 

, we get . 

It requires heat to water. If no heat is added, 

evaporation will absorb and cause a drop in . 

Conversely, condensation gives out . 

Fog is condensation on in droplets settle. 

Fog in the upper air is called . Clouds formed very 

high consist of because temperature. These 

are called clouds. Cumulus clouds or thunderheads 

are due to . Nimbus clouds are . When cloud 

particles , we get rain. If the temperature is below 

, we get snow. 

Sleet consists of ram. Hail is sleet which . 


1. What is absolute humidity? 

2. Name as many words as you can which mean, or are 
caused by, moisture in the air. 

3. What is meant by saturated air? 

4. What is meant by 90% relative humidity? 

5. What conditions of temperature and relative humidity are 
ideal for human beings? for plants? 

6. Why is the absolute humidity greater over and near the sea 
than inland? 

7. The relative humidity decreases, usually, as the day ad- 
vances. Explain. Does the absolute humidity decrease also? 

8. What is the dew point? 


9. What must be the condition of the air in order that it start 
to rain? to snow? 

10. Why is one much more quickly chilled in a wet garment 
than in a dry one? 

11. Why is one cooled by a fan? 

12. What is meant by the cycle of water in nature? 

13. What conditions are necessary for condensation? 

14. How are fogs formed? 

15. What is the principal cause of cloud formation? Explain. 

16. Why are the skies clear in a region of high pressure? 

17. Why are the skies cloudy in the doldrums? 

18. What is the principle of the hair hygrometer? 

19. Explain how to find relative humidity by the sling psy- 

20. If the difference between the wet and dry bulb ther- 
mometers is very small, what does that tell about the relative 

21. What portion of the atmosphere has the highest absolute 

22. What part of the troposphere has the highest relative 
humidity? Why? 

23. At what part of the day is the relative humidity highest? 
lowest? Why? 

24. What is the difference between frost and frozen dew? 

25. Why is dew or frost seldom formed on cloudy nights? 

26. Why is dew usually formed on grass? 

if Optional Exercises 

27. Explain, using figures different from the text, how rela- 
tive humidity decreases with increase of temperature, while ab- 
solute humidity may remain the same. 

28. In what way does dew resemble rain? In what way is it 

29. Frost often forms on the windows of a house that is occu- 
pied. Why does it not form when the same house is unoccupied? 

30. What part of the atmosphere has practically no moisture? 

31. Why are London fogs particularly frequent? 


Dispersion of 

White Light into 

the Colors of 

the Spectrum 


355. We see the sun at rising as a golden sphere, at mid- 
day a globe of dazzling white, and at setting, if the air is 
dusty, it may disappear below the horizon as a ball of fiery 

As we ascend the mountain slope, the noonday sun takes 
a bluish tinge, and if we were to ascend far above the earth, 
we should find the sky black and the sun a brilliant white. 

The ever-changing color of the sun, as it mounts in the 
sky or shines through clear or cloudy air, is the result of 
selective reflection of light. 
White light is composed of 
many colors, each having its 
own wave length, and if one or 
more of these is taken away, 
the resulting light will have a 
different color (Fig. 278). A 
colored cloth often seems to 
have a different color during 
the day and night, because 
during the day it reflects light 
received from the sun, whereas 
the artificial light used in the evening contains colors not 
present in the sun. 

The shortest waves of light are the blue; and, like the 
small waves or ripples on water, they are easily turned aside 
by obstacles in their path, such as particles of dust. The 
longest visible rays are red; and these, like the great waves 
of the sea, pass by obstacles which scatter shorter waves. 

An object may appear one color by reflected light (that 


FIG. 278. White light is a mixture 
of many colors. These may be 
separated by a glass prism. 



Scattered by 


making sky blue 


is, when we look at it), and a different color by transmitted 
light (that is, when we look through it). A thin gold leaf is 
yellow by reflected light but green by transmitted light. 
A glass of soapy water, viewed from above, looks bluish 
white, but when the sun is seen through the soapy water, it 
appears red or reddish yellow. The short blue waves are 

scattered by the small soap 
particles, whereas the longer 
red ones pass by them. 

356. Colors of the sky. The 
sky appears to us blue, and 
the sun yellow. This is due to 
the scattering of the blue rays 
by tiny particles of dust and 
moisture; only the longer 
waves, reds and yellows, can 
get through. The color of an 

b J 6Ct is due to the .^ Jt 

sends to our eyes. Since the 
dust and cloud particles, present everywhere in the lower air, 
send blue waves of light to our eyes, this air, or as we call it, 
the sky, seems to be blue (Fig. 279). Near sunrise and sunset 
the light from the sun passes obliquely through much more 
air and therefore much more dust, which scatters more and 
more of the blue end of the spectrum and thus makes the sun 
appear less and less blue; that is, more and more red. 

After a volcanic eruption the sun is always deeper red 
because of the quantities of volcanic dust in the air. 

The beautiful colors of clouds are obtained by a com- 
bination of transmitted and reflected light effects. When the 
sunlight comes through a cloud, the reds and yellows get 
through. Other clouds, looked at, appear blue or green be- 
cause the cloud scatters or reflects blue and green, while the 
reds and yellows are permitted to pass through. 

*357. Refraction. When a ray of light passes obliquely from 
one optical medium to another of different density, it is bent 




making sun 


FIG. 279. Why the Sky Is Blue 



or refracted (Fig. 280). Inasmuch as the lower air is denser than 
the upper, every heavenly body that is not directly overhead seems 
to be higher in the heavens than it really is (Fig. 281). The nearer 




Ray bent 

X "*" normal 

>^ Ray bent away 
if from normal on 
entering air 

FIG. 280. Refraction of Light 


FIG. 281. Near sunrise and sunset 
the sun appears to be higher in the 
sky than it really is. 

to the horizon the body is, the greater the displacement because 
of the greater thickness of air. 

We must remember that we think we see an object in a straight 
line along the path of the rays of light from that object to our 
eyes. At sunrise and sunset the sun is seen above the horizon, 
when it actually is below. In that way refraction increases the 
length of the day. At the equa- 
tor this increase amounts to a 
few minutes, but at the poles 
the long summer day is in- 
creased by about 100 hours, 
and the polar night is shortened 
by the same amount. 

It is because of refraction, 
away from the normal, that an 
observer on a mountain top sees 
other, lower mountains higher 
than they really are (Fig. 282). 

FIG. 282. An observer on the 
mountain at the left sees a lower 
mountain higher than it is because 
rays of light from the mountain to 
his eye are refracted away from the 
normal as they enter less dense air. 

*358. Looming. The air nearest the earth is densest. If it is 
abnormally denser than the air ten or twenty feet higher, we get 
the illusion called looming (Fig. 283). The ray ZP bends as it strikes 
the denser layer of air; and by the time it reaches the observer at 
E, it is moving along a path EZ f . The observer at E therefore 



thinks that Z is at Z' . Looming is an early morning or winter phe- 
nomenon, due to rapid radiation of the land at night and the cool- 
ing of the air near it. Ships at sea are often discovered, while yet 
below the horizon, by looming. 

FIG. 283. Looming 

*359. Mirage is an interesting case of refraction through air of 
varying density (Fig. 284). On a flat stretch of desert, it often 
happens that the air nearer the sand is hotter, and therefore less 
dense, than the air directly above it. Oblique rays from distant 

FIG. 284. Mirage on the Desert 
The horseman thinks he sees a body of water in the distance. 

objects are refracted by the air as it changes its density, and an 
illusion of a distant object inverted is obtained, as if it were a 
reflection from a surface of water. Caravans are deceived by mirage 
into the belief that they are approaching water. 

*360. Dispersion of light. White light is a mixture of innumer- 
able wave lengths of light of all colors. We usually distinguish 
seven colors, violet, indigo, blue, green, yellow, orange, and red, 
but there are many more than seven wave lengths. When white 


light is refracted, each color is refracted differently, so that the 
colors separate from each other. We call this dispersion. The red 
is bent least, the violet most. (See Fig. 278.) 

Dispersion of white light gives us a spectrum containing 
the colors mentioned above. Sometimes in front of a cyclone, 
cirrus clouds, that outrun and foretell the coming storm, 
stretch across the sky, and light or colored rings encircle 
the sun or moon. These are called halos and result from 
dispersion of light by ice crystals in the cirrus cloud. The 
red, since it is least bent, will occupy the inside of the ring 
and the blue the outside. As the cloud thickens with ap- 
proach of the storm, the ring becomes smaller. This corrob- 
orates the very general belief that the greater the number of 
stars to be seen inside the ring, the greater the number of days 
before the arrival of the storm center. 

The colored rings seen about the street light, when one 
views it through a frost-covered windowpane, are similar 
to the halos seen around the sun and moon. 

The tiny water particles that constitute a fog also cause 
dispersion; but the nearness of these particles to the ob- 
server causes the rings to be very near the light, and these 
are generally called coronas. 

361. The rainbow is produced by dispersion of the light 
of the sun by raindrops. We shall understand how this is 
brought about if we analyze what happens when the light 
passes through a single drop (Fig. 285). 

A ray of white light entering a drop at any angle but 
normal is refracted, the blue being bent more than the other 
colors, the red least. Besides being refracted, some of the 
light is also reflected by the curve of the drop as shown. The 
eye, following along the line of the rays, projects their 
source as shown, so that we get the red on the outside and 
the blue on the inside. From each drop only one color will 
enter the eye unless the eye is moved. But other drops, in 
slightly different positions, will send other colors and so we 
get the entire rainbow. This is the primary bow. 

Primary Bow 


FIG. 285. The Rainbow 

In order to see a rainbow, the sun must be at the ob- 
server's back and its elevation must not be too great, for if 
it is, the rays will pass over the observer's head, because the 
average angle between the sun's rays and the rays entering 
the eye is about 40. 

Similar bows are sometimes seen in the spray of water- 
falls and fountains. 

A less distinct secondary bow, outside the primary and 
with colors reversed, is sometimes seen. It is formed by 
raindrops so situated that light is twice reflected within the 
drop before passing out to the eye (Fig. 285). The secondary 
bow is less distinct because some light is always lost every 
time it is refracted or reflected, and the secondary is re- 
flected twice. 

A tertiary rainbow is sometimes, but seldom, seen outside 
the other two, with colors the same as the primary. It is 



v^v^v ^ 
$$$'$# ' 



caused by light three times reflected within drops, and hence 
it is very faint and seldom seen. 

The reason why one does not always see the three rainbows 
is that the light is not strong enough. The primary bow, 
being most intense, is often seen, whereas the tertiary, being 
least intense, is seldom seen, 
but it is always there just the 

362. Lightning. Whenever 
a substance is broken up or 
torn apart, electrical charges 
are left on the two surfaces, 
positive on the one, negative 
on the other. During the up- 
rush of air in a thunderstorm, 
the raindrops are torn apart, 
the larger bits of drops being 
positively charged, the 
smaller, negatively. The rain 
that falls, consisting of the 
larger drops, is usually posi- 
tive. The voltage, or electri- 
cal pressure, increases as the 

drops increase in size. The small negative drops are carried 
up, forming thereby three electrical layers : high up, a nega- 
tively charged mass of droplets, then a positively charged 
mass, and the earth itself with an induced negative charge 
(Fig. 286). As the voltage builds up, it finally overcomes 
the high resistance of the air and a spark passes, usually 
between the positive and negative clouds. This ionizes the 
air in between, making it a better conductor, and a rush 
of electricity follows again and again, until the electrical 
pressure is too low to push across the gap. 

The lightning flash may be, and usually is, between the 
two parts of a cloud, or between a cloud and the earth, 
or, rarely, between two totally different clouds. Tropical 

FIG. 286. The Air Just Before a 
Lightning Discharge 



thunderstorms, which often have the most violent electrical 
displays, always take place in a cumulus cloud, in which 
we have the uprushing current of air. Hence the discharge 
is between one part of the cloud and another part of the 
cloud, and there is no danger whatsoever. 

All the freak effects of 
lightning are due to heating. 
Houses are set on fire. Metal 
objects are melted. Trees 
are stripped of their bark, 
or entirely shattered, by the 
heating and expansion of 
water in the layer carrying 
the sap (Fig. 287). Objects 
are thrown down when their 
supports are melted, or 
broken by expansion. 

*The discharge of electric- 
ity through air causes several 
chemical changes to take place. 
One important reaction is the 
fixation of nitrogen, causing 
it to combine with oxygen. The 
compound formed is soluble in 
water and furnishes nitrogen to plants. This is one of the few 
sources of fixed nitrogen, which is one of the substances essential 
to fertility of the soil. 

The belief that lightning does not strike twice in the same 
place is a dangerous error. The fact that lightning strikes in 
any place argues the existence there of favorable conditions, 
and it is more likely to happen again if a thunderstorm 
passes near enough. 

363. Dangers from lightning. In the days of the ancients, 
lightning was regarded as a javelin hurled by the king of 
the gods, and it is likely that the popular idea today is 
somewhat similar; that some sort of long, sharp object is 

FIG. 287. A Tree Struck by Lightning 


being hurled from the sky toward the earth, and that if by 
chance a person is in its path, he will be struck. This is a 
superstition that ought to be destroyed. But aside from 
that, if the real situation were known, much of the fear of 
lightning that people have would be dispelled, and most of 
the dangers easily averted. 

An atmosphere charged with electricity is like a boiler 
full of steam under pressure the electrical particles re- 
pel each other because they are all alike. Now, in the case 
of the boiler there is no danger until a leak develops; or 
suppose someone on the outside of the boiler starts to bore 
a hole in it. As soon as the metal is weak enough, the 
pressure of the steam bursts the thin piece of metal and 
forces it out of the boiler with explosive violence. The 
person who has been boring the hole will be struck by the 
steam and badly burned. No person can be hurt except 
the one who bored the hole. His injury is not due to chance, 
but to something he did. 

Similarly with the electrically charged atmosphere, the 
whole atmosphere from clouds to earth is charged under 
electrical pressure. It discharges whenever and wherever it 
can overcome the electrical resistance between cloud and 
cloud, or cloud and earth. Metals are good electrical con- 
ductors ; they have very little resistance, and when a piece of 
metal is near the cloud and joined to the moist earth, it fur- 
nishes a path whose resistance can be easily overcome, and 
the cloud discharges that way. It is likely that lightning rods 
discharge passing clouds and prevent electrical displays by 
relieving the pressure quietly. 

A tall tree with its roots in contact with moist earth is a 
conductor, but not so good as metal. Its resistance is greater; 
not so great, however, but that sufficient electrical pressure 
can overcome it, and in the process great friction develops, 
heat is generated, and the tree is burned. Standing under 
the tree one is not likely to be struck by lightning because 
the electricity is passing through the tree; but a branch may 


be burned off and fall, or the rending of the tree by expan- 
sion may cause injury to anyone near by. 

How can the discharge strike a person? That person must 
be a conductor with resistance less than that of the air. 
Suppose he holds on to a water pipe which is well grounded 
in moist earth, and is therefore an excellent conductor. The 
electricity may discharge that way, if the total resistance 
of person and metal is not too great; hence, the person would 
be struck; that is, the electricity would pass through him. 
But if the person were standing right next to the pipe and 
the pipe alone were struck, the person would be unaffected. 

Wet objects are often better conductors than dry ones. 
One should, during a thunderstorm, keep dry and not in 
contact with metallic objects which are grounded, and he is 
perfectly safe. Wire clotheslines are a particular source of 
danger; posts and leaders outdoors, radiators and water 
pipes indoors. Do not grasp wires of radios and telephones. 
Open windows are not more dangerous than a closed room, 
since electricity penetrates every material. 

As a general thing it is safer indoors during a lightning 
storm, especially if the building has lightning rods or 
grounded metal leaders. 

If one must be outdoors, it is safer in a valley than on a 
mountain, but not under a tree. However, if there are trees, 
the single, isolated, high tree is the most dangerous. 

364. Protection from lightning. The fear of lightning and 
the desire for protection have led to the adoption of many 
measures, the most common of which is the lightning rod. 

All solids are better conductors of electricity than air; 
hence buildings and trees are more often struck by lightning 
than the open ground near them. The greater the number 
of trees or buildings, the greater the number of paths for 
the electricity and the smaller the discharge through each 
one. On this account, a house in the city is safer than the 
isolated farmhouse, and a tree in the forest is safer than a 
"lone tree." It is rare indeed for a modern steel building to 



be struck. In fact it is highly probable that each grounded 
steel building acts, like a lightning rod, to carry off the 
electrical discharge quietly, so that it never accumulates to 
lightning pressure at all. 

The lightning rod usually consists of a metal ribbon, or 
flattened tube of copper, laid over the roof of a building, 

Connections at Leaders, Gutters 
=7 and all other Metal Parts 





^ Copper 

FIQ. 288. Connections for Lightning Rods 

with frequent pointed branches rising into the air. Electrical 
discharges occur on points rather than on surfaces. There 
must be sufficient metal to conduct the heavy current; 
otherwise the heating of the wire will be excessive and it 
may melt. It must be out of contact with combustible 
material like wood, which might be set afire by the heat. 
The lightning rod must be buried sufficiently deep in the 
ground to reach moist earth (Fig. 288). In the case of a 
petroleum tank, the safest protection is a grounded wire 
cage entirely surrounding the tank. 

365. Thunder. Thunder may be likened to the noise 
made by an explosion. The lightning flash heats the air 
along its path, and the sudden push caused by the expan- 
sion starts a compressional wave or sound. Echoes cause 


rolling thunder. Sound travels about a mile in five seconds, 
whereas light is instantaneous. When the lightning, there- 
fore, is very near, the thunder clap quickly follows the 
flash. If the storm is a mile away, it will take five seconds, 
after the flash, to hear the thunder. 

366. The aurora, sometimes called the aurora borealis, 
the aurora polaris, and northern lights, is a beautiful electrical 
display common in regions near the pole, but often seen in 
northern United States. It is believed to be due to the dis- 
charge of electricity into the upper air, at heights of 50 to 
150 miles, where the air is very rare. These discharges in- 
crease with sunspot maxima, and they are now believed to 
be sometimes alpha particles, but more often electrons, shot 
from the sun toward the earth. The aurora, then, is similar 
to electrical discharges in rarefied gases, like the neon lamps 
used for signs. As seen in the United States, northern lights 
usually consist of a more or less distinct arch of light, ex- 
tending east and west, and crossed by streamers of white or 
colored light, like a number of searchlights coming from a 
far northern point. These streamers change their length and 
position so rapidly that they are called "the merry dancers." 
They radiate from the north magnetic pole. 

Brilliant auroral displays are often accompanied by 
severe electrical and magnetic disturbances throughout the 
country. Telegraph and telephone services are interrupted, 
radio communication is bad, and the magnetic compass be- 
comes so variable as to be useless. 

Completion Summary 

The sky is blue because of the scattering of blue rays of 
light by . The sun is red at sunrise and sunset be- 
cause the thicker layer of air blue rays, and the 

light that gets through the air is lacking in rays. 

The sun is seen above the horizon after sunset and before 
sunrise, because of . 

Looming and mirage are also phenomena. 


*Dispersion of white light breaks it up into the - - colors 
because each color is at - - angle. 

Halo is caused by in a - - cloud. 

The rainbow is due to dispersion by - . The secondary, 

with reversed, is fainter than the primary; and the 

tertiary is so faint . 

When raindrops are - - by a - - of air, both 

- are electrically charged, the smaller drops - 

and the larger - . The smaller - - carried up, form 
a layer above the - . The charge finally becomes 

- and a spark - . This is lightning. Lightning 
causes damage because it - , not because - - bolt 
or other solid missile. It does not strike by chance. It passes 
through the best - - it can find. Moist earth is a good 
conductor. A tall tree, with its roots - - will, therefore, 

. A tall building - ; but if it has metallic - 
in contact , it will carry - - to earth. If there is 
enough metal, so that the resistance is not too great, there 
will be no excessive heating, and nothing - . This is 
the principle of the - . It is likely that the large number 

of - - in a big city discharges passing quietly 

and prevents . 

Thunder is the - - caused by the push of . 

The aurora is believed to be an electrical from 

the sun into the - . 


1. Explain the ease with which blue rays are scattered. 

2. Why is the sky blue? 

3. Why is the sky red at sunset and sunrise? 

4. How does a volcanic eruption affect the colors of the sky? 

5. Why do some clouds look red, while, at a different time, 
the same clouds might be yellow? 

6. What is a halo? 

7. What is the cause of a rainbow? 

8. Why is the secondary rainbow less distinct than the 


9. Why do we so seldom see a tertiary rainbow? 

10. Explain how a tree is shattered by lightning, and show 
that the lightning does not act like a huge axe. 

11. Under what conditions would a person be struck by light- 
ning? Explain, showing that the person is not struck, but that the 
injury is due to the passing of a current of electricity. 

12. State rules for safety from lightning in a storm. 

13. Why is a city house much safer from lightning than one in 
the country? 

14. Explain how to construct a lightning rod. 

15. What is the cause of thunder? 

16. How far away is the storm, if the thunder is heard ten 
seconds after the flash? 

17. What is the aurora borealis or northern lights? 

18. What effects has the aurora on scientific instruments? 

* Optional Exercises 

19. Explain how the sun can be seen after it has dropped 
below the horizon. What effect has this on the length of the day? 
What effect has it on the length of time the sun is seen above the 
Arctic Circle? 

20. Explain mirage with the aid of a diagram. 

21. Explain the relation between refraction and dispersion of 

22. Explain the colors of a halo. 

23. Why do we not always see a rainbow when the sun shines 
through a shower? 

24. How is lightning produced? 

25. What are the chemical effects of lightning? 

26. Explain why standing under a single, tall tree during a 
thunderstorm is more dangerous than being in a forest. 



367. Weather and climate defined. Weather is the con- 
dition of the air at a given time and place, with reference to 
temperature, pressure, moisture, state of the sky, and winds. 
These conditions, called the elements of the weather, are 
constantly changing, and as a consequence the weather is 
proverbially fickle. 

After sunrise, as the day advances, the temperature 
normally rises, reaching its maximum between 1 and 3 P.M., 
after which it falls till near sunrise the following day. With 
these changes in temperature come changes in relative hu- 
midity, which drops as the temperature rises, and also 
changes in barometric pressure, wind direction, and wind 

Climate is often defined as " average weather" over a 
long period; but in reality, not only the average but im- 
portant extremes must be taken into account. For example, 
if it were 100 F. in the daytime and 30 F. at night, the 
average would be 65 F.; and, over a period of time, we 
might think of the climate as mild in that respect, whereas 
with extremes of 30 F. and 100 F. it is not at all mild. 

We use the term weather in referring to atmospheric con- 
ditions at a given instant or for a short period, as a day, a 
week, or a month. We even speak of summer or winter 
weather; but when we wish to know what the climate is, 
we seek information about every atmospheric factor that is 
likely to affect our comfort and health, or our business, 
throughout the year, and year after year. 

368. Weather changes. Although the fickleness of the 
weather is proverbial, there are important controls, which, 



acting with different intensities and in different combina- 
tions, give us the different kinds of weather; and it is knowl- 
edge of these which enables weather forecasts to be made. 
The most important weather controls are: 

1. The alternation of day and night 

2. The succession of winter and summer 

3. The more or less systematic passage of lows and highs 
The first two are fairly regular at any place, though 

differing widely for different places, whereas the third varies 
in period and intensity. 

The daily and annual changes of the weather are more 
pronounced near sea level than at higher altitudes; in the 
interior of the continent than at the coast; in the polar 
regions than near the equator. 

Convectional cyclones are more frequent over land, more 
vigorous in the daytime and in summer, whereas noncon- 
vectional cyclones are more frequent and intense in winter. 
Both types are of longer duration over the sea, because of 
the greater humidity there, which makes the air less dense 
and starts the rising current necessary for the cyclone. 

In the United States during March and April, when the 
land is warming up most rapidly, it is a common occurrence 
to have days of blustering winds succeeded by nights of 
calm. This is due to the rapid warming of the lower air 
during the day, which causes convection, whereas at night, 
when the lower air becomes cooler than the upper, con- 
vection ceases and the winds die down. 

369. Weather in the tropics. Night has been called the 
" winter of the tropics/' because the variation in weather 
from day to night is greater than from summer to winter. 

In the doldrum belt the days are uniformly warm, owing 
to the nearly vertical rays of the sun. The rapid convec- 
tional ascent of the air in the morning is followed late in 
the afternoon by torrential downpours of rain, followed in 
turn by cloudless nights. The nearly equal day and night, 
combined with the low percentage of cloudiness, accounts 


for the great daily range of temperature. Cyclonic interrup- 
tions are of secondary importance. 

In the trade-wind belts, over the sea, there is a constancy 
of weather conditions not found elsewhere. The extreme 
range of temperature scarcely exceeds 10; and the wind 
blows continuously from the same direction and with about 
the same strength day and night. On land the range of 
temperatures increases, and over both land and sea there 
is little rainfall, except where the winds are compelled to 
rise over ascending land. 

Though there is some variability of wind direction and 
some precipitation in the trade-wind belts, owing to the 
cyclones which develop there, the constancy of the wind 
and the deficiency in rainfall are the marked characteristics 
of this belt. However, continents and islands and, to a more 
marked extent, mountain ranges that lie across the path of 
the trade winds are usually well supplied with rain. As illus- 
trations of this, note the abundant rainfall of eastern Aus- 
tralia, eastern Africa, and eastern Brazil; the northern slopes 
of the West Indies and the Hawaiian Islands ; and the eastern 
slopes of the Andes and the mountains of Mexico. In all 
these cases, the winds blow over the water just before they 
are forced to rise over the land, which in all these cases is 
high. As the air rises, it expands and cools until it reaches 
the dew point, and then precipitation takes place. 

Regions on the borders of the trades have monsoon 
changes of weather. If these regions are near the doldrums, 
there is the alternation of the light winds and abundant rains 
of the doldrums and the constant winds and light rains of 
the trades. If they lie near the high-pressure calms of the 
horse latitudes, then the characteristic conditions of the 
trades alternate with those of the horse latitudes. 

370. Weather outside the tropics. In the zone of pre- 
vailing westerlies weather changes are irregular and mainly 
of cyclonic control, with marked differences in the two 
hemispheres. In the southern hemisphere, where there is 


little land to interrupt them, the westerlies attain a con- 
stancy approaching that of the trades, and so high a velocity 
that they are called the " roaring forties. " In winter the 
cyclones are more frequent and succeed each other with 
almost periodic regularity. 

In the northern hemisphere, where the land is massed, 
there is a strong contrast between the weather of the land 
and the water areas of the prevailing westerlies; the land 
areas have much greater extremes of weather conditions, 
both daily and seasonal. The greatest seasonal ranges of 
temperature are found in the interiors of the northern conti- 
nents. In northern Siberia there is a range of about 200 F. 
In central United States a range of 130 F. is not uncommon. 

*As a result of the massing of the continents in the northern 
hemisphere, the North Atlantic and North Pacific oceans are low- 
pressure centers during the northern winter, and high-pressure 
centers during the northern summer. Over northern America and 
Eurasia the pressure is low in summer and high in winter. It is 
from these seasonally permanent pressure centers that the cyclones 
of lower latitudes are projected. 

In the frigid regions, although temperature changes are 
determined chiefly by the appearance and disappearance of 
the sun, the other weather elements are controlled mainly 
by the passage of cyclones. The precipitation, though less 
abundant, is mostly in the form of snow which accumulates 
upon the land. If more snow falls than disappears by melt- 
ing and evaporation, its accumulation results in the forma- 
tion of an ice sheet, like that which covers Greenland and 
the Antarctic continent. 

371. Weather prediction. After a thorough understand- 
ing of the relative values of the factors determining weather 
in any region, it is possible to predict, with a high degree of 
accuracy, the changes of weather likely to occur. The degree 
of accuracy attainable varies with the season and with 
geographic position. Under the doldrums and trade winds, 
where the daily change is dominant, weather prediction 


may be made with an assurance almost amounting to a 
certainty that it will be fulfilled. Indeed, the weather 
changes there are so regular and certain that the weather 
is not a topic of conversation. 

In regions where the weather is due mainly to cyclones, 
it is not possible to predict with nearly so high a degree of 
accuracy. Yet even here the relative values of the factors 
are so well known and the systematic movement of cyclonic 
disturbances so well understood that predictions may be 
made with the reasonable expectation that a large per- 
centage of them will be fulfilled. These predictions for any 
station must take account of: (1) the systematic movement 
of cyclonic disturbances, their strength of development and 
place of origin, and direction and rate of movement ; (2) the 
season; and (3) local topography. 

372. Weather in the cyclone. To understand the weather 
conditions which prevail about lows and highs, it is neces- 
sary to remember the directions of the winds about these 
disturbances and the effect upon the humidity resulting 
from a change of temperature. 

In the United States, cyclones, as we have seen, move 
eastward, and the winds blow in toward the cyclonic center 
in counterclockwise spirals. At any station the wind will 
not, as a rule, be blowing directly toward the center, but 
a little to the right of it (Fig. 289). Therefore, in front of 
the cyclone, the winds are blowing from a warmer to a 
cooler region (south to north) and their relative humidity is 
increased. As they approach the center of the low, the air 
rises, and its humidity is further increased by cooling from 
expansion. This may be sufficient to bring the air to satura- 
tion. As a result of these conditions, a rising temperature 
with cloudiness or precipitation generally characterizes the 
front of the low and may be predicted as a well-developed 
cyclone approaches. 

In the rear of the cyclone the winds are moving from 
colder to warmer regions, north to south; and as a result, 


the relative humidity of the winds is lowered and the skies 
are therefore clear. 

As a result of this difference in conditions behind and in 
front of the cyclone, the increase in humidity, due to ascent, 
may bring the air in front of the cyclone's center to the 
saturation point, and yet not saturate the less humid air in 
its rear. Consequently, falling temperatures and clearing skies 


FIG. 289. An Ideal Low 

The heavy arrow indicates the movement of the low toward the northeast. 
Note cloudiness with rain or snow in front of low. In the rear of the low note 
clearing skies. 

may be expected after the center of a well-developed cyclone 

The direction of the shifting of the wind depends, as we 
have seen, upon the position of the path of the cyclone's 
center, whether north or south of the station. (See p. 399.) 
Ordinarily the strength of the wind increases as the cyclone 
approaches, and decreases as the cyclone recedes. 

In winter the strong indraft of cold air in the rear of a 
cyclone, if accompanied by snow, is known in the United 
States as a blizzard. 

373. Weather in the anticyclone. Since the movements of 
the air about a high are the reverse of its movements about 


a low, it follows that the conditions in respect to temperature 
and humidity which prevail about a high are likewise the 
reverse of those which prevail about a low. In front of a 
high the winds are northerly, and behind a high the winds 
are from a southerly quarter, while at the center of the high 
the air is sinking. Consequently, fair and cooler weather is 
usually predicted as the high approaches, and rising tempera- 

FIG. 290. An Ideal High 

The heavy arrow indicates the movement of the high toward the northeast. 
Clear, cold weather characterizes the front of the high and warmer, cloudy 
weather appears at the rear. 

tures with possible cloudiness or even precipitation as the 
high recedes. 

Since the winds start from the center of the high, unlike 
the low, the winds weaken as the high approaches and 
strengthen as it recedes. For example, a person directly in 
the middle of a high would feel very little wind, since air is 
moving away from him. As with the low, the direction of the 
shifting of the wind is determined by the position of the 
station with reference to the path of the center of the high. 

In winter, if a high follows closely in the wake of a well- 
developed low, the fall in temperature may be abnormal. If it 
is as much as 20 F. in 24 hours, reaching a temperature of 


freezing or lower, it is called a cold wave. In southern United 
States the term is applied to changes somewhat less than 
20 F., even if it does not drop below freezing. 

374. Weather service. The United States Government 
has established a weather service extending to all settled 
parts of the country. This service, which is the work of the 
Weather Bureau, a division of the Department of Agri- 
culture, has its central office in Washington, D.C. Its corps 
of observers to the number of more than three thousand, 
paid and voluntary, are distributed throughout the country. 
Regular observations of the weather are made at more than 
two hundred stations, as nearly as possible at the same 
instant, 8 A.M. and 8 P.M., 75th meridian time, and are re- 
ported by telegraph to the central office at Washington and 
to each other. The most important observations are: pres- 
sure; temper atureSj current, maximum, and minimum; direc- 
tion and strength of wind; amount and kind of precipitation 
during the past 24 hours ; and percentage of cloudiness. 

375. Weather maps. When these data are collected and 
plotted on a map of the United States, the result is a weather 
map. (See Fig. 262.) The daily map is published at the cen- 
tral office in Washington, and also at one or more substations 
in every state. It not only sets forth the weather conditions 
existing at the time of observation, but also serves as a basis 
for prediction of the weather for the 24 or 36 hours following. 

Each local map supplements the general prediction for 
the entire country with a forecast for the particular locality. 
To be of value for purposes of forecasting, weather maps 
must be distributed the day issued, since weather conditions 
are constantly changing. 

Since our weather is mainly of cyclonic control and since 
the cyclonic disturbances move eastward across the country, 
the weather map as a basis of weather prediction is of more 
value to the eastern than to the western part of the country. 
On the Pacific coast it is of little value, since there are few 
stations farther west to report coming changes. With further 


extension of wireless telegraphic service and the radio, the 
value of the weather service to the western coast will be 
correspondingly enhanced, because information will be ob- 
tained from vessels on the Pacific. 

376. Value of weather predictions. Every observer is 
familiar with the daily and seasonal changes of temperature, 
also with the fact that there are other important changes 
that are irregular in their occurrence. More and more people 
are learning to appreciate the relation of these unperiodic 
changes of the weather to the eastward march of cyclonic dis- 
turbances, and to appreciate the great value of our weather 
predictions. Each year brings a wider use of these predictions 
and the more general rejection of the predictions of charla- 
tans who make year-long forecasts. 

Among the first to realize the benefits of our weather 
forecasts were the shipping interests of our southern and 
eastern coasts and of the Great Lakes. Not infrequently 
censuses have shown that marine property to the amount 
of more than $25,000,000 has been held in port because of 
storm warnings issued. Few masters of vessels now leave 
port without knowing the latest forecast of the weather. 

Shippers of perishable goods are also interested in weather 
predictions. Estimates from shippers place the value of 
property saved by the warning of the cold wave of Janu- 
ary 1, 1898, at nearly $5,000,000. Aviators, skippers, farmers, 
planters, truck growers, and fruit growers are interested in 
being forewarned of the changes in the weather, especially 
when these changes mean destructive winds, floods, or 

It is our confident belief that, with a more extended field 
of observation and a better knowledge of upper air con- 
ditions, the present practical limit for safe predictions (36 
hours) may be considerably increased. 

*377. Weather signs and proverbs. There are two distinct 
classes of weather signs. The first is based on century-long observa- 
tions by those whose occupations have led them to observe weather 


changes closely; the second class includes a mass of superstitions 
that have been strangely preserved and transmitted. The signs 
of the first class have usually found expression in trite sayings 
that have come to be known as weather proverbs. As an aid to 
memory these proverbs are commonly expressed in rhyme. 

" Rainbow in the morning, sailors' warning; rainbow at night, 
sailors' delight" is a proverb that is true only in those regions 
where cyclonic storms move eastward. If the rainbow is seen in the 
morning, the storm center is apt to be westward, and its further 
progress will bring it nearer. 

" Mackerel scales and mares' tails make lofty ships carry low 
sails" is applicable the world over. The long, wispy clouds called 
" mares' tails," and the sky flecked with cirro-cumulus clouds 
known as a " mackerel sky," are the result of the high-level over- 
flow of air in front of a cyclone. Consequently they presage a 
coming storm. "Mist rising o'er the hill brings more water to the 
mill" the world over. 

Weather proverbs are usually of only local application, though 
many are worldwide. When local, in order to appreciate them 
one must be acquainted with the local conditions. 

378. Climatic controls. Since climate is but average 
weather, those conditions which control weather likewise 
control climate. The most obvious, and perhaps the most 
important, climatic controls are: 

1. Latitude 

2. Height above sea level 

3. Distance from the sea 

4. Position with reference to mountain ranges 

5. Position with reference to prevailing cyclonic paths 
Although climate is defined as the average condition of 

the air with reference to the various climatic elements, it 
does not follow that when these averages are the same, the 
climates are alike or even similar. 

New York City and San Francisco have about the same 
average temperature for the year, but New York has hot 
summers and cold winters, whereas San Francisco has equable 
temperatures throughout the year. The central Mississippi 


Valley has about the same annual rainfall as the coast of 
California; yet in the interior the rains are distributed 
through the year, whereas on the western coast they are con- 
fined to the winter months. 

Of vastly greater importance than averages are the extremes 
of climatic conditions, and the distribution of these condi- 
tions through the year. 

379. Climatic zones. Temperature being the most im- 
portant climatic element, and depending, as it does, mainly 
upon latitude, the earth may be divided into east-west 
zones, each of which furnishes a distinct type of climate. 
Within any zone there may be considerable variation from 
the type, yet there is sufficient similarity to justify the di- 
vision into zones (Fig. 291). 

The customary division, whereby the zones are bounded 
by parallels, gives us zones of light rather than climatic 
zones; therefore the tropics and polar circles are not 
boundaries for torrid, temperate, and frigid climates. A 
more reasonable boundary is the isotherm. It has been sug- 
gested that the average annual isotherm of 68 F. be taken 
as the poleward boundary of the torrid zone, and the summer 
isotherm of 50 F. as the poleward boundary of the temperate 

The temperature of 68 F. is about the temperature we 
desire for our houses in winter, and the temperature neces- 
sary for so-called tropical plants; a temperature of 50 F. is 
necessary for trees and for the maturing of the hardier 

The temperate zone is the widest zone, and wider in the 
northern than in the southern hemisphere. This is due to the 
excess of land north of the equator, as land is a better ab- 
sorber of insolation than the sea. The frigid zones or, more 
accurately, the polar cold caps have an average temperature, 
even in the hottest month, below 50 F. 

In classifying climates, the separation into temperature 
zones does not take into account the very important element 



of rain, and we shall therefore have to subdivide each zone 
into climatic types. 

380. Tropical rainy climates. The temperature of all 
climates of the torrid zone is high, 75 F. to 100 F., except 
where mountains rise into cold altitudes. 

The almost vertical rays of the sun heat up the lower air 
in the early forenoon and cause rapid convectional currents. 
These rise by noon to such altitudes that their cooling by 
expansion produces condensation of their vapor, the forma- 
tion of clouds, and rain in the early afternoon. The rains 
are of almost daily occurrence throughout the year. 

The other climate elements call for a subdivision into these 

1. Constantly wet forest 

2. Savannas with wet and dry seasons 

3. Monsoon savannas, with a short dry season 

1. Tropical rain forest climate. Besides the high temperature, 
which continues into the night and from day to day, the air is 
charged with moisture which comes down every afternoon in a 
heavy cyclonic storm. The total annual rainfall in this climate is 
100 inches or more, and dense forests are common. 

Man finds this climate oppressive and enervating because of 
the combination of temperature and humidity. Tropical rain 
forest regions include the Amazon Valley in South America ; and, 
in Africa, the valley of the Congo River and part of the west coast. 
Southern Asia and the East Indies are tropical rain forest. 

2. Tropical savanna climate has a definitely separated wet and 
dry season, giving fewer and less dense forests and more extensive 
regions of tall grass like the prairies. Temperatures are high, al- 
though part of the dry season is slightly cooler. Rainfall is ex- 
treme: very much during the wet season, and very little during the 
dry season. The total rainfall may reach 50 inches. The rainy 
season occurs during periods of high altitude of the sun. The 
savannas of the northern hemisphere are having a rainy season 
4 while those of the southern are undergoing a dry season. 

Savannas are situated farther from the equator than the rain 
forest areas, between the doldrums and the trades; and it is the 


shifting of these belts which causes the dry and wet seasons; the 
trade wind causes a dry and the doldrums a wet season. Repre- 
sentative regions are the Llanos of the valley of the Orinoco River 
in Colombia and Venezuela, the Campos of Brazil, the Sudan in 
Africa, and the grasslands of northern Australia. 

3. Monsoon savanna climate. Wherever the monsoon winds 
blow, a modified savanna climate prevails. During the dry period 
the land breezes are very strong because they strengthen the 
trade winds. In India the season starts in October. The climate 
is clear and cool. By February it gets warm and dry, but this 
season is short and is soon followed by the rainy season, when the 
sea breezes blow from the ocean. Because the winds are forced to 
ascend high mountains, the rainfall here is the heaviest in the 
world, 500 inches per year at the foot of the Himalayas. Monsoon 
savannas differ from the tropical savannas in that the rains are 
not the doldrum variety, but are of sea-breeze origin. 

381. The dry climates. A climate is considered dry when 
evaporation is greater than rainfall; there is no permanent 
ground water nor are there permanent streams, except such 
as merely flow through the region, their source being in some 
more humid area. 

There is often a region of semiaridity separating the dry 
from the humid regions. These semiarid areas are called 
steppes. But there is no sharp line of separation between 
them. In the United States a region is considered dry if 
the annual rainfall is less than 10 inches, and semiarid if it 
is between 10 and 20 inches; but such a criterion does not 
hold throughout the world; for while a 25-inch rainfall in 
Nebraska might leave the region humid, the same rainfall 
in equatorial Africa still leaves the region parched, be- 
cause of the rapid evaporation due to heat. We have to 
consider then, two types of dry climate: the equatorial and 

The temperatures of the dry climates are extreme, high 
in the daytime and low at night. The relative humidity is* 
very low, from 10% to 30% during the day, permitting the 






sun's heat to penetrate the clear, cloudless atmosphere. Con- 
versely, at night radiation is rapid and the land cools. It is 
not uncommon in Arizona to have a temperature of 130 F. 
during the day, followed by freezing temperature at night. 
The air is often filled with fine dust, since it is in such 
regions that wind action is at a maximum. 

382. Equatorial deserts. The trade winds that pass over 
land areas are drying winds; and they are the chief source 
of equatorial deserts. Such a belt is found in Australia, 
Africa, South America, and in our own southwest, where 
the rainfall is under two inches a year. In northern Chile the 
average rainfall for a 20-year period was less than one tenth 
of one inch per year. On the Sahara Desert, the rainfall is 
less than five inches. But not only is the rainfall on deserts 
very small but it is very uncertain. In the Chilean desert 
it may not ram for five years at a stretch. In some of the 
desert areas rain may come in sudden downpours, which, 
because of the sparse vegetable cover, becomes a torrent, 
destructive hi its velocity. Much of the water evaporates 
rapidly, the rest sinks into the ground and forms springs 
and playa lakes which, however, soon dry up. 

The skies are clear although occasionally a cloud develops 
and even a thunderstorm; but the rain may evaporate with- 
out reaching the ground. 

Relative humidity as low as 2% with temperatures over 
100 F. are known on the Sahara. Temperatures in summer 
run up to 110 F., and the highest ever recorded was 136 F. 

383. Deserts of the temperate zone are not situated 
there because of their latitude, but rather because they are 
far inland on the continents, or separated from the ocean 
by highlands. Hence the great deserts of the temperate zone 
are in the central areas of Asia and North America, the two 
largest continents. 

The chief difference between these deserts and those of the 
torrid zone is the temperature. In the temperate zone, deserts 
have a severe cold season. 



384. Steppes or semiarid climates. On the margins of the 
deserts in both the torrid and temperate zones, there are 
semiarid areas in which the rainfall is about twice as great 
as in the desert. But farming is almost impossible, because 

FIG. 293. The Steppes of Western United States 

the rainfall is not dependable unless supplemented by irri- 
gation. Such regions are more often given over to grazing, 
because grass will grow when it rains and turn to hay when 
it becomes dry and warm. A large section of western United 
States is steppe land (Fig. 293). It is there that great irriga- 


tion projects, such as those at Roosevelt and Boulder Dams, 
are reclaiming vast areas of our country. 

385. Moist temperate climates. In the equatorial climates 
the important factor is the rain; hence climates there are 
classified as rainy or dry. But in the temperate zone the 
temperature becomes more important, and the seasons, in- 
stead of wet and dry, are summer and winter. In the torrid 
zone there is always enough heat, and the important need, 
then, for plant growth is water. In the temperate zone the 
most important factor in growth of plants is the amount of 
sunshine, and farming is done chiefly in summer. 

The succession of cyclones and anticyclones which causes 
important changes in the weather is an important feature 
of these regions. 

*386. Mediterranean climate has dry summers and moist, mild 
winters. Skies are clear and, in general, the climate is considered 
almost ideal for a winter resort. The Mediterranean lands, southern 
California, South Africa, and South Australia have Mediterranean 

It will be noticed that these areas are all located near the edges 
of the tropical climates or on the western coasts. They are in the 
paths of the prevailing westerlies, which are moist, and on the 
shifting of the wind belts they lie in the tropics; this gives them 
dry summers with clear weather and mild winters with cyclonic 
changes, but these are not so common as farther north. But even 
in winter the weather is good ; rains are few but heavy, and the sky 
is not often overcast. 

Nearness to large bodies of water makes for mild temperature 
changes: 40 F. to 50 F. in the winter and 70 F. to 80 F. in the 
summer. But humidity is low in the summer, and the heat, even 
when it is 95 F. as in some places in California, is not oppressive ; 
and then, these hot days are followed by cool nights, around 60 F., 
and day after day this same good weather is repeated. 

*387. Moist subtropical climates have more rain than the 
Mediterranean climate, and the rain is spread over the entire year 
or is concentrated in the summer, rather than in the winter. South- 
eastern United States (Gulf States), part of the Argentine, eastern 


China, and Japan are representative regions. In general these 
lands lie on the eastern sides of continents, where they are bathed 
by warm ocean currents. 

Temperatures are like those of Mediterranean regions. Summer 
temperatures are high and the air is moist, 70% to 80% relative 
humidity. The nights as well as the days are hot and uncomfort- 
able because radiation is prevented by clouds. 

In winter the temperature is mild, about 60 F. in the daytime 
and 40 F. at night. The growing season is long and well suited to 
oranges, pineapples, and other crops requiring a long time to 
mature. But frost is not unknown; in southern United States it is 
quite common, and it results in great loss to the fruit growers. 

388. Marine west coast climate. The western coasts have 
the benefit of the tempered westerlies coming from the ocean. 
This gives throughout the year an evenness of temperature 
not found on the eastern coasts of the temperate zone. There 
is more rain, sometimes up to 100 inches, and it is concen- 
trated in the winter months, because in winter the colder 
land chills the moist winds from the ocean; and that brings 
winter fogs as well. 

The northern and southern hemispheres differ widely in 
the climates of then- western coasts, as a result of ocean 
currents. Alaska and northwestern Europe are warmed many 
degrees above normal by winds from the great warm currents 
in the Pacific and Atlantic oceans, while Chile, western 
Africa, and Western Australia are cooled as a result of the 
branches, along these coasts, of the cold Antarctic current. 

The climatic influence of warm and cold currents is much 
greater on windward than on leeward coasts. In the wester- 
lies the windward coasts are the western coasts, whereas in 
the trade belts the windward coasts are the eastern coasts. 

389. Continental and marine climates. Continental humid 
climates are found in central and eastern United States and 
Europe and in eastern Asia. The interiors of all continents 
are marked by great variability of temperature, which de- 
creases as we approach the coast. 


Winters and summers are severe. In the summer the 
temperature may exceed 100 F., while winters may drop 
to 30 below zero. Chicago, in this area, experiences these 
extremes. Rainfall is small inland and there are frequent 
periods of drought, but the low temperature makes drought 
less disastrous. 

Temperatures at the seacoast are not subject to these ex- 
tremes for the following reasons : 

1. Water rises in temperature much less than land for the 
same insolation. 

2. Water radiates much less than land. 

3. Ocean currents distribute the heat more uniformly. 

4. There is more cloudiness over the ocean. 

*390. Mountain climates. As we ascend a mountain in any 
latitude, all the climatic elements change from those prevailing at 
the base of the mountain. It gets colder, absolute humidity de- 
creases, and relative humidity increases until the dew point is 
reached, and then decreases. Precipitation increases for a time as 
we ascend, and then gradually fails; and winds increase in strength 
and constancy. 

On the whole, all of the climatic elements become more constant 
with altitude; and this sameness is the most marked characteristic 
of mountain climates. 

The windward and leeward sides of mountains, and particularly 
of mountain ranges, are likely to present very different types of 
climate. The windward side, owing to ascending currents, which 
expand and cool, has an excess of rainfall, while the leeward side 
with descending currents, which get warmer, is likely to be dry. 
In the trades the eastern slopes receive the rainfall, whereas in 
the westerlies the eastern slopes are dry. 

The arid regions of Nevada and western Argentina are examples 
of arid regions to leeward of mountains in the westerlies; and the 
arid western coasts of Mexico and Peru lie to leeward of moun- 
tains in the trade-wind belts. 

The southern slopes of the Himalayas receive most of their 
rainfall while the southwest monsoon blows; and the northern 


slopes of the Atlas Mountains of northern Africa are the windward 
and therefore the rainy slopes. 

*391. Polar climates are characterized by freezing tempera- 
tures most of the year, day and night. But nearer the polar circles, 
in summer, when there are 24 hours of daylight, it may reach 
75 F., and plants grow for a short period. 

Precipitation is less than 10 inches over most of the area; but 
this does not make for desert conditions everywhere, since the 
precipitation is snow and little evaporation takes place. In fact, 
much of the area is covered by an ice cap. 

The polar cold cap is much more extensive in the southern than 
in the northern hemisphere, possibly because of the aphelian 

An important factor in the polar climates is the long polar day 
and night. For about 6 months it is day; and for the following 6 
months it is night. 

*392. Climates of the past. It is now known that there have been 
great changes of climate during past geological ages. Evidences of 
these changes are found in the character of the rocks and in the 
fossils found in them. Glacial climates have existed in the United 
States as far south as New York (page 120), Cincinnati, and 
St. Louis, as well as in northern Europe, South America, Africa, and 
Australia. Evidences of warm climates have been found in Alaska, 
Greenland, and Spitzbergen. 

393. Glacial climates. Evidences of former glacial climates are 
recognized chiefly in the character of the deposits left by the ice 
sheet. These deposits, known as glacial drift, are unassorted, an- 
gular, faceted (rubbed flat), and striated (scratched and grooved). 
The deposit of the last glacial period is a confused mixture of clay, 
sand, gravel, and boulders. Since it was so recently formed, it is the 
surface deposit of much of northern United States and Europe. It 
is loose and unconsolidated. 

Similar deposits have been found, in various countries, in rocks 
laid down during past geological periods and now covered by 
great thicknesses of other rock. These deposits, now consolidated, 
are known as tillite; and, wherever found, they are recognized as 
unquestioned evidence of glacial climates of the past. Such de- 
posits are made only by glacial action. 

*394. Warm climates. All theories concerning the evolution of 


the earth agree in the belief that the sea was originally hot, even 

Reef -making corals thrive only in waters that are warm 
seldom below 68 F. In the present seas they are found mostly in 
the tropics, no farther north than the Bermuda Islands. This is 
due to the great, warm Gulf Stream, that flows past these islands. 

It is not surprising then to find that vast masses of rock of the 
early periods of the Paleozoic Era are made of coral. We find such 
rock in New York, Tennessee, Alaska, and Spitzbergen, indi- 
cating that in the distant past these regions were bathed by warm 

It is worthy of note, too, that the cold periods of the earth's 
history coincided rather well with mountain formation, while 
there were warm periods, more often, during times of widespread 

For example, toward the end of the Permian period, when the 
Appalachian Mountains were uplifted, continental glaciers were 
common in many parts of the earth. On the other hand, the Penn- 
sylvanian was a time of low, swampy land ; and it was at that time 
that there were great tropical forests covering the land. 

The evidence of this is that the fossil trees of the Pennsylvanian 
coal do not show rings of growth as do the trees of temperate 
climates. In our modern tropical forests we also have trees which 
do not show these rings, because their growth is not suspended 
by a season of cold. Hence we can assume that the climate of 
the Pennsylvanian was of the tropical rain forest type. 

Some of these fossil trees have been found north of the Arctic 

*395. Aridity of past ages. In arid regions evaporation ex- 
ceeds rainfall, and lakes in such regions are salt. At the same 
time, beds of salt and gypsum are deposited on the borders, as at 
our Great Salt Lake and the Caspian and Dead seas. No such 
deposits form in humid regions. 

In the distant past these regions were humid. We find evidence 
of the existence of a much larger fresh-water lake, ancient Lake 
Bonneville, which has now shrunk to the Great Salt Lake, because 
of a change in climate from humid to arid. 

Similar changes must have occurred in all regions where we 
find deposits of salt and gypsum in the rocks of past ages. Such 


deposits are found underlying much of the area of New York, 
Ohio, Michigan, Kansas, Oklahoma, and Louisiana; and it is 
worthy of note that the greatest salt deposits of the world, par- 
ticularly at Stassfurt, Germany, are of the Permian period. During 
the Permian, the land was rising rapidly and this may have been 
responsible for the world-wide aridity. These regions are today 
humid, indicating another change in climate since ancient geo- 
logical times. 

Completion Summary 

Weather depends upon - - air. 

Climate is - weather, together with - - extremes. 
In the tropics, weather is - . Every day there are 
-, while nights 

In the trade-wind belt, weather is . Rainfall is 


In the regions of prevailing westerlies, cyclones. 

Weather prediction is - - necessary in most regions 
except where - - cyclonic storms. 

As a cyclone approaches, the weather will be - - with 
probable - , and as it passes, - - and - . 

The United States Weather Bureau prepares - - day, 
a weather map and - - prediction for - . These are 
of much - - value for - - eastern - - country. 

Climatic zones based on - - may be established by 
using isotherms instead of parallels. In such zones will be 
found several climates, controlled chiefly by - . 

In the tropics we find several kinds of rainy , and 

several kinds of - climate. 

*The tropical rainy climates include ; wet and 

dry - and - - savannas. 

The dry climates include and . 

In the temperate zone we also have - - and , 

whose situation has no relation to their - , but rather 
because they are continent, or - sea by . 


*Mediterranean climate is ideal. It is usually found on 

coasts, whereas on the eastern coasts we often find climate, 

not very comfortable, but good for . 

In marine west-coast climates we find tempera- 
ture and rain. These climates are modified by 

which, in the hemisphere, are different . 

Continental humid climates are usually found in the 

temperate zone, in the and eastern . In the 

central sections temperatures . Near the coast tem- 
peratures and rainfall . 

*0n mountains climatic conditions are . On the wind- 
ward side . 

The important factors in polar regions are and the 


In past geological periods climates . Glacial conditions 

are shown by . Warm climates are indicated by - , 

which we find even in . Tropical rain forest must 

have prevailed in the when coal . In the 

Permian it must have been , which gave us the extensive 

salt . 


1. Climate is average weather. Why is that statement not 
quite true? 

2. Distinguish between weather and climate. 

3. What are the important weather controls? Which are 

4. Why are spring days windy, while the nights are often 

5. What is meant by "night is the winter of the tropics " ? 

6. Describe the weather of the doldrum belt. 

7. Why are weather predictions in the trade-wind belt un- 

8. Why are weather changes more pronounced near sea level 
than at higher altitudes? 

9. Why are weather changes more pronounced inland than 
near the coast? 


10. Why is weather more variable in polar than in equatorial 

11. Where do we find monsoon changes of weather? 

12. What is the chief weather control in the regions of pre- 
vailing westerlies? 

13. Where do we find the greatest temperature range in the 
United States? How do you account for it? 

14. What is the chief weather control in the polar regions? 

15. Why is weather a common topic of conversation in the 
temperate zone? 

16. What is the direction of the wind in front of a cyclone in 
the westerlies? 

17. Why is the front of a cyclone apt to have cloudy or rainy 

18. Why does relative humidity decrease in the rear of a cy- 
clonic storm? 

19. How does the velocity of the wind change as a cyclone 

20. What weather prediction should be made as a high ap- 

21. What weather prediction should be made as a high recedes? 

22. Why does the wind weaken as a high approaches? 

23. How does a cold wave develop? 

24. Why is the weather map of more value to eastern than to 
western United States? 

25. How far ahead can the weather be predicted? 

26. Of what commercial value is the weather forecast? 

27. Name the climatic controls. 

28. Cite an example to show that two places with the same 
average for one of the climatic factors may not have the same 

29. Upon which one of the climatic factors does the usual 
division into zones depend? 

30. Along what isotherms would it be advisable to separate the 
temperature zones? 

31. What are the chief conditions prevalent in the tropical 
rainy climates? 

32. What is a dry climate? Why cannot the amount of rainfall 
be used as an index of a dry climate? 


33. What are steppes? 

34. Why are the temperatures in a dry climate extreme? 

35. What is the chief cause of equatorial deserts? 

36. What is the smallest rainfall in the world? Where? 

37. Why are the skies clear in deserts? 

38. What is the cause of deserts in temperate zones? 

39. How do temperate-zone deserts differ in temperature from 
tropical deserts? 

40. Where are the steppes of the United States? What can they 
be used for? 

41. Which climatic factor is of paramount importance in the 
temperate zone? How does that affect agriculture? 

42. What other important factor is the chief determinant of 
weather in the temperate zone? 

43. Account for the even climate of western Washington and 

44. Why are the rains of the west coast concentrated in the 

45. Explain the difference in marine west-coast climates of 
the British Isles and Chile. 

46. What is the climate of New York State? 

if Optional Exercises 

47. Why are northern North America and Eurasia low-pressure 
centers during the summer, and high-pressure centers during the 

48. Explain why we get falling temperatures and clearing skies 
after a cyclone passes. 

49. Why is the weather prediction not always correct? 

50. Discuss weather proverbs, and show that they usually have 
only local application. 

51. Why is the subdivision of the earth into temperature zones 
inadequate for climates? 

52. Name a characteristic tropical rain forest region. What is 
the rainfall in this region? 

53. In what way does the tropical savanna differ from the 
tropical rain forest? What is the rainfall? Name a typical savanna 


54. What is the cause of the excessive rainfall in northern 

55. Why is Mediterranean climate ideal for tourists? What 
regions have this climate? 

56. How does a moist subtropical climate differ from the 
Mediterranean type? Name a region in the United States that has 
this climate. What crops are well suited to the climate? 

57. What is the chief characteristic of mountain climate? What 
differences are found on windward and leeward sides? 

58. How do mountain climates differ in the westerlies and in 
the trades? 

59. How does it happen that anything at all will grow in a 
polar climate? 

60. What is the evidence of glacial climates of the past? 

61. How can the presence of enormous masses of coral in early 
Paleozoic formations be explained? 

62. What relation do we find between times of uplift and 

63. Present evidence to prove that the regions of the United 
States in which we find our chief coal deposits had a tropical rain 
forest climate at some past time. 

64. What evidence convinces us that the climate of the Great 
Salt Lake region was at one time humid? 



*396. Climatic regions. Situated in the zone of prevailing 
westerlies, the climate of the United States is chiefly under cyclonic 
control. But its wide range in latitude, its variation in distance 
from the sea, and the difference of altitude of various sections 
result in climates of all varieties, except savannas and the polar 
types. Minnesota and Maine have lower temperatures than Florida 
and Louisiana because they are farther north. Kansas and 

Ull Marine W. Coast 

S Desert ^ Subtropical 

ESI Semi Arid E23 Mediterranean's.^ 

EM! Continental "*" * ' 

FIG. 294. Climates of the United States 

Nebraska, lying in the interior of the continent, have greater 
ranges of temperature and less rainfall than Oregon and Maryland. 
Denver, in the foothills of the mountains, has a more equable 
climate than St. Louis, which is at about the same latitude but at 
a lower elevation. 

The distribution of types of climate can be seen at a glance by 
inspecting Fig. 294. The Pacific coast region has a marine west- 

* This entire chapter is optional. 


coast climate in the north; California is chiefly Mediterranean, 
with desert here and there, especially in the south. 

The Great Basin is chiefly desert and semiarid land. 

The rest of the country from the Rocky Mountains to the 
Atlantic may be divided into two climatic zones : the southern half 
is humid subtropical, and the northern half, humid continental. 

These regions will now be taken up in greater detail. 

*397. The Pacific coast region. This zone extends inland from 
the Pacific coast about 200 miles, to the backbone of the Sierra 
Nevada and Cascade Mountains. Like all regions situated near 
the sea, it is characterized throughout by an equable temperature. 
The isotherms, instead of running east-west as they usually do, 
run almost parallel to the coast (Fig. 295). 

The continuation of the Japan Current, the North Pacific Drift, 
cooled by its long journey through North Pacific waters, washes 
the entire length of our Pacific coast. The winds passing over this 
current increase slightly the temperature of the northern coast 
and lower decidedly the temperature of the southern part, but 
there is seldom a frost in the lowlands of the southern part of 

But there is a wide difference in rainfall between north and 
south. The westerly winds come from the Pacific, laden with 
moisture at all seasons. In summer they blow upon lowlands 
warmer than themselves, and therefore yield no rain until they 
begin to rise up the mountain slopes. In winter the cooler land 
induces rainfall, even over the lowlands, thus making winter rains 
the most marked characteristic of the Pacific coast. On the coast 
of Washington, where high mountains are near the sea, we have 
the greatest rainfall of the United States, more than 100 inches, 
whereas in southern California, with its coastal plain and its 
nearness to the high-pressure calms, it is less than 10 inches. The 
climate of the north, including a little of northern California 
together with all of Oregon and Washington, is of the marine west- 
coast type, marked by evenness of temperature throughout the 
year and by heavy rains concentrated in the winter. 

Much of California has a Mediterranean climate, with dry 
summers and moist, mild winters. Temperatures may drop to 40 F. 
in the winter, but that is extreme. In the summer 70 F. to 80 F. is 
the rule; and even if it does reach 95 F., the heat is not oppressive, 


owing to the low relative humidity, and the hot days are followed 
by cool nights, around 60 F. Skies are clear in the summer and 
one fair day follows another, making this region ideal as a resort. 

The cultivated lowlands of the south are parched during the 
growing season, but, fortunately, their nearness to the mountains 
makes irrigation possible, and as a result of the unbroken succes- 
sion of sunshiny days, they are the most valuable fruit lands in 
the United States. 

More and more of the arid lands of California are being reclaimed 
by irrigation; and, with the completion of the Boulder Dam Proj- 
ect, more than a million acres of the Imperial and Coachella 
valleys will be under cultivation. 

Along the coast fogs are common, especially in winter. Severe 
storms are almost unknown. Thunder is rarely heard upon the 
coast; on the mountain slopes thunderstorms break and lightning 
flashes are seen, but at distances from the coast too great for the 
sound of the thunder to be heard. 

*398. The arid regions. This region embraces the Great Basin, 
the highlands lying between the Sierra Nevada and Cascade 
Mountains on the west and the Rocky Mountains on the east, as 
well as the western portion of the Great Plains. Its most marked 
characteristic is its dryness. It lies on the leeward or western side 
of the mountains, and the winds from the Pacific, which are forced 
to rise over these mountains, lose their moisture on the western 
side and have little rain left for the eastern slopes. Furthermore, as 
these winds come down from the mountains, they are compressed 
and therefore warmed, which lowers their relative humidity, and 
they become drying winds. It is only after they have crossed the 
greater part of the region and begin their ascent of the western 
slopes of the Rockies that any rain falls. Occasional cyclonic storms 
yield some rain, but over much of the region the rainfall is insuf- 
ficient for agriculture, without irrigation. It varies from 20 inches in 
Washington to 3 inches in Arizona. Much of the Great Basin is 
distant from sources of water and must therefore remain arid and 

Some of the semiarid lands have been cultivated by " dry- 
farming," a process which has caused extensive soil erosion and 
destructive "dust storms," which, it is said, will reduce the land 
ultimately to a desert. Some of it has been overgrazed by cattle; 


in other places the surface cover of grass has been ripped off, 
to attempt cultivation, and in some cases the unwise use of irriga- 
tion has brought alkali to the surface. 

The whole process of the utilization of these waste lands has 
come to the fore again, in connection with dust storms, soil erosion, 
flood control, irrigation, and water power; and it is being borne in 
upon us that all these problems are closely connected. 

Skies over the Great Basin are prevailingly clear, and the daily 
range of temperature is great. Winters are cold and summers 
extremely hot. Cold winter cyclones sweep down from the north- 

Rainfall is nowhere in the region sufficient to support heavy 
forests. In the north, where there is rain, it falls mostly in winter, 
the growing season being almost without rain; but owing to the 
deep and retentive soil, hi which capillary action brings water 
up from unusual depths, this part of the region yields abundant 
wheat harvests. Where irrigation is available, apples and other 
fruits of temperate latitudes are grown. 

The chinook winds coming down the mountain slopes, warming 
as they drop, keep the narrow mountain valleys free of snow. On 
this account these valleys are much sought by wild and domestic 
animals for winter grazing. 

*399. The humid subtropical south includes the south Atlantic 
and Gulf States as well as Arkansas, Oklahoma, Kentucky, and 
Tennessee. Winters are temperate and summers are oppressive, 
because of high humidity rather than heat. The coastal states are 
bathed by the Gulf Stream, giving them a climate resembling that 
of the Amazon, which is hot and humid. Temperatures during the 
day run up to 100 F. Montgomery, Alabama, has a maximum of 
107 F. in August; and Savannah, Georgia, 105 F. in July. 

The nights are oppressive because of the humidity, and the 
overhanging clouds prevent the land from cooling off by radiation, 
in strong contrast to California, where the nights are cool. 

Winters are mild, averaging around 50 F., though there are 
extremes that may bring frost, due to the winter-monsoon effect 
of the north and northwest winds. In the daytime the temperature 
may reach 60 F. or even 70 F., and the day is bright and sun- 
shiny, only to be followed by a cloudy day or a cold one, as the 
wind shifts to the northwest. 


Late summer and fall brings possible hurricanes from the south- 
east, which, fortunately, are not numerous. 

Rainfall near the coast is as high as 60 inches, well distributed 
throughout the season. It is often accompanied by lightning 
storms of local origin. Florida has over 60 lightning storms a 

*400. The humid continental climate of the north. This region 
includes the so-called northern states, from the semiarid lands of 
the Great Plains east to the Atlantic (Fig. 294). It therefore in- 
cludes the northern part of the Great Plains and Prairies, the Appa- 
lachian Plateau, the Piedmont Plateau, and the north and middle 
Atlantic States. 

Most of the region is characterized by cold winters and hot 
summers, much more extreme in the interior than on the Atlantic 
seaboard and near the Great Lakes, although the tempering effect 
of the ocean is not nearly so great as it is on the Pacific. On the 
Pacific coast the westerly winds blow from the sea, whereas on the 
Atlantic coast the same westerly winds blow from the land to 
the sea. There is a temperature range of 160 F. in North Dakota, 
while on the coast it is only half that. 

In the west, near the semiarid regions, there are abundant grass- 
lands; but the rainfall, which on the western fringe is only 20 
inches, increases to the east, and is everywhere sufficient for agri- 
culture. Most of our farm lands are in this region. 

There is a strange and unexplained absence of forest in the 
prairie lands of the central states, in spite of the sufficiency of 

Rains and winds are under cyclonic control. In the central 
regions winds are variable and attain high velocity where un- 
checked by forest. Winter cyclones come from the northwest. 
They bring snow rather than rain, especially in the lake region. 

Rains are more frequent in the early summer, when the sun's 
heat starts convectional storms. This is very important for crops 
requiring maximum water in the early stages of growth. 

Winter cold is more extreme than summer heat, and the 
weather is very changeable, because it is under control of the cy- 
clones sweeping down from the northwest. 

The blizzard and the cold wave are characteristic of the humid 
continental climate. The blizzard is a high, cold wind, filled to 



blinding with a mass of fine snow. It is common enough in the 
interior and not unknown on the coast. 

In a cold wave there is a rapid drop in temperature, as much as 
20 F. in 24 hours, ending close to 32 F. It is developed at the front 
of an anticyclone advancing from the northwest. 

Weather in the summer is more regular than in the winter. 
Cumulus clouds are common, and thunderstorms in the afternoon 
may be frequent, somewhat like weather in the tropics. 




FIG. 295. Isotherms of the United States for January 

Hot waves result from southerly winds, and the heat wave is 
broken when the wind shifts to the northwest. 

Spring and fall show fickle alternations of summer and winter 
weather, as cyclones are followed by anticyclones with their low 
temperature. Fall presents fine clear days and freezing nights. 
Then come warm spells with a hazy atmosphere, known as Indian 

These seasons are more characteristic of the northern portions of 
the continental climate belt than of the southern. The chief differ- 
ence is the greater length of summer in the corn belt to the south. 

*401. Exceptional climatic conditions. In defining climate as 
average weather, it was noted that while that may be true in 
general, it often happens that the exceptional characters of the 



weather, rather than the average, are of greater importance. For 
example, our buildings must be constructed to withstand the 
strongest wind, and our heating systems to provide against the 
lowest temperature rather than the average. 

As we have seen, profitable agriculture is dependent not so much 
upon annual rainfall as upon rainfall during the growing season. 

In order to obtain a clear understanding of the climates of the 
United States, it is necessary to examine maps showing averages 



FIG. 296. Isotherms of the United States for July 

for given periods as well as maps showing departures from the 

From the January chart of temperatures (Fig. 295) one can see 
the wide difference in the winter temperature of Key West, 70 F., 
and of North Dakota, F. This difference, while in part due to 
difference of latitude, is to a much greater extent due to the 
difference between coast and continental conditions. Along the 
Atlantic coast the change of temperatures is from 70 F. at Key 
West to 15 F. in Maine, while along the Pacific coast it runs from 
50 F. to 40 F., showing the influence of the marine west-coast 

The July temperature chart (Fig. 296) tells a different story. 
No longer is the highest temperature found at Key West, on the 


coast, but in Arizona, which is farther north. This is due to the 
clear skies of the arid land. 

From Florida to Maine, on the Atlantic coast the difference is 
about 25 F. as compared to 55 F. in January, while on the Pacific 
coast the difference for July is about the same as for January, the 
isotherms in both cases running almost parallel to the coast. The 
interior of the continent, which is colder than the coast in January, 
is warmer in July. 

The lowest temperature in the United States, 63 F., is 
reached in the interior of the continent near the Canadian border. 
Low temperatures on the Atlantic coast are about 25 lower than 
those on the Pacific coast. 

The tempering influence of the sea is again well shown by a 
comparison of the average of the highest temperatures of the 
coasts, 95 F., with that of the interior, 105 F. The highest tem- 
perature, 125 F., is recorded in the interior desert regions of 
Southern California and Arizona. Temperatures of 105 and over 
are common over the Great Plains, but the dry heat is not so 
oppressive as that of the Gulf and Atlantic coasts, where it is very 

The range of temperature is the difference between the summer 
maximum and the winter minimum. The greatest range is found in 
the northern interior continental climate, whereas the lowest range 
occurs at Key West, in a tropical climate. In general, range of 
temperature increases with increase of latitude and with distance 
from the sea. 

The range of temperature along the Pacific coast varies little, 
being only 15 F. greater in the extreme north than in the extreme 
south, while the Atlantic coast varies in range from 50 F. in the 
south to 110F. in Maine. The range of temperature for most of the 
Gulf coast is about 85 F., whereas that for Montana is twice as 

*402. Freezing temperatures. The number of days with average 
temperatures below freezing varies from none in the Pacific, Gulf, 
and South Atlantic coast regions, to 165 days in Minnesota and 
North Dakota. Of much greater interest to farmers and fruit 
growers, however, are the dates of occurrence of earliest and latest 
killing frosts. 

In the fall, with the lengthening night and increasing slant of 



the sun's rays, there comes a time when the daily minimum tem- 
perature falls almost to freezing. The passage of a low across the 
continent is then likely to be followed by frost. This is due to the 
cold indraft of north winds at the rear of the low, where the sky is 
clear and the winds light. 

The date of the first killing frost in the extreme north central 
part of the United States is about the first of September (Fig. 297). 
As the winter season marches southward and toward the coasts, 
the first killing frost occurs later and later in these directions as 

FIG. 297. Date of First Killing Frost 
Note its southeast movement. 

late as December 15th in central Florida. In spring, when the noon 
altitude of the sun is increasing and the days are lengthening, there 
comes a time when it ordinarily does not get below freezing. But for 
weeks after this, a passing low, with its cold indraft of northern 
winds behind, may bring freezing temperatures. At such times 
falling temperatures and clearing skies forewarn of frost. 

Since spring moves northward and landward from the coasts, 
the average time of latest killing frost is earliest at the south, and 
earlier at the coast than inland (Fig. 298). Along the Gulf coast 
it occurs before February 15, and it is delayed in the extreme north 
part of central United States until June 1. 

The absolute date of latest killing frost is considerably later 




FIG. 298. Date of Latest Killing Frost in Spring 
Note its northwest movement. 


FIG. 299. Average Annual Rainfall in the United States, in Inches 

than the average date in all sections, being much nearer March 1 
on the Gulf coast, and July 1 in Minnesota. 

*403. Distribution of rainfall in the United States. From the 
rainfall chart (Fig. 299) we are able to locate the regions of great- 
est and least rainfall during the year. 


The least rainfall, 3 inches, occurs in southwestern Arizona. 
Most of this amount may fall in a single day, or indeed, in a few 
hours during a single thunderstorm. 

The greatest annual rainfall in the United States, more than 100 
inches, occurs in northwest Washington, and while most abundant 
in winter, is fairly well distributed throughout the year. The annual 
rainfall on the Pacific coast decreases southward; in central 
California it is less than half that in Washington. On the Atlantic 
coast the maximum rainfall is near Cape Hatteras, decreasing 
northward and southward. 

For agriculture, a rainfall of two to four inches per month is 
desirable during the growing season. Sometimes it rains more than 
that in a single day. Such torrential downpours are injurious to 
crops and to the land. The soil is washed away, and streams are 
flqoded and overflow their banks, causing destruction of life and 
property. Such heavy downpours are known as cloudbursts. 

The recorded rainfall includes snowfall; 10 inches of snowfall is 
estimated as equivalent to one inch of rain. 

404. Distribution of snow. Every part of the United States, 
except southern Florida and southern California, receives some 
snowfall. It is least at the south, although occasional heavy snow- 
falls occur there. It is more than 40 inches in the region of the 
Great Lakes and in the Rocky Mountains. A fall of 13 inches oc- 
curred at Baton Rouge, Louisiana, during a single storm in 
February, but such snows usually melt within a day or two after 

The greatest annual snowfall in the lowlands of the United 
States, 130 inches, occurs in the northern peninsula of Michigan; 
the moisture is supplied from the adjacent lakes. The greatest 
average annual snowfall of the entire country, not including Alaska, 
occurs in the Sierra Nevada Mountains. The moist westerlies from 
the Pacific, compelled to rise in passing over the mountains, 
precipitate, on an average, 378 inches of snow at Summit, Cali- 

The Rocky Mountain region has a heavy annual snowfall, 
though less than the Sierra Nevada and Coast ranges. It is mainly 
the melting of these snows in the Rockies that furnishes the great 
irrigation projects with their supply of water. The floods in the 
Missouri, and other eastward-flowing streams with sources in these 


mountains, occur in May and June, when the normal rainfall is 
augmented by the melting snow. 

The snowfall in the northern plains and prairie regions is 
variable. Some winters it is excessive; others, light. When abun- 
dant in the wheat-growing sections, a good crop is expected, since 
the snow serves as a protection from the cold, and also leaves the 
soil in good condition. 

In the lumbering sections of the north, from Minnesota to Maine, 
the profits of the season are directly related to the snowfall, because 
the lumber is moved on sledges over the snow. 

*405. Number of days with precipitation. The number of rainy 
or snowy days during the year varies widely in different sections of 
the country. In general, it is least in the interior and increases 
toward the coasts, and is greater in the north than in the south. 
The greatest number, 180, occurs in northwest Washington; then 
follows the Great Lakes region with 170 days. In the southwest 
desert region the number falls to 13. For most of the agricultural 
sections the number varies from 100 to 140. Forty consecutive 
rainy days are reported in northwestern United States, and 150 
days of consecutive drought in the arid region of the southwest. 

*406. Humidity. The absolute humidity of the air is greater in 
southern United States than in northern; it is greater in summer 
than in winter, and greater near the coast than in the interior. 

The relative humidity is, on an average, lowest on the Colorado 
plateau, where it is about 40%, and highest on the eastern and 
western coasts, about latitude 40 N., where it is about 80%. In 
the Gulf region it is about 75%, and approximately the same in 
the region of the Great Lakes, although, as a rule, continental 
interiors favor low relative humidities. 

The percentage of cloudiness agrees well in winter with the 
relative humidity, but in summer one of the areas of greatest 
cloudiness is over the Colorado plateau, where the average relative 
humidity is low. This is probably due to the strong convectional 
currents set up during the summer season when the air rises to 
heights sufficient for saturation. 

*407. Winds. As before stated, the winds are stronger on the 
coast and over the prairie regions than over forests and moun- 
tainous regions. For most of the country the season of strongest 
winds is spring, and the month of weakest winds is August. 


Aside from tornadoes and hurricanes, during which, for a few 
seconds, the velocity of the wind may exceed 100 miles per hour, 
the strongest winds are about 70 miles an hour inland, and 90 
miles an hour on the coast. 

Though the direction of the wind is variable in all parts of the 
United States, in valleys, there is a decided up or down the valley 
tendency in wind direction. On the coasts, in winter there is a 
predominance of land winds. This is especially true of the Gulf and 
Atlantic coasts. On the Pacific coast the meeting of the land winds 
and the prevailing westerlies produces " along shore" winds. In 
summer the conditions upon the Atlantic and Pacific coasts are 
reversed. The Pacific then has strong ocean winds, while the in- 
blowing winds upon the Atlantic coast are met by the westerlies, 
and "along shore" winds from the southwest are produced. 

The winds which bring cloudy weather and precipitation vary 
with the section. They are generally winds blowing from the 
nearest great body of water. On the Atlantic and Gulf coasts, and 
over most of the interior of the United States, they are east and 
southeast winds, while on the Pacific coast they are generally 
southwest winds. In winter, in the northern section of the United 
States snow often accompanies winds from a northerly direction. 
Winds from southerly directions, in front of the low, bring higher 
temperatures and yield rain, while the colder winds in the rear 
yield snow. 

Completion Summary 

The chief climatic control of the United States is . The 

Pacific coast has climates : in the north , and in the 

soutn . 

In the Great Basin the climate is , practically all over. 

The country to the east of the is in the north; and 

along the southern coasts . 

The prevailing westerlies give the west coast an climate. 

In the north it is of marine west-coast type, with winters ; 

in the south the climate is Mediterranean, with summers 

and winters. The prevailing westerlies, rising over the 

mountains, drop on the western side of and as they 

descend on the eastern slopes, they are dry; hence the arid region 
. These chinook winds provide good for grazing. 


The coast regions of our southern states have - - climate. 
Humidity and the summers are therefore for human 
beings. Lightning storms - , and hurricanes - . 

Most of the northern and eastern states have - - climate. 
The winters - - and the summers - . Weather is - 
because under - - control. Blizzards, cold waves, hot waves, 
and thunderstorms are phenomena of - . 

In studying the climate of a region, exceptions from the aver- 
ages are sometimes more important than the averages themselves. 
Variations in the interior are great, especially in temperature. On 

the coasts it is much less, but the Atlantic coast varies 

than the Pacific, because of the - . 

The greatest annual rainfall, amounting to , is ; 

the least, - , is - . 

Snowfall is heavy in - regions as well as . 


1. What is the chief climatic control in the United States? 

2. What factors cause variations in the climate of the United 

3. What are the climates of the Pacific coast? 

4. Why is the Great Basin chiefly arid? 

5. What are the climates of eastern United States? 

6. Why do the isotherms of the west coast run parallel to the 

7. What effect has the North Pacific Drift on our west coast 

8. Why does the west coast have most of its rains in winter? 

9. Explain the difference in rainfall between Washington and 
southern California. 

10. Describe the climate of California. 

11. Why is southern California so well suited to fruit growing? 

12. What advantages does the Boulder Dam Project bring to 

13. Follow the prevailing westerlies from the ocean across the 
west coast and over the Great Basin, and explain the rainfall of the 
regions over which these winds blow. 


14. Describe the destructive effects of dry farming on the semi- 
arid lands. 

15. Describe the climate of the Great Basin. 

16. Why is it said that the problems of dust storms, soil erosion, 
flood control, irrigation, and water power are closely connected? 

17. How do chinook winds affect the climate of the valleys 
in western mountains? 

18. How does the Gulf Stream affect the climate of the south 
Atlantic and Gulf States? 

19. Why are summer nights hot in the Gulf States and cool in 
California, at about the same latitude? 

20. How do the winter monsoons affect the climate of the 
southern states? 

21. What is the cause of the prevalence of lightning storms in 

22. What is the climate of the south? 

23. What is the climate of the north? 

24. Why is the humid continental climate more extreme in the 
interior than on the Atlantic coast? 

25. Why is it that the Atlantic Ocean has not nearly so much 
effect on the climate of the coast as the Pacific? 

26. Where is the greatest temperature range in the United 
States? Explain. 

27. What is the range of rainfall in the northern states? 

28. Why are the northern states our chief agricultural lands? 

29. Account for the changeable weather of the northern 

30. What is a blizzard? In what regions is it common? Why? 

31. What is a cold wave? Where is it common? Why? 

32. What is Indian summer? When and where do we get it? 

33. How does the southern part of the continental humid 
climate belt of the United States differ from the northern part? 

34. How do you account for the greater range of temperatures 
on the Atlantic than on the Pacific coast? 

1 35. Where do we find the highest temperatures in the United 
States? the lowest? 

36. What is the date of the first killing frost in the northern 
states? in Florida? 

37. How may a cyclone produce a frost in spring? 


38. Where do we find the greatest rainfall in the United States? 

39. Where do we find the least rainfall in the United States? 

40. What is a cloudburst? 

41. What rainfall is ideal for agriculture? 

42. Where is the greatest annual snowfall? How much? 

43. Why does abundant snowfall presage good wheat crops? 

44. In what region do they have the greatest number of days 
of precipitation? 

45. What is the reason for the cloudiness on the Colorado 
Plateau in summer, although the relative humidity is low? 

46. Why are thunderstorms practically unknown on the west 

47. From what direction do storms in your region usually 

48. What direction of wind is most likely to bring snow in 

49. What is the yearly rainfall in your region? 

50. What is the yearly range of temperature in your section? 



408. The relation of the sea to the land. The sea wears 
away the land, at its margins, by wave action; and indirectly 
the erosion of the land depends upon the sea in two ways. 
(1) The water which falls as rain is derived from the sea by 
evaporation; and (2) the level of the sea determines the 
velocity of the running water which brings about erosion of 
the land. 

The sea is a great international highway and plays an 
important part in the trade of the world. It is no longer a 
barrier between nations, since great steamships are little 
affected by storms. Equipped with radio, ships communicate 
with each other and with stations on land, and this removes 
the isolation that was formerly experienced in crossing great 

The digging of canals across isthmuses tends to change 
routes of travel and commerce at sea. The Suez Canal 
shortened the route from Europe to India and China, and 
the Panama Canal has cut by thousands of miles the sea 
distance between New York and San Francisco. 

The surface of the sea is commonly regarded as having 
a very nearly uniform level, known as "sea level," from 
which land elevations and sea depressions are measured. 
When large mountain masses are situated near the coast, 
their gravitational attraction draws the sea up on the land 
and thereby upsets the uniform level of the sea. This change 
in level may amount to several hundred feet in different parts 
of the earth. On the coast of India near the Himalaya Moun- 
tains, the water stands much higher than water in mid- 


ocean or water along a lowland coast like western Europe or 
eastern United States. 

*The area of the earth's surface covered by the sea has changed 
constantly during past ages, but since the Rocky Mountain Uplift 
at the end of the Mesozoic Era, about 60 million years ago, the 
continents have assumed practically their present outlines. 

Most of the land is covered by sedimentary rocks, which proves 
that it was at some time covered by the sea, and some of the 
present sea bottom has at one time been land. 

The Mississippi Valley and central Canada have several times 
been drowned, all the way from the Gulf of Mexico to the Arctic. 
On the other hand, the east coast of North America extended 
several hundred miles farther out than it does now, being 
occupied, in fact, by an extensive range of mountains; and again, 
recently the present coast has been drowned. 

409. Divisions of the sea. The continuous body of salt 
water called the sea, covering about three fourths of the 
earth's surface, has five divisions called oceans. 

The Pacific is the largest ocean, comprising three eighths 
of the sea area. Its greatest width is about 10,000 miles at 
the equator. On its Asiatic shores it is characterized by 
numerous border seas, islands, and many rivers; and on its 
American shores, by high mountain ranges parallel to the 
shore, but by few rivers. 

The Atlantic is second in size, with an area about one 
quarter of the sea area. Its average width is 3,600 miles. The 
North Atlantic (north of the equator), on both the American 
and European shores, has many bays which give it an ir- 
regular shore line with many good harbors. The coasts slope 
gradually toward the sea and there are many rivers. The 
South Atlantic has a more regular shore line with few good 

The Indian Ocean has an outline that is roughly circular. 
It has one eighth of the total sea area and a diameter of about 
6,000 miles. The Indian Ocean is bordered by large bays, and 
has a northern and western boundary of very high mountains. 


The Arctic Ocean is an extension of the Atlantic. It has 
a width of about 2,500 miles. It occupies about one thirtieth 
of the sea area; the greater part of it is covered most of 
the year with drifting ice. The water at the center of the 
Arctic Ocean, near the North Pole, is nearly two miles deep. 

The Antarctic Ocean lies within the Antarctic Circle. In 
this region there is a continent covered by an ice cap 
thousands of feet thick; in other words, there is a glacial 
epoch in the Antarctic. 

The South Pole is located on land with an elevation of 
two miles. The continental glacier, thousands of feet thick, is 
the source of the icebergs of the southern seas, just as the 
glacier covering Greenland furnishes the icebergs of the North 

410. Distribution of the ocean waters. The greatest ex- 
panse of water is in the Southern Hemisphere; the island of 
New Zealand is at the center of the water hemisphere. 
London, England, almost exactly opposite New Zealand on 
the globe, is the center of the great land area of the earth. 

411. Deep sea basins. Before the seas existed, the conti- 
nents must have been formed of huge masses of granitic 
rock, which, being of lower density than the basaltic rocks, 
floated higher in the dense liquid earth mass. The basalt 
was of larger quantity, and therefore the sea bottom, which 
seems to be basalt, is of greater area than the land mass. 

The continents must at one tune, then, have existed as 
great plateaus, three miles above the ocean floor. 

The average depth of the sea is 2.5 miles, with a maximum 
depth of almost 7 miles in the Swire Deep, east of the Phil- 
ippine Islands. 

The sea bottom is much more regular than the land surface, 
owing, no doubt, to the fact that the land surface is being 
continually eroded, while there is no such process on the sea 
bottom; quite the contrary, deposition is going on there. 
Here and there we find mountain peaks, most of which are 
volcanic cones. In other places are corresponding deeps, 




many of them parallel to mountain ranges on the continents, 
and possibly complementary to them. Such, for example, is 
the Tuscarora Deep, about 28,000 feet deep, parallel to the 
Japanese Mountains. 

In the Atlantic Ocean we have the Nares Deep, off 
Porto Rico, about 28,000 feet deep. Over 50 of these deeps 
are known (Fig. 300). 

412. The continental shelf. The seas in many places 
are overflowing their basins and flooding a portion of the 
continental mass, called the continental shelf (Fig. 301). 
This slopes gently toward the deep ocean so that the water 
on the shelf is shallow, not more than 600 feet. It is here 
that the sediments brought down by the rivers are de- 
Continent Shore Continental Continental Deep 
Zone Shelf Slope Sea 

FIG. 301. The Marine Zones 

posited and smoothed out in horizontal layers, which, when 
they become consolidated, form sedimentary rocks. It is the 
continental shelf which is sometimes uplifted to form a 
coastal plain. Continental shelves are well developed along 
the eastern coasts of North and South America, in places 
more than 100 miles wide. On the western coast they are 
much narrower. The British Isles are on the continental 
shelf of northern Europe. There is evidence that much of 
the continental shelf has been above the sea level. Several 
of the valleys of large rivers flowing into the Atlantic may 
be traced seaward, across the continental shelf, by valleys 
or canyons which were formed by the river when the conti- 
nental shelf was dry land. 

413. Composition of the sea water. About 3.5% of the 
sea water consists of dissolved salts. About three fourths of 
this is common salt (sodium chloride). The bitterness of sea 


water is due to magnesium chloride and sulphate (known 
as Epsom Salts) and calcium sulphate, called gypsum. 
Beside these, there is a little magnesium bromide, potassium 
iodide, and calcium bicarbonate. From the last, marine ani- 
mals fashion their shells of calcium carbonate; and it is this 
which ultimately accumulates and forms limestone. 

There are about 30,000 million million tons of salts in the 
ocean, enough to cover the entire earth with a layer 150 
feet thick. The Mediterranean and other inland seas have a 
higher salinity than the oceans, especially where the evapo- 
ration is greater in the trade- wind belts. 

The sea is getting more salty as tune goes on, because 
the rivers constantly bring more salt from the land, while 
evaporation carries away only pure water. 

*Attempts have been made to estimate the age of the oceans by 
calculations based on the amount of salt and on the rate at which 
rivers deliver salt to the sea. The following equation will show how 

this is done. 

total salt in seas 

Age of seas = 

salt in rivers per year 

On this basis the age of the ocean turns out to be something less 
than 100 million years. But the assumption is here made that the 
amount of salt carried to the sea has been the same, year by year, 
whereas it is more than likely that this is not true; that in the 
beginning much salt was dissolved out of the rocks and it is 
becoming less and less. On that basis the age of the seas is much 
greater than 100 million years. 

414. Sedimentary deposits in the sea. The rivers are con- 
stantly bringing vast quantities of sediment and dissolved 
salts to the sea. Waves cut into the land and add more, and 
some is contributed by the wind. The solid matter thus 
received is assorted, transported, and deposited in beds 
which ultimately become sedimentary rocks. These deposits, 
consisting of gravel, sand, and mud, are dropped on the 
continental shelf. 

Gravel beds are found at the mouth of a swift river, since 


the river must drop the heavier pieces as soon as its velocity 
is checked by the sea. Gravel may also be found where wave 
action is violent. Sand may extend out many miles from 
shore. Mud beds made of the finest particles are located 
beyond the sands in the open sea or in the quiet water of 
bays; for only there can the finer matter settle. 

*A large part of the dissolved calcium bicarbonate of sea water 
is taken up by plants and animals. The plants, particularly cal- 
careous algae, extract the carbon dioxide from the bicarbonate in 
the water. Animals decompose the bicarbonate and use the cal- 
cium carbonate thus formed in constructing their shells. These 
processes take place only in warm, clear, shallow waters, because 
sunlight does not penetrate muddy water or very deep water; 
hence plants cannot grow, and if there are no plants, animals 
cannot thrive. 

It is therefore on the continental shelf, in water less than 600 
feet deep but not very near the shore, that most marine life thrives. 
In this zone, therefore, limestones are formed from the precipitated 
calcium carbonate of plants and the shells of animals. 

There is no sharp line of division between gravel, sand, 
mud, and shells, but these grade into one another, and, of 
course, the same gradation will be found in the sedimentary 
rocks formed from those deposits, which will be conglom- 
erate, sandstone, shale, and limestone. 

None of these sediments reach the deep sea unless they are 
carried by the wind. Wind-blown deposits include some sur- 
face soil from the land, pumice and other volcanic ash, and 
particles of meteoric iron; but most of the deposits of the 
deep sea are skeletons of tiny floating animals; all of the de- 
posits make up a fine mud, called ooze. Much of it is glo- 
bigerina ooze because of the preponderance of microscopic 
globigerina. Much of this ooze is red from the iron com- 

415. Temperature of sea water. The surface water of the 
sea is warmest ; and since warm water is less dense than cold 
water, it remains on the surface. In polar regions the cold 


air cools the surface water, which sinks. The bottom water 
may be as low as 29 F. in the sea, while in a lake made of 
fresh water it can never get below 39 F., at which the fresh 
water reaches its maximum density. At the equator the 
temperature of the surface is about 80 F., while in enclosed 
seas it may rise as high as 95 F. 

At New York the winter temperature is about 55 F. and 
the summer temperature about 65 F. 

With increasing depth the water gets colder. Even near 
the equator the temperature, at less than half a mile, is below 
40 F., while at the bottom of the sea the temperature is 
about 35 F. Ocean currents cause irregularities in tempera- 
ture: the cold water from the poles creeps along the bottom 
toward the equator. 

416. Ice in the sea. When sea water is cooled, the ice that 
first forms, at 32 F., is pure and free from salt. Then the 
salt water freezes at about 27 F., the temperature depend- 
ing upon the per cent of salt. 

In the colder regions ice forms along the shores and also 
on the deep sea, often to a thickness of 8 or 10 feet. 

The ice formed in the winter is usually broken in pieces in 
the summer. These floating pieces, called field or floe ice, are 
often crowded and jammed together into an ice pack, which, 
because of lateral pressure, is raised considerably above the 
water. This sea ice may be driven upon the land by waves 
and tides and become 20 feet thick by accumulation of snow. 
Rock fragments from overlying cliffs and from the shore are 
picked up by this ice, which is known as an icefoot. In winter 
the grinding of this ice foot, up and down the shores, smooths 
and rounds the rocks of these coasts. In the summer it melts, 
breaks up, and scatters the rocky material, often over long 

Glaciers entering the sea from the land in" polar regions 
break off at the shore and send off large masses of ice, known 
as icebergs. Some icebergs are a mile or more in length and 
500 feet above water. As ice is about nine tenths as dense as 


water, nine tenths of the mass of the iceberg is below the 
surface of the water. 

These icebergs transport boulders and pebbles and drop 
them on the sea bottom in the warmer and more open seas. 
Some geologists have regarded the Grand Banks, off New- 
foundland, as due to deposits from icebergs. 

In the Southern Hemisphere icebergs have been en- 
countered north of the Falkland Islands and also near Cape 

In the Northern Hemisphere icebergs are frequently seen 
near the Newfoundland Banks and reach farthest south, 
near New England, in May and June. They are dangerous 
because the most frequented ocean lane passes through this 
region. Since 1912, when the Titanic sank after collision with 
an iceberg, the United States Coast Guard has maintained a 
patrol in the dangerous zone. The patrol has been so effec- 
tive that not another life has been lost in this way. From 
September to January there is no ice to be seen in these 

Completion Summary 

The sea is divided into five oceans : 
, and . 

The edge of the continent covered by the sea is called 
-. The important part of the shelf, where rocks 

are formed, is not deeper than 

The rest of the sea bottom consists of the , on which 

no sedimentary rocks are formed. 

Sea water contains about % of dissolved , 

including salt, , and . These were formed 

from weathering and solution of of the land and 

brought down to the sea by . The sediments brought 

to the sea are deposited in near , the coarse 

, the sands , and the muds . Farther 

out, where the water , limestone is formed. 

The surface waters of the sea have temperature. 


Farther down the temperature - . At latitude 41 N, 

near New York, the temperature in winter is and in 

summer . 

Where Arctic glaciers sea, they icebergs. 

These are to shipping. 


1. In what two ways does erosion of the land depend upon 
the sea? 

2. What evidence is there that the land was ever submerged 
by the sea? 

3. What is the relation between the sea and the oceans? 

4. What part of the earth's surface is at present submerged? 

5. Name the oceans. 

6. How does the Pacific differ from the Atlantic in shore lines? 

7. State a theory to account for the sea basins with the con- 
tinental masses at higher elevations. 

8. How does the sea bottom differ in profile from the surface 
of the land? 

9. What is a deep? Name one, giving its depth and location. 

10. What is the relation of the continental shelf to the sea and 
to the land? 

11. What evidence is there that the continental shelf was dry 
land at some past time? 

12. Name three salts present in sea water. Which one is the 
source of an important rock? 

13. Why is the sea becoming more salty? 

14. Where are sedimentary rocks formed? 

15. In what relation to the shore and to each other do we find 
the different sediments? Why is this so? 

16. What is the chief deposit of the deep sea? 

17. Compare the surface temperature of sea water with that 
at the bottom, near the equator and in polar regions. 

18. What is the variation in temperature of the surface of the 
sea in temperate regions? 

19. Why is the temperature of the sea so low at the bottom? 

20. How could pure water be obtained from the sea in cold 


21. What is an ice foot? 

22. How are icebergs formed? 

23. How has the danger from icebergs been overcome? 

*Optional Exercises 

24. Give evidence to support the statement that the sea has 
time and again flooded the land. 

25. Show how the age of the earth might be calculated from the 
amount of salt in the sea. Is this really the age of the earth? 

26. Why is limestone always found rather far offshore; at least 
farther out than shale? 

27. Why do we never find sandstone, conglomerate, or lime- 
stone in deep sea deposits? 

28. Discuss marine erosion due to ice in the polar regions. What 
deposits may result from this erosion? 



417. Waves. A gentle breeze causes ripples to form on 
the surface of the water over which it blows; a strong wind 
changes these ripples into great waves. But it is not the 
mass of water which moves forward, only the wave motion; 
just as the up and down movement of the hand holding one 
end of a loose rope, the other end being fastened to a wall, 
causes a wave to travel along the rope. The rope does not 
move forward, only the wave motion. So with a water wave : 
the water particles move chiefly up and down, while they 
pass their motion on to neighboring particles of water, and 



FIG. 302. Movement of Water Particles in a Wave Advancing on the Shore 

return to their former positions. Since water is not so rigid 
as rope, however, there is some forward motion of the 
particles. This may be observed by examining a small boat 
as a wave approaches and passes. The boat merely rises and 
falls while the wave moves by. This is illustrated in Fig. 302. 

The forward motion of the water is most rapid in the 
crest of the wave, and the backward motion is most rapid 
in the trough. The forward motion is slightly in excess of 
the backward motion and because of this, when winds are 
steady in the same direction, currents are produced which 
flow in the same direction as the wind. 

On the front of the wave the water rises and on the back 



of the wave the water falls. As the wave moves, new water 
enters in front and leaves on the back of the wave. 

418. Size of waves. During a storm, waves may be as 
much as 500 feet long, measured from the crest of one wave 
to the crest of the next; and up to 50 feet high, from the 
crest to the trough of one wave. The height and force of the 
waves depend upon the force of the wind, the length of time 
it continues to blow, the depth and breadth of the water, 
and the form and direction of the coast line. In small bays 
the waves can never be so high as on the open sea. 

419. Groundswell. High waves are often driven from an 
area of storm winds into a region of gentle winds, hundreds 
of miles away. They diminish in height but keep their ve- 
locity and length. They are known as groundswells. 

420. Breakers. When a wave approaches a gently sloping 
shore, the wave length is diminished and the wave height 
is increased. The front of the wave, because of lack of water, 
becomes steeper than the back; and as the wave continues 
to move into water of less depth, the crest curls, falls for- 
ward, and forms a line of breakers. The water now moves 
forward. The loose material of the bottom is churned up 
and piled higher and higher, forming a sand bar at the line 
of breakers. 

Rocks or bars near the surface of the water may be located 
by the breakers, which therefore are a warning of danger. 

The height of the wave determines the place where it 
will break, since a wave extends as much below the surface 
as it does above. A high wave will break farther out than a 
low one. 

421. Surf and undertow. When the waves break on the 
shore, the water is thrown forward and runs up on the beach 
as surf; and it returns along the bottom as a current called 
the undertow. When the waves reach the shore obliquely, a 
longshore current is formed (Fig. 303). 

422. Pounding of the waves. Waves are agents of erosion, 
that is, they break and grind material along the shore and 




FIG. 303. When the waves advance on the shore obliquely, longshore currents 
are formed which move sediments parallel to the shore. 

transport it small distances. The work of breaking and 
grinding is done by the impact of the breakers along the 
shores. In summer, in the Atlantic the average blow of a 
breaker is about 600 pounds on every square foot of surface. 
In winter this rises very much, ranging from 2,000 to 6,000 
pounds per square foot. 

The impact of the water alone accomplishes some erosion, 
but the chief work of the waves is accomplished with the 
aid of rock fragments, sand, pebbles, and boulders, which 
the waves move along or pick up and dash against the cliff. 
The mixture of water and rock acts like a hammer and some- 
times like a file, breaking and grinding the cliff as well as 
rounding off the pieces of rock used as tools. 

Weak rocks exposed along the shores are broken down 
and removed. The more resistant rocks are loosened by 
undercutting and, because of the joints in the rock, fall as 
angular blocks (Fig. 304). These in time become rounded 
and reduced in size. Large masses of rock, too large at first 
to be moved by the waves, are reduced by the pounding of 
smaller fragments, which the water drives against them, 
until they, too, are shattered. Then the waves use these 
new fragments as tools. Thus, huge masses of rock are 
reduced in turn to boulders, pebbles, sand, and finally to 
silt, which can be carried by the undertow. 

423. Features developed by wave erosion. The work of 
the waves at the cutting level may be compared to that 



FIG. 304. Chimney Rocks or Stacks 

of a horizontal saw. They cut a notch in the cliff and the 
unsupported rock finally falls, leaving a steep face known 
as a sea cliff. As the blocks are reduced in size to finer 
and finer material, a beach is formed at the foot of the 

If there are vertical joints, the waves will widen them, 
removing blocks and leaving here and there isolated columns 
called chimney rocks or stacks (Fig. 304). The "Old Man of 
Hoy" on the coast of the Orkney Islands is an example. If 
the joints are irregular, sea caves and arches maybe formed 
(Fig. 305). 

As the waves do their work of erosion, like a horizontal 
saw, a tablelike surface of rock is left behind, called a 
wave-cut bench (Fig. 306). Pieces of rock dragged back and 
forth along the wave-cut bench continue to cut it down and 
smooth it off (Fig. 306). The fine matter covers the bench 
to produce a beach between the high- and low-water marks. 
This debris is finally dragged so far out that it is no longer 
subject to wave action, and there it is piled up into a wave- 
built terrace (Fig. 306). 

This terrace is finally built up high enough for waves to 



FIG. 305. Sea Caves 

High Tide 
Low Tide 

FIG. 306. Some of the Features Developed by Wave Erosion on the Shore 


FIG. 307. A Barrier Beach Built by Wave Action 

churn it up and pile it higher into a low ridge at the line of 
breakers. This feature is a barrier beach, which may be 
built up above the surface, parallel to the shore, by storm 
waves (Fig. 307). Between the barrier and the shore a body 


of quiet water, called a lagoon, is trapped. Rockaway Beach, 
on the south shore of Long Island, is a barrier beach. It is 
growing westward because of longshore currents. 

The free end of a barrier beach is called a spit. It is often 
curved inward by currents, and often built across a bay, 
entirely closing it and interfering with navigation. Sandy 
Hook, near the entrance to New York Harbor, is a spit. 
Deposits brought down by streams help to fill up the bay or 

At the entrance to New York Harbor, for example, dredg- 
ing is necessary at all times, to deepen the channels through 
which large boats pass. A fleet of dredges is constantly at 
work, not only in the bay but also in the narrow arm of the 
sea called the East River. 

Small irregularities in the shore line develop because of 
differences in the resistance of the rocks and in their ex- 
posure to the attacks of the waves; but, as a rule, the shore 
line becomes more regular, since the more exposed head- 
lands are worn away, and bay heads and lagoons are filled 
in by rock waste from the shore and also by deposits brought 
by streams and emptied into the lagoon (Fig. 308). 

424. Tides. Along the shores of the ocean and its gulfs 
and bays, the water rises slowly for about 6 hours and 13 
minutes, and then falls slowly for about the same time, 
making an average of 12 hours and 26 minutes from high 
water to next high water, or from low water to next low 
water. This periodic rise and fall of the level of the sea, twice 
in every 24 hours and 52 minutes, constitutes the tides. 

This makes the hour of high water at any particular place 
vary from day to day. If it is high water at the ocean shore 
this afternoon at 4 o' clock, the next high water will occur 
at 4:26 tomorrow morning, again at 4:52 tomorrow after- 
noon, and so on. 

425. Variation in tidal range. The amount of rise and 
fall is greater along most continental coasts than in mid- 
ocean, and is greatest in bays which have broad openings to 



A barrier beach has been built up 
by the waves. 

the sea and are narrow toward their heads. The tidal range 
at Key West, Florida, is usually not more than 2 feet, while 
in the Bay of Fundy it is often more than 50 feet. 

The amount of the rise and 
the fall of the sea, at any 
particular place, also varies. 
The tidal range may increase 
from day to day for about a 
week and then decrease for the 
same period, making a maxi- 
mum and minimum range 
twice a month. At Governor's 
Island, in New York Harbor, 
the tidal range may be as 
small as 3. 4 feet and as great as 
5.3 feet, during a single week. 

426. Flood and ebb tides. 
The change of level of the sea 
is accompanied by tidal cur- 
rents called the running of the 
tides. When the tide is running 
from the open ocean into bays, 
it is flood, or incoming, tide; 
when the tide runs to the open 
ocean again, it is the ebb, or 
outgoing, tide. During the few 
minutes when the flood tide 
changes to ebb tide, or ebb to 
flood, slack water occurs. 

427. Tidal races. When the 
tidal currents pass through 

a strait, such as a narrow inlet into a bay, or between an 
island and the mainland, the currents often run many miles 
an hour. Such currents are called tidal races, and are often 
so strong as to interfere with navigation. The tidal currents 
"race" through Hell Gate, the narrow passage from the 

Storm waves erode the outer edge 
of the barrier beach and hurl the sedi- 
ment over into the lagoon. Streams 
are filling the lagoon with delta de- 
posits, while vegetation encroaches 
from all sides. 

The waves have deepened the water 
offshore, dragging the sediment farther 
out and destroying the barrier beach. 
The shore line is regular. 

Once more the waves begin their 
attack on the cliff. 

FIG. 308. Straightening of the Shore 
Line by the Waves 


East River into Long Island Sound, at the rate of 5 or 6 
miles an hour. 

428. Tides in rivers. The tidal wave often runs up rivers 
to a point many feet above sea level. The tide runs 150 
miles up the Hudson River to Troy, 5 feet above sea level, 
where the tidal range is more than 2 feet. The tide is felt 
70 miles up the St. John River, in New Brunswick, where 
the elevation is 14 feet above sea level; and at Montreal, 
280 miles up the St. Lawrence River. 

*The action of tidal currents in narrow rivers is very different 
from the action of tidal currents on open seacoasts. In rivers, when 
the water stands above the average level, the tidal current flows 
upstream along with the tidal wave; when the water stands 
below the average level, the tidal current flows downstream, 
opposite to the direction of the tidal wave. Since the rate of flow 
depends upon the difference in level, the flow is most rapid at high 
and low water, instead of being slack water at these times, as on 
open coasts. Hence the tidal current flows upstream for some time 
after high water has passed and the water level is falling; and the 
tidal current flows downstream for some time after low water is 
reached and the water level is rising. In broad, deep mouths of 
rivers, slack water does not occur at high and low water as on 
open coasts, nor at average level, as in narrow, shallow rivers, but 
at some intermediate level. 

429. Tidal bore. In the estuaries of many rivers, broad 
flats of mud or sand are nearly exposed at low water. The 
tidal wave, when entering these rivers, often rises so rapidly 
that it assumes the form of a wall of water. Such a wave is 
called a bore. Tidal bores occur in some of the rivers of 
China, where in one case the water travels up the river at 
every high tide, often reaching a height of 12 feet. After the 
bore has passed, an after-rush often carries the water up 
several feet higher. 

Bores have been observed on the Severn in England, on 
the Seine in France, on the Amazon in South America, and 
on a few other rivers. 


*430. Causes of the tides. To understand the causes of the tides 
with all the factors involved requires a very complicated piece of 
mathematics, entirely unsuited to a book of this kind ; but, if most 
of the details are omitted, it is possible for us to get at least a 
superficial understanding of the subject. 

According to Newton's Law of Gravitation, and the Laws of 
Motion, the tides are due to the gravitational effects of the sun 
and moon on the sea, together with the effects of centrifugal force. 
Now let us see what these statements mean. The Law of Gravita- 
tion tells us that the sun and moon attract the earth and the water 
on it and tend to draw it toward them. 

The attraction depends upon the mass of the bodies, and also 
on their distance apart. A larger body has a greater attraction: if 
it is twice as large, it will exert 
twice the attractive force ; and on 
this basis the sun has the greater 
gravitational effect. But, if the 
body is twice as far away, it 
exerts less attraction: not one 
half, but one quarter; or, in other 
words, distance is more important 
than mass. For this reason, the F 3og 

moon, although much smaller 

than the sun, has more attraction for the earth, because it is much 

When a body is revolving about a point, it has a tendency to fly 
away from the point. This is called centrifugal force. It is illustrated 
by mud flying from a wagon wheel because the centrifugal force is 
greater than the force which holds the mud on the rim (Fig. 309). 
The greater the distance from the center, the greater the centrif- 
ugal force, because the velocity is almost zero at the center of ro- 
tation and increases as we get farther away from the center. If 
we understand these two things, gravitational attraction and cen- 
trifugal force, we are ready to investigate the causes of tides. 

Let us first summarize the principles we need to know. 

1. Gravitational attraction between two bodies increases in 
proportion to their masses. 

2. Gravitation decreases with the square of the distance. 

3. Principle (2) is more important than (1). 



4. Centrifugal force is developed on a rotating body. 

5. It increases with the distance from the center of rotation. 
The direct tide is simple to understand; but there is another high 

tide directly opposite on the other side of the earth, at the same 
time. To understand this indirect tide we need to understand one 
more point. 

The moon does not revolve about the earth, as is commonly 

'Common Center of Gravity 
of Earth and Moon 

FIG. 310. The moon and the earth revolve about the axis, R. 

thought, but both earth and moon revolve about a common axis, as if 
they were attached to a rigid rod (Fig. 310) just like two weights, 
one very large, the earth, and one small, the moon. The earth is 
about 80 times the mass of the moon ; so the axis of Revolution, 
which is at the common center of gravity, R, is much nearer the 

High Tide 



High Tide Due to 
Moon's Attraction 

FIG. 311. High tides are developed at Hi and T 2 , while there are low tides at 

Li and 1/2. 

center of the earth than it is to the moon. In fact, the point is 
about 100 miles within the earth itself. This rotary movement 
produces centrifugal force and tends to throw the water off the 
earth; but, of course, the attraction of the earth itself holds on to 
the water. 

In Fig. 311 the high tide at HI will be due to the attraction of 


the moon on the water. At this point we need not consider the 
centrifugal force, because HI is very near R and the force is small. 

At H 2 there is another high tide, because there the moon's 
attraction, which would help hold the water toward the earth, 
is small because of the great distance from the moon. But the 
centrifugal force is much greater, because H% is much farther from 
R than HI. 

To sum up, the high tide at HI is due principally to the moon's 
attraction; and at H z it is due principally to the centrifugal force. 
At LI and L 2 we have low tides, since these places are farther from 
the moon than HI, but not so far as # 2 . 

431. Effects of the earth's rotation. So far, we have been assum- 
ing that the earth and the moon have no other motions, except 

- "0 

Moons Position 
the Following Day 

FIG. 312. Successive direct high tides are 24 hours and 52 minutes apart, be- 
cause it takes 52 minutes for point A to travel to position Ai. 

around their common center of gravity. If that were so, the high 
tides would remain at the same places. But actually the earth's 
rotation on its own axis has an effect on the tides. 

Supposing the moon stood still, while the earth rotated. Then, 
as a place came directly under the moon there would be a direct 
high tide and twelve hours later that place would have an indirect 
high tide. But the moon revolves about the earth-moon center of 
gravity, and in the same direction as the earth rotates. In other 
words, the moon is moving away from a given point on its orbit, 
and in order to catch up, the earth will have to turn more than one 
complete circumference; and it takes just 52 minutes to accomplish 
that. Therefore successive direct high tides are 24 hours and 52 
minutes apart (Fig. 312). 

Tidal movements are interfered with by the continents, by 
shallow water, by strong winds, and by atmospheric pressure. This 



explains in some measure why the actual local tides in so many 
places fail to agree with the theory. 

432. Establishment of the port. The earth rotates rapidly on its 
own axis, carrying the tidal wave with it; but the moon holds it 
back, and this causes the tidal wave to lag. The interval of time 
between the passage of the moon across the meridian, which should 
produce high tide, and the actual time of the high tide, which 
comes late, is called the " establishment of the port." This has a 
different value for every port. At New York it is 8 hours and 13 

433. How solar tides affect lunar tides. The explana- 
tion of solar tides is similar to that of lunar tides. The sun's 

Nea^ Spring New 3 

' SUN^(J 



FIG. 313. Spring and Neap Tides 

At spring tide, when both sun and moon act together, the high tides are 
higher and the low tides lower than at neap tide. 

effect is about half that of the moon. When sun and moon 
act together, the tides are stronger; when they oppose each 
other, the tides are weaker (Fig. 313). 

Twice a month, at times of new moon and full moon, the 
lunar and solar tides fall together and so produce a higher 
tide than usual, called spring tide. It must be noted that 
this tide has nothing whatever to do with the spring season. 
At first and last quarters of the moon, the solar high tide 
falls at lunar low tide, and solar low tide falls at lunar 
high tide. The effect of this is to lessen the tidal range; that 
is, the high tides are not so high and the low tides are not so 
low as usual. This condition of least range is called neap 
tide (Fig. 313). 


*434. Inequality of tides. The two successive high tides of a 
given place are usually of unequal height. They are of equal height 
only when the moon is over the equator and as this happens on 
only two days of the month, two weeks apart, the two successive 
high tides are usually unequal. The maximum inequality of the 
successive high tides occurs when the moon is farthest north or 
south of the equator, and amounts to several feet at some places. 

435. Effects of tides. Tides have an important effect 
on navigation. Ocean liners must take them into account; 
for in some harbors there is not enough water to float a 
large vessel over a bar or other obstruction at low tide, and 
therefore it is necessary to wait for high tide, to enter and 
leave the harbor. 

The erosion caused by tidal currents is known as tidal 
scour. It keeps inlets in barrier beaches open, as may be 
seen along the shore of New Jersey, and often maintains deep 
waterways in bays, to the advantage of navigation. In other 
places the tidal currents may deposit material and hinder 

Tidal currents maintain circulation and remove sewage 
drained into bays by neighboring cities. Vessels are aided 
or hindered in their movement by tidal currents and some- 
times they are brought to danger on rocks and shoals, es- 
pecially in fogs. 

Tidal currents transport material along shore from more 
exposed positions, headlands, to quiet water at the heads 
of bays, and this tends to straighten the shore line. 

436. Ocean currents. Every continent is washed by 
ocean currents, and every ocean has its distinct circulation. 
Currents from equatorial regions carry warm water into 
polar regions, and other currents carry the cold polar waters 
toward the equator. 

While each ocean has its separate circulation, yet the 
separate schemes of circulation fit into the general scheme 
as cogwheels in a vast machine. 

The Pacific Ocean, which for most purposes is considered 


as one ocean, is, by reason of its circulation, divided into 
two distinct parts, the North Pacific and the South Pacific. 
The Atlantic and Indian oceans, lying, like the Pacific, on 
both sides of the equator, are also divided into northern 
and southern oceans, by reason of their distinct circulation. 

437. Systematic movement. Ocean currents, like air cur- 
rents, obey FerrePs Law, in that they turn to the right of a 
straight course in the northern hemisphere, and to the left 
in the southern. This results in a distinct eastward drift 
toward the margin of the south polar ocean, and a less dis- 
tinct eastward movement about the Arctic Ocean. In other 
oceans the northern divisions have a clockwise circulation, 
whereas the southern divisions have their circulation counter- 
clockwise (Fig. 314). 

The movement of the waters in all oceans is chiefly about 
the margins, leaving the great central areas undisturbed. 
In these areas of quiet water, seaweed and other floating 
matter accumulate, thus producing what are known as 
Sargasso seas (Fig. 317). These seas are avoided by masters 
of sailing vessels, who find it difficult to get out of the drift- 
covered waters when driven into them by storms. Columbus 
thought, when he came to the Sargasso Sea in the Atlantic, 
that he had come upon land. 

438. Cause of currents. All winds, however fitful, brush 
the surface water along with them. If they constantly vary 
in direction, no systematic or continuous currents can re- 
sult. When the same direction is held for several days, a 
distinct drift with the wind is observed. 

Continued east winds over Lake Erie have at times so 
heaped up the water toward the west end of the lake that 
Niagara Falls has practically run dry, whereas a continued 
west wind brings an unusual volume of water over the falls. 
We are told, too, that strong east winds sometimes drive 
the waters back from one of the arms of the Red Sea and 
make it possible to cross this basin "dry shod." 

Other minor causes may operate to produce locally cur- 




rentlike movements by causing differences of level in the 
ocean surface. The excessive rainfall in the doldrum belt, 
combined with excessive evaporation in the trade-wind 
belts, tends to cause northward and southward movements 
of the surface waters; and great storms, like the Galveston 
storm, pile the water up against the land, to be returned as 
local currents. Differences of temperature produce vertical 
currents when the surface water is colder than the water 
below, but horizontal variations in temperature can scarcely 
cause perceptible motion. 

439. Ocean currents caused by the trade winds. Since 
continuous wind from a given direction sets the water 
drifting with it, the trade winds, blowing, as they do, always 
from the same direction, would seem the logical cause of 
those world-wide movements found in all oceans, known as 
ocean currents. 

Near the eastern sides of the oceans, where the trade 
winds are not well established, the currents are weak and 
somewhat irregular, but farther west, away from the dis- 
turbing influence of the continents, the currents both north 
and south of the equator are pronounced and continuous. 

The current under the northeast trades is known as the 
north equatorial current, and that under the southeast trades 
the south equatorial current. These are found in the three 
oceans that lie athwart the equator; and between them is 
found an eastward-moving current known as the equatorial 
counter current, to be explained later. The north and south 
equatorial currents may be considered the birthplaces of 
the world- wide system of ocean circulation. 

440. Poleward currents. The equatorial currents are 
barred in their westward movements by islands and conti- 
nents across their paths. They are thus forced to turn pole- 
ward along the western shores of the oceans. Whether they 
turn northward or southward is determined by the outline 
of the coast. 

While the currents are moving along and near the equator, 


the earth's rotation has but slight deflecting influence; and 
it is probable that, if not interrupted by land barriers, the 
equatorial currents would continue their westward course 
around the earth. 

As soon, however, as they begin to flow into other lati- 
tudes, the rotation of the earth is effective in turning them 
from a straight course, to the right in the northern hemi- 
sphere and to the left in the southern. 

These poleward currents are warm currents and carry the 
warm water from equatorial regions into colder latitudes. 
At the same time they spread out, lose their velocity, and 
are then known as drifts, which move to the margins of the 
polar oceans, then eastward to the eastern shore of the ocean 
in which they have their origin. 

441. Equatorward currents. By continued deflection 
these eastward moving currents, now cooled by loitering 
in high latitudes, are in part turned toward the equator 
along the western coasts of the continents. Returning thus 
to the trade-wind belts, in which they assume their west- 
ward direction, the circulation about the nonpolar oceans 
is complete. Other branches of the eastward-moving cur- 
rents are, by the configuration of the land or sea bottom, 
made to take other courses. 

The equatorward currents are cold or cool currents and 
bring lower temperatures toward, or even to, the equator, 
and so cause the eastern sides of all oceans in the lower lati- 
tudes to be cooler than the western sides. 

*442. Circumpolar currents. Under the winds of the circum- 
polar whirl, the waters of the polar oceans move with them, 
counterclockwise in the Arctic Ocean and clockwise in the Antarctic. 
The movement of the currents about the Arctic Ocean is not so 
well developed, and not so strong, as that about the Antarctic, 
because of the numerous islands in the north that interrupt them. 
Branches from the circumpolar movement in the north are sent off 
southward into the Pacific and the Atlantic. These cold currents, 
deflected to the right, follow closely the eastern coasts of Asia and 



North America, until they sink beneath the warm currents be- 
tween the parallels of 40 and 50 N. Because of the unobstructed 
course of the Antarctic Drift, it flows eastward along the border 
of the Antarctic continent with a greater velocity than the Arctic 
Drift has. The " brave northwesterlies," that blow in this high 
southern latitude, are likewise responsible for the greater velocity 
of the currents there. 

443. Creep. The movement of the deep polar waters to- 
ward the equator is known as creep. In this way the cold polar 
waters are carried even to the equator, and the low tempera- 

FIG. 315. Currents of the Indian 
Ocean in January 

Note the counterclockwise cur- 
rent near India. 

FIG. 316. Currents of the Indian 
Ocean in July 

Note the clockwise current near 

tures of deep equatorial seas are accounted for. We cannot 
observe the creep; but, as more surface water is carried into 
polar regions than returns as surface currents, the excess must 
be equalized by under-surface return currents. 

*444. Monsoon currents. If any doubt existed as to the suffi- 
ciency of the winds to produce ocean currents, that doubt would 
be removed by a study of those currents which change their 
direction with the change of direction of the monsoons. 

While there are monsoons at the horse latitudes, the winds there 
are neither of sufficient strength nor constancy to be effective in 
producing ocean currents. It is in the monsoon belt, over which 


the heat equator migrates, that we find conditions favorable for 
the production of ocean currents. 

*445. Currents of the northern Indian Ocean. About the 
northern Indian Ocean, when the southwest monsoon blows, the 
water is set drifting in a clockwise direction. As these winds 
weaken, the drift slackens; and soon after the northeast monsoon 
begins, the direction of the drift is reversed. It continues as a 
counterclockwise circulation while the northeast monsoon con- 
tinues, changing again to the clockwise direction with the return 
of the southwest monsoon. These changes of direction of the ocean 
currents can be accounted for only by the reversal of the winds. 

*446. Equatorial countercurrents. In the Pacific Ocean, where 
the heat equator lies prevailingly north of the terrestrial equator, 
the southeast trades, changed to southwest winds north of the 
equator, set up an ocean drift to eastward. 

This is the equatorial counter current. It is fairly distinct through- 
out the year, though better developed during the northern summer. 
Its explanation is the same as that of the clockwise movement 
about the northern Indian Ocean during the southwest monsoon. 

Because of the narrowness of the Atlantic Ocean at the equator, 
the countercurrent is not so well developed as in the Pacific. 

447. Currents and navigation. Sailing vessels lay their 
courses to suit the winds and oceans currents, and even 
steamships do not scorn to take advantage of the great 
ocean circulation. 

Sailing vessels from New York to English ports take 
advantage of the northeast Atlantic Drift; on their return 
they use the trades. Those bound from New York to Rio de 
Janeiro must lay their courses far to eastward of the eastern 
cape of South America, lest the equatorial currents carry 
them northward again while in the doldrums, where winds 
are apt to fail. 

Ships sailing from Atlantic ports for Australia sail east- 
ward around the Cape of Good Hope, to take advantage of 
the Antarctic Drift, while those returning also sail eastward 
past Cape Horn, to have the advantage of the same drift. 

Vessels bound from Honolulu to San Francisco sail north- 


ward beyond the trades and equatorial current, then east; 
returning, they take a more southerly route. 

448. Currents and life. The distribution of many marine 
forms is determined by the temperature of the water, 
which in turn is in part determined by ocean currents. 
Corals serve well to illustrate. The waters of the Galapagos 
Islands, west of South America, are too cold for corals, 
although these islands lie under the equator. The Peruvian 
Current, bringing water from the Antarctic regions, makes 
these waters cold. Contrasted with these islands are the 
Bermudas, in the Atlantic, in latitude about 35 N, which 
are chiefly coral rock and are bordered by reefs of living 
coral. The warm waters are brought to these islands by the 
Gulf Stream. 

The seeds of many plants are distributed by means of 
ocean currents, and insects and the smaller animals are 
carried upon drifting material in these currents. 

449. Currents and climate. The direct climatic influence 
of ocean currents is confined to the ocean and immediately 
bordering lands. Indirectly their influence may be felt hun- 
dreds of miles inland. This is markedly true of lands lying 
to leeward of currents that are abnormally cold or warm. 

The North Atlantic Drift. The most pronounced and far- 
reaching of all ocean currents, in its climatic influence, is 
perhaps the Gulf Stream. The winds from over this broad 
sheet of warm water not only bring abundant rainfall to the 
British Isles and Norway, but so temper the cold of these 
high latitudes as to make them comparable in temperature 
to our own eastern coasts, 20 farther south. 

The North Pacific Drift. The continuation of the Japan 
Current tempers the climate of Alaska and British Colum- 
bia in like fashion. 

These great drifts, in both oceans, send branches south- 
ward along the western coasts of the continents; and when 
they reach the latitude of northern Mexico and Africa, 
their effect is to temper the heat of these coasts. 


The cold currents that follow closely the eastern coasts 
of North America and Asia, being to leeward of those con- 
tinents, do not affect the climate so far inland. However, 
the bleakness of Labrador and Kamchatka is in some de- 
gree traceable to these currents. 

In the southern hemisphere the western coasts are cooled 
and the eastern coasts are warmed by the ocean currents, 
but their influence is less pronounced than in the northern 

450. Currents and harbors. The harbor of Hammerfest, 
at the north of Norway, and well within the Arctic Circle, 
is about as free from ice as that of Boston, 30 farther 
south. In the one case, we see the effect of the warm Gulf 
Stream; in the other, of the cold Labrador Current. 

In the Pacific Ocean, the barrier of the Aleutian Islands, 
together with the narrowness of Bering Strait, prevents the 
North Pacific Drift from entering the Arctic Ocean. As a 
result, the bays on the north coast of Alaska, in the same 
latitude as Hammerfest, are practically closed by ice 
throughout the year. 

The Russo-Japanese War had for one of its objects the 
securing for Russia of the open harbor of Port Arthur. The 
harbor of Vladivostock, Russia's chief port on the Pacific, 
in about the latitude of New York, is for a long time every 
year closed by ice, owing to the cold current coming down 
through Bering Strait. 

451. The Gulf Stream. This greatest and most important 
of all ocean currents derives its name from the Gulf of 
Mexico, from which it issues. It is, in fact, a continuation of 
the combined equatorial currents. 

The North Equatorial Current in the Atlantic is turned, 
by the land masses in its path, wholly into the northern 
division of this ocean. Much of its waters passes among the 
islands of the West Indian group, while the remainder 
passes to the eastward. 

The eastern cape of South America is so situated that it 



divides the South Equatorial Current in two; part of it 
turns southwest along the coast of Brazil, as the Brazilian 
Current, while the other part enters the Gulf of Mexico be- 
tween the West Indies and the mainland of South America. 
This water issues through the Strait of Florida as the Gulf 
Stream. It is truly a stream, flowing between banks of water. 
At that point it is deep and narrow, scouring the bottom of 

FIG. 317. The Gulf Stream 

the strait, and it flows with a velocity greater than that of 
the lower Mississippi River. 

Joined by the waters that come through the West Indian 
group of islands and the waters which pass outside, the 
Gulf Stream is greatly increased in volume. It passes parallel 
to the Carolina coasts and near enough to send off return 
eddies, which build the Carolina capes. Spreading and de- 
creasing in velocity, the Gulf Stream becomes the North 
Atlantic Drift. 

The frequent and dense fogs off Newfoundland are pro- 
duced by warm winds from the North Atlantic Drift blow- 


ing over the cold Labrador Current. The line of meeting of 
the cold and warm water is known as the cold wall. 

Completion Summary 

Wave action undercuts - and causes the collapse 

. This - - sea cliff, and the - - forms a 

beach. Vertical joints - - stacks, and - sea caves. 
The land in this way is cut down below - , and roughly 
smoothed off into a - . The fine material is dragged 
out - - and wave-built terrace. The bottom is worked 

over by waves and piled up into . Behind the 


*This lagoon is gradually - - by - - from the land, by 
the growth of - and by the action of the waves on the barrier 

There are - high tides every day; at 12 

hours and 26 minutes. Tidal range varies, on the shore, 
from a few - - up to - - feet. Flood tide is the 

- incoming - ; ebb - . Slack water - . 
The currents become - - in narrow channels. The tidal 
current entering rivers with - - becomes a wall of water, 
called a - . 

*Tides are caused principally by the together with 

. The direct tide is due chiefly to moon. The indirect 

tide, 180 - , is caused chiefly by - , which is a greater 
force than - - moon. For, since the water there is farther 
away from the center of rotation, the - - is greater than it is 
on the side near the moon; and at the same time the moon's 

- on the water is less - - than - . 
If the earth did not rotate, or the moon did not - , high 
tides would come at the - - time every day. The - - of 
the moon in the same - - earth causes the tides to be - 
later each day. 

The sun - - tides, but moon. When, however, 

both sun and moon straight line with , the 


high tide is and the low tide is . It is then 

called tide. This happens at times of and 

- moon. At - - and of the moon, the tidal 

range . These are called neap tides. 

Ocean currents are caused by , and like , 

they are deflected in accordance with Law. The 

trade winds, being continuous, - currents. 

The north equatorial current starts in the warm 

seas, moving - , is turned - by , then 

east by , and finally - - by the . This 

circulation has - direction. 

Cold polar water seems to creep toward , along 

*Wherever there are strong monsoon winds, currents are . 

The climate of many coasts by . The Gulf 

Stream flows - - near the - coast of the United 

States, then turns - - and bathes the shores of , 

making the climate of Europe distinctly . 


1. Describe wave motion in water. 

2. What produces waves? 

3. How high are the largest waves? 

4. What is the groundswell? 

5. What are breakers? Where do the waves break? 

6. What causes the undertow? 

7. What is the relation of the undertow to a longshore cur- 

8. Describe the work of wave erosion. 

9. Describe the formation of a sea cliff. 

10. How is a beach formed? 

11. Why are stacks not always formed by wave erosion? 

12. Describe the formation of a sea cave. 

13. Why do the waves cut like a horizontal saw? 

14. What is a wave-cut bench? 

15. What is a wave-built terrace? 


16. How is a barrier beach formed? 

17. What is a spit? Why is it often curved? 

18. How is a lagoon formed? What finally happens to it? 

19. What is the time interval between tides? 

20. If it is high tide at 3.10 A.M. today, when will the next high 
tide occur today? tomorrow? 

21. Why is the tidal range small at some places and great at 
others? Where is the greatest tidal range in the United States? 

22. What is flood tide? ebb tide? slack water? 

23. What is a tidal race? 

24. How far up a river does the tide extend? 

25. What is a tidal bore? 

26. How do the sun and moon together affect the tides? 

27. What is spring tide? 

28. What is neap tide? 

29. How would an observant person, living at the seashore, 
connect the rise and fall of the tide with the moon? 

30. How does the tide affect navigation? 

31. When do we get the lowest water, at spring or at neap tide? 

32. What is tidal scour? 

33. What is the effect of tidal currents on the shore? 

34. What effect has the earth's rotation on ocean currents? 

35. What is a Sargasso sea? 

36. What is the cause of ocean currents? 

37. What causes the north and south equatorial currents? 

38. Why do the equatorial currents turn toward the poles? 

39. Explain how the earth's rotation affects the equatorial cur- 

40. Describe in detail the course of an ocean current. 

41. Why are the eastern sides of the oceans, in low latitudes, 
cooler than the western? 

42. What is creep? 

43. How do currents affect marine life? 

44. What effect has the Gulf Stream on the western coasts of 

45. What effect has the Japan Current on our western coast? 

46. Show by example the effect of currents on harbors. 

47. What is the cause of Newfoundland fogs? 


if Optional Exercises 

48. How can a tidal current flow upstream, after high water? 

49. Why has the moon greater gravitational effect on the earth 
than the sun has? 

50. What is centrifugal force? On what part of a rotating body 
is it greatest? 

51. Explain how there can be two high tides on the earth at the 
same time. 

52. Is there any tidal effect on the rock portion of the earth? 

53. Why do we not have high tides at exact intervals of 12 

54. Explain by diagram spring tide and neap tide. 

55. Show by example the effect of monsoon winds on an ocean 

56. What is the cause of the equatorial countercurrent? 

57. What is meant by establishment of the port? 



452. Kinds of shore lines. An examination of the shores 
of the continents reveals two kinds of shore lines: regular 
and irregular. These are represented in Fig. 318, showing 
part of the coast of Alaska, and Fig. 320, showing part of 

FIG. 318. A Regular Shore Line Due to Recent Emergence of the Land, 
at Nome, Alaska 

the coast of Maine. These two shore lines have a different 
origin, and we shall now try to study the features of each 
kind of shore line, and how it is developed. 

453. Emerging continental shelf. The continental shelf, 
as we have seen, is composed of loose sediments, for the 
most part, which are almost horizontal and quite flat. 
When it is uplifted slowly, it forms a coastal plain and the 
shore line will be quite regular (Fig. 318). 

The water is very shallow for quite a distance from shore, 
and waves churn up the bottom and pile some of the loose 
material at the line of breakers. This builds up higher and 




FIG. 319. An Emerging Continental 

Shelf, with Barrier Beaches Parallel 

to the Shore 

higher, especially in stormy 
weather, until it is above sea 
level. It finally takes the form 
of a long, sandy island paral- 
lel to the shore, called an off- 
shore bar or barrier beach. The 
body of quiet water between 
the bar and the shore is called 
a lagoon (Fig. 307). The la- 
goon is usually about a mile 
across; but occasionally it 
may be 10 miles or more if 
the continental shelf slopes 
very gently. It is seldom more 
than 20 feet deep. The shore 
line, which started very regu- 
lar, is now somewhat irregular. 
Soon the waves have done 
all they could to the bottom 
outside of the bar, and they 
are free to break against the 
bar itself. They cut down the 
seaward side of the bar, carry- 
ing some of it out, and, dur- 
ing storms, hurl some of it up, 
over the bar into the lagoon 
beyond. By this process the 
bar is eroded and gradually 
moved toward the shore, fill- 
ing in the lagoon. At the same 
tune streams from the land 
carry their deposits into the 
lagoon, and help fill it up. 
Vegetation begins to encroach 
from the landward side, con- 
verting the lagoon into a tidal 




Boothbay topographic sheet, U. S. G. S. 

FIG. 320. The Irregular Coast of Maine 

marsh. Finally the waves remove the bar and the sediments 
in the lagoon entirely, and the shore is once more regular. 



The east coast of the United States from New York to 
Florida has recently emerged and therefore is faced by a 
series of barrier beaches. Atlantic City and Miami Beach 
are situated there (Fig. 319). The lagoon at Miami is called 
Biscayne Bay. 

454. The submerged coast. When a coast is submerged, 
it is very irregular, because the sea enters into all the river 
mouths and fills in the depressions of the river valleys. 
This produces long narrow bays called estuaries, long nar- 
row peninsulas and headlands, and numerous islands near 
shore (Fig. 320). 

The projecting ends of the peninsulas and the islands are 
first attacked by waves, giving us wave-cut cliffs with long 
blocks hidden in the deep water. Caves and stacks may de- 
velop later, and then the blocks are broken up to form the 
beach. Loose material is carried into the quieter water of the 
bays or estuaries and deposited right across the mouth as 
spits. Such a spit is Sandy Hook at the entrance to New York 
Harbor. As the spit increases in length, it closes the bay, 
sometimes entirely, forming a bay bar (Fig. 321). The tidal 
current usually maintains an opening in the bar (Fig. 322). 



FIG. 321. Bay-Mouth Bars, Martha's Vineyard, Massachusetts 

Behind the bar a body of sheltered water, a lagoon, forms. 
This is filled by sediments from streams, while the bay bars 
are extended between islands to form a barrier beach all 



along the coast, giving it a more regular shore line. The la- 
goons develop into marshes and are gradually rilled up. The 
indentations of the shore have now been eliminated, and the 
waves continue their work on the more or less straight line 
of cliffs (Fig. 323) . If there is more than one kind of rock on 
the shore, the weaker rock will be worn more rapidly and the 

FIG. 322. Drowned Shore, Showing Spits, Bay Bars and Lagoons 

*' ' 


FIG. 323. Drowned Shore in Maturity 

harder one will still form promontories, so that the develop- 
ment of a regular shore line will be indefinitely postponed. 

Irregular shores, particularly those due to recent drown- 
ing, have good harbors because the water is deep and the 
surrounding hills form a haven. 

*455. Fault-plane shores. In many parts of the world a fault in 
the bedrock is the cause of the submergence. The seaward side of 
the fault is submerged, and the landward side forms a long row of 
rocky cliffs that make a very regular shore line. Harbors on such a 
coast are likely to be small, shallow, and far apart. Such shores 
make commerce even more difficult than the offshore bars do, be- 



cause there is the added difficulty of moving freight up and down 
the face of the cliffs. 

Figure 324 is a block diagram of such a coast. The fault plane, 
or the plane of the break, is marked 1, 2, 3, 4. The unshaded por- 
tion is water, standing at the level 5, 6, 7, 8, and submerging one 

FIG. 324. Diagram of a Fault Plane Shore 

After Cotton 

side of the fault. The surface of the fault plane, marked 2, 3, 6, 5, 
forms the row of cliffs above water. 

Beaches and continental shelves on such fault-plane coasts are 
rare, except on those that are approaching old age, but they will 
be built, in time, as the cliffs are weathered and eroded and as 
sediment brought in by streams accumulates. Drowned valleys 
are absent. Fault-plane shores occur in New Zealand, on the 
shores of the Red Sea, and on the west coast of Africa. 

456. Mountainous shores. Young, folded mountains, par- 
allel to and near a shore, would form a very regular shore 
line if they had emerged enough to raise the passes well 
above the sea. The western coast of the United States has the 
Coast Range quite near the shore, and the shore line re- 



Lot. 42 

Pt. St. George 

sembles the smooth curves of the offshore bars much more 
than it does the irregular coast of Maine. It is also like a 
regular shore in the scarcity of good har- 
bors. Figure 325 shows about 125 miles 
of the California coast, without an im- 
portant harbor. There are but two large 
rivers that have eroded channels through 
the mountains and formed important 
harbors on the western coast of the 
United States. They are the Columbia 
and the Sacramento. The scarcity of good 
harbors is a characteristic of many moun- 
tainous coasts. When, however, the 
mountains are submerged so that the 
passes are drowned, the valleys will also 
be drowned, forming an irregular shore 
line like that shown in Fig. 326. 

Trinidad Head 

Cape Mendocino 

' Punta Gorda 
t 40, 75' 

FIG. 325. Shore Line 
in California 

*457. Fiord shore lines. In cold regions 
where ice is of enormous thickness, valley 
glaciers sometimes gouge out the river valleys far below sea level. 


5 10 15 2C 

FIG. 326. Submerged Mountain Ranges 

The east coast of the Adriatic south of Istria. The ranges run parallel 
with the shore. 




FIG. 327. The irregular coast of Nor- 
way has innumerable fiords. 

When the ice recedes, the lower 
ends of these troughs will be 
submerged by the sea. These are 
fiords. They are long, narrow, 
branching bays of great depth, 
with U-shaped cross section and 
steep sides. The lower Hudson 
River is a fiord without steep 
sides. The coasts of Norway and 
Alaska are full of fiords, which 
make a very irregular coast line 
(Fig. 327). 

458. Coral reefs. Southern 
Florida, the Hawaiian Is- 
lands, and the shores of all 
oceans of the torrid zone, ex- 
cept the eastern shores of the 
Atlantic and the Pacific, are 
fringed with jagged coral 

The reef-building coral is a 
small animal living in colonies 
attached to the ocean floor. 
It requires clear, warm, salt- 
water currents to bring it 
food, and it requires light, 
which it cannot get much be- 
low a depth of 120 feet. It 
extracts limestone from sea 
water and deposits it in the 
lower part of its body. By the 
growth and decay of count- 
less corals, the rocky base 
may be built up nearly to the 
surface of the sea. The waves 
break off branches of the coral 



and grind them to coral sand, which finally consolidates to 
a granular limestone. The waves and the wind may build up 
a low reefy not more than 20 feet above the level of the sea. 
Where the reef is close to the shore, as along eastern 
equatorial Africa, Brazil, Cuba, and the Hawaiian Islands, 
it is called a fringing reef. The outer border, better supplied 

FIG. 328. An Atoll 

with food, grows more rapidly than the inner. Within, the 
corals die and the rock is dissolved, until a lagoon may 
develop inside the barrier reef, as it is now called. The Great 
Barrier Reef of the northeast coast of Australia is about 
1,200 miles long. 

An atoll, or ring of coral around a central lagoon, may be 
formed where the coral has grown on the top of a shoal that 
came to within 120 feet of the surface (Fig. 328). Or, accord- 
ing to Darwin's theory, the atoll is a coral reef around a 
sunken island, as for example, a volcano. The growth of the 
coral equaled the rate of sinking. When the rate of sinking, 
or rise of water, exceeds the rate of growth, the coral are 
drowned. The Chagos Islands in the Indian Ocean are the 
unsubmerged portions of a very extensive coral region. 

Coral islands are naturally very low, though some few 
show that they have been elevated. Plant life may be abun- 
dant, though of few varieties. The coconut palm furnishes 
food, clothing, and utensils to the unambitious natives. 

459. The submerged Atlantic Coastal Plain. The Atlan- 
tic coast of the United States shows features of uplift as 


well as submergence. It was first uplifted, giving us the 
Coastal Plain with the long barrier beach, extending all 
along the coast and around Florida; and then recently it 
has been partly submerged and this has developed irregu- 
larity in the shore line by forming deep bays and blocking 

them up with bay bars 
(Fig. 329). 

The shore is low and 
marshy from New York 
south, but the land rises 
gently to the fall line. 
The strata beneath the 
soil are the same as those 
of the continental shelf 
and contain many ma- 
rine fossils, which prove 
the former submergence 
of the region. 

Although the shore is 
irregular, it has very few 
good harbors, because of 
the offshore bars and 
shallow lagoons which 
have developed. 

The major irregulari- 
ties of the coast, such as 
New York Bay, Dela- 

Barnegat sheet, U .S.G.S. and coast chart (reduced) 

FIG. 329. Part of the Submerged 
Atlantic Coastal Plain 

ware Bay, Chesapeake Bay, and several others, are among 
the most important harbors of eastern United States (Fig. 
330). In these protected bays we find the great cities of 
Boston, New York, Philadelphia, Baltimore, Norfolk, and 

The drowned valleys of the Atlantic coast were eroded, 
as a rule, in the unconsolidated sediments of the uplifted 
coastal plain, which accounts for their V-shaped cross section 
and the general narrowing of the submerged portion as we 



follow them inland. The 
drowned valley of the Hudson 
River, above the Harlem, was 
eroded out of bedrock by 
stream and glacier. 

460. The Maine coast. Fig- 
ure 320 shows a part of the 
Maine coast. There are many 
offshore islands and rocky 
promontories, and many estu- 
aries. Estuaries, as you recall, 
are wide-mouthed, drowned 
rivers and therefore subject 
to tides. Ordinary rivers have 
no tides. 

The islands are rocky, but 
there is much good farm land. 
Beaches are few, showing that 
wave erosion has not gone far, 
but harbors are numerous. 
There are many lighthouses to 
warn shipping of the numer- 
ous rocky headlands and sub- 
merged rocks, but wrecks have 
been comparatively few, be- 
cause of the numerous harbors. 

461. What is a harbor? The 
terms haven, harbor, and port 
are each defined as a sheltered 
recess in the shore line. It is 
true that each of the three re- 
quires safety for the vessels, 
but the harbor is more than 
a haven; it requires means for 
loading and unloading vessels 
and for landing passengers. 

FIG. 330. Harbors of the Submerged 
Atlantic Coastal Plain 


A port is really a gateway or entrance and is defined, in 
law, as a place where persons and merchandise may legally 
enter or leave a country, i.e., where customs officials are 
stationed, to see that the laws of the nation concerning 
such entry or departure are carried out. In this sense, a 
port may be far from any body of water. 

The location of a harbor is necessarily limited to places 
where it is convenient to transfer passengers and mer- 
chandise from vessels to land or vice versa. 

462. Historic importance of harbors. There has been a 
close relation between harbors, trade, and the spread of 
civilization, ever since the Phoenicians carried their alpha- 
bet and the products of Egyptian and Asiatic civilizations 
from Tyre and Sidon to Greece, Carthage, and the western 
Mediterranean world. Rome was situated near the most 
convenient harbor on the borderland between Greek and 
Etruscan civilizations. Although on the surface the Punic 
wars, between Rome and Carthage, had other causes, the 
real underlying cause was the trade rivalry between two 
nations, each of which had a good harbor. After Carthage 
was destroyed, Rome became mistress of the Mediterranean 

During the Middle Ages and the Renaissance the products 
of the civilizations of the East and West were distributed 
largely by those cities that had good harbors, the Italian 
cities, Venice and Genoa, and the German cities, Hamburg 
and Bremen. 

In modern times the United States, England, Holland, 
and Germany owe no small part of their rapid advance in 
power and wealth to their numerous harbors. 

463. What makes a good harbor? A short description 
of the landing of passengers or freight from ocean vessels 
in a good harbor and a poor one will make it clear why 
nations with good harbors monopolize the trade of the world, 
why people prefer to land at those ports, and why its mer- 
chants can undersell their rivals in other countries. 


When a large vessel approaches Cherbourg, France, a 
rather poor harbor, it must anchor miles offshore and transfer 
its passengers and freight to small boats, which then steam 
into the harbor and tie up against the wharf. Passengers 
can then walk down gangplanks and freight can be trans- 
ferred by derricks or other mechanical devices. There 
are harbors even more open than Cherbourg, and if the 
weather is stormy, the transfer at sea is dangerous and 
very slow, because the lighter bobs up and down, and it 
becomes difficult to step into it. It is not unusual for a per- 
son to fall into the water in the process. 

Contrast this with landing at New York, Southampton, 
London, Liverpool, or Hamburg, which are fine harbors. In 
the first place, a storm has practically no effect on the land- 
ing, because the docks are situated in places protected from 
the winds, and each dock is covered so that one is practi- 
cally in a building. Gangplanks are lowered, and passengers 
walk onto the dock. Freight is handled by derricks and 
traveling cranes, which sometimes load material directly 
into waiting cars on tracks parallel to the shore (Fig. 331). 

It does not cost much to ship goods through a good 
harbor, because it is not transferred and handled several 
times, and therefore the merchants of that city and those 
shipping that way can undersell others. 


1. Protected from wind and wave 

2. Sufficient anchorage without rock bottom 

3. Deep water at entrance and alongside docks 

4. Docks must be long enough. 

5. There must be numerous docks. 

6. Adequate facilities: elevators for grain; pumps for 
tankers; clamshell buckets for coal; derricks and traveling 
cranes for heavy freight 

7. Open throughout the year. No ice in winter 

8. Access to a rich hinterland 




FIG. 331. London Dock, Showing Facilities for Handling Freight and 


The importance of (7) became one of the causes of the 
Russo-Japanese War. The Russians had no ice-free harbor, 
and their desire to acquire one brought them into conflict 
with Japan. During the war important units of the Russian 
Navy were icebound in the harbor of Vladivostock, which 
perhaps brought about the defeat of the rest of the navy at 
the hands of the Japanese fleet. 

464. New York Harbor. "The City of New York has be- 
come the metropolis of North America because of the natural 
advantages of its geographical location, rather than be- 
cause of the acumen of its business men." 

Let us see whether the facts justify this statement (Fig. 

New York is situated at a corner in the shore line. The 
shores of Long Island have an easterly trend, as indicated 
by Coney Island and Rockaway Beach; and the adjacent 
shore of New Jersey has a southerly trend, as indicated by 



FIG. 332. New York Harbor 

Sandy Hook. The harbor, which comprises the Upper Bay, 
the East River, and the lower Hudson River, is practically 
surrounded by land more than 100 feet high on all points of 



the compass, and is therefore protected from wind and wave. 
The water is deep enough to accommodate the largest ocean 
liners, right alongside the docks, although dredging is 
constantly carried on in the Narrows. There are scores of 
docks longer than the biggest vessels. 


FIG. 333. The largest transatlantic vessels can be accommodated 
in New York Harbor. 

The facilities at the docks are unexampled; mechanical 
devices of all kinds are found, and storage facilities are near 
at hand (Fig. 333). 

The harbor is never frozen over. 

Add to all that its nearness and accessibility to eastern 
and central United States via the Hudson River, the Erie 
Canal, the Great Lakes, and the innumerable railroads, and 
it is easy to understand why New York Harbor is the best 


in the world, and why it has become the center of the trade 
of the United States and the wealthiest and most populous 
city of the world. 

465. Submerged valley harbors. The harbors of New 
York, Philadelphia, San Francisco, Seattle, Montreal, Que- 
bec, Liverpool, Bristol, London, Shanghai, Hamburg, and 
scores more of the important harbors of the world belong 
to this class. The submerged valleys were formed by stream 
erosion. Even the great harbor of San Francisco, sometimes 
classed as a mountain-range harbor, is really the submerged 
valley of the Sacramento-San Joaquin River, which reaches 
the ocean through the " Golden Gate" that the river cut 
through the Coast Range. 

Advantages. As a rule these harbors are large, deep, and 
well protected. Many of them have connecting waterways 
that extend far inland. For example, the St. Lawrence con- 
nects the harbors of Quebec and Montreal with the Great 
Lakes and more than 1,500 miles of navigable water. 
Hamburg receives freight from the Austrian frontier, and 
Shanghai from far across China. The large volume of water 
that flows into and out of some of these harbors at each 
change of tide keeps the entrance to the harbor open. 

Disadvantages. Some drowned valley harbors have to 
contend with a shallow entrance, as at Liverpool, where a 
bar across the mouth of the Mersey prevents the passage 
of large vessels except at high tide. Some of them have a 
crooked entrance, as was formerly the case with New York 
Harbor. The modern ocean liner, 1,000 or more feet long, 
could not steer through the crooked entrance; so the old 
Ambrose Channel, which was only 16 feet deep at low tide, 
had to be widened, deepened, and straightened. 

This was done at a cost of about $6,000,000, and we now 
have a channel 2,000 feet wide, 40 feet deep at low tide, and 
seven miles long, with but one curve. 

Some submerged valley harbors have excessively high tides. 
At Liverpool they formerly had considerable difficulty hi 


loading and unloading vessels that were raised or lowered 
20 feet between tides. The difficulty was partially overcome 
by building an immense floating raft or " landing stage" 
with drawbridgelike approaches, similar to the hinged ap- 
proaches that enable us to drive motor cars onto ferry boats. 

466. Crater harbors. The harbor of Ischia, in the Bay 
of Naples, occupies the crater of a dormant volcano. Such 
harbors are usually well protected, but they frequently have 
a rocky bottom, which causes the loss of many anchors. The 
island of St. Paul, near Ischia, also has a crater harbor, and 
so has the island of St. Thomas in the West Indies. The 
depression is due to the shrinkage of the cooling lava that 
formed the crater. 

467. Delta harbors. Among the important delta harbors 
are those at Para, in the mouths of the Amazon; New 
Orleans, on the Mississippi; Calcutta, on the Ganges; and 
those on the Yellow River of China. They have the advan- 
tage of a large area of navigable water in the distributaries 
and of long, connecting inland waterways, but are subject 
to two disadvantages: (1) the shifting of the principal 
mouth, and (2) the formation of bars at the mouths of the 
distributaries. The latter was overcome, in the Mississippi 
delta, by building jetties, which narrowed the channel, and 
so caused the river to scour out the sediment between them 
and to carry the river's load out into deeper water. In time 
a bar across the entrance will be formed in this deeper 
water, and then the jetties will have to be lengthened again. 

468. Fiord harbors. The water in fiords is usually very 
deep, close to shore, because of the U-shaped cross section. 
This enables ships to lie close to shore. These harbors are 
well protected and often have a wide, deep, and fairly 
straight entrance. 

Their high, steep sides tend to make loading and unloading 
vessels difficult and to make the harbor inaccessible. Moun- 
tains sometimes surround fiords, increasing the difficulty of 
access. Few fiord harbors are important. 


This is partly because of the disadvantages mentioned 
and partly because the demands of commerce are not great 
in these high latitudes. The harbors at Oslo and Bergen, 
Norway, are the most important of the kind, since they 
have the advantages, but not the usual disadvantages, of 
fiord harbors. 

469. Lagoon harbors. Some lagoon harbors are located 
behind barrier beaches, others behind sand spits, and still 
others behind coral reefs. In each case the harbor is well 
protected from all ordinary storm waves but not from 
landward storm winds. The entrance to lagoon harbors is 
sometimes shallow, narrow, crooked, and dangerous, par- 
ticularly when the barrier is a coral reef. Harbors behind 
sand reefs are usually very shallow, and, if the area of the 
harbor is large, may have strong tidal currents through the 
inlet. Some inlets to harbors behind such barriers are often 
being filled by sediments, while others are being opened. It 
is believed that there was an inlet through Sandy Hook, 
near Atlantic Highlands, in Revolutionary times. 

Jamaica Bay (Fig. 332) is a sand-spit harbor, as is also 
that at Erie, Pennsylvania, on Lake Erie. The harbor at 
Atlantic City, New Jersey, is enclosed by offshore bars. 

Biscayne Bay, at Miami, Florida, is an example of a 
coral barrier reef harbor. That of Hamilton, Bermuda, is 
an atoll harbor. Pearl Harbor, Hawaii, and the island of 
Guam are the most important coral-reef harbors belonging 
to the United States. 

470. Island harbors. Many harbors are protected from 
wind and wave by islands. The harbor of Vancouver is 
behind a large island that protects it from the prevailing 
northwest wind. 

The harbor of Callao, Peru, is behind an offshore island. 
The latitude of Callao is 15 south, and therefore the region 
is subject to the southeast trades most of the year and to 
the northwest hooked trades for the remaining months. 

Many of the piers of Boston Harbor lie along the sub- 


merged valleys of the Charles and the Mystic rivers, but 
a larger area is protected by an island of glacial origin. 

471. Artificial harbors. Harbors have been built at both 
ends of the Panama Canal and on the Pacific side of 
the Isthmus of Tehuantepec. A well-protected harbor has 
been devised at La Plata, near Buenos Aires, by building 
great breakwaters in the open roadstead of the Plata River. 
The naval harbor at Dover, England, is one of the best 
artificial harbors in the world. Here, great concrete break- 
waters enclose a deep area of about a square mile. The 
harbor of Hilo, Hawaii, has a breakwater two miles long; 
and one of about the same length at San Pedro, California, 
makes a harbor for Los Angeles. Breakwaters have also 
been built at Naples, Italy, and at Haifa, in Palestine. 

Completion Summary 

When is uplifted, it makes a shore line. 

Wave action then churns up and deposits at 

the line of breakers. Finally the waves the barrier 

beach, pushing it toward , the becomes nar- 
rower until it is and the shore is regular again. 

A drowned shore is at first. Wave erosion under- 
cuts , producing and . Right under the 

surface of the water, a is developed. The rock waste 

is into a and . Sand bars are formed 

across , which are ultimately by deposits 

from land and , straightening . 

Uplifted mountains may produce a regular if 

In warm, shallow water, corals build . An atoll 

seems to be built around . 

The Atlantic Coastal Plain has been submerged 

in part, so that we find features of the emergent , 

like , as well as features of the submergence, like 

. Estuaries were formed by . 

A good harbor must be , deep water , 


and must have facilities ; it must ice , 

and have access . 

Submerged shores harbors. 

, - , and - owe their good harbors to 

drowned shore lines. 

Lagoons on uplifted - - sometimes make harbors, but 

these have many disadvantages. They are not , 

, and the entrance - . 
Artificial harbors are often formed by building . 


1. Why does a newly uplifted coastal plain have a regular 
shore line? 

2. How is a barrier beach formed? Why is it parallel to the 

3. What determines the width of a lagoon? 

4. Describe the action of the waves on a barrier beach. 

5. Why is a lagoon only a temporary shore feature? How is it 
finally destroyed? 

6. Which shore of the United States is emergent? Cite features 
to prove it. 

7. Why is a newly submerged coast irregular? 

8. What is a spit? Why does it often curve? 

9. What is a bay bar? Where is it formed? 

10. How does a drowned shore finally become regular? 

11. When is an emergent mountainous shore irregular? regular? 
Cite an example of each. 

12. Considering the answer to question 11, when will there be 
good harbors on a mountainous coast? poor harbors? 

13. What is a barrier reef? Where is the Great Barrier Reef? 

14. What is an atoll? 

15. Was the Atlantic Coastal Plain uplifted before or after it 
was submerged? 

16. What features were developed by the uplift and by the 

17. What is an estuary? 

18. What is a harbor? 

19. What makes a good harbor? Name three good harbors. 


20. Compare the facilities and advantages of New York Harbor 
with the characteristics of a good harbor and rate it on the basis of 

21. Name 5 of the best harbors in the world. 

22. State at least two advantages of each harbor mentioned 
in the answer to question 21. 

23. State two disadvantages of drowned valley harbors. 

24. What are the advantages of a delta harbor? the disad- 

25. Name two delta harbors. 

26. Name a fiord harbor. What are its advantages? 

27. Why do lagoons usually make poor harbors? Name a lagoon 

28. Mention an island harbor. 

29. Mention two artificial harbors. 

^Optional Exercises 

30. Give a short Me history of an emergent shore line. 

31. Why are bars formed between headlands on a drowned 
shore, while they are parallel to the shore on an emergent coast? 

32. How is a barrier beach formed on a drowned shore? 

33. Under what two conditions would a shore be irregular? 

34. What kind of shore line is formed by a fault? What kind of 
harbors does this produce? 

35. How are fiords formed? What kind of shore line do they 
make? What kind of harbors? 

36. Show the historic relation between good harbors and civili- 

37. What is a crater harbor? What disadvantage has it? 


Maps and Map Projections 

A map is a representation of a portion of the earth's surface 
on a plane or flat surface. Every great nation employs a body of 
men engaged in surveying and map making. In the United States 
the General Land Office has mapped most of the country in order 
to allot and sell the public domain. The United States Geological 
Survey is making an accurate large-scale map to show geological 
and relief features, rivers, lakes, and coasts. The portion of the 
earth represented on a map is indicated by latitude and longitude. 

The scale of a map is the ratio between the length of a line on 
the map and the actual distance the line represents. A scale of 
1 : 63,360 is equivalent to 1 inch to the mile, since 1 mile = 63,360 
inches. On the United States topographic maps, the scale most 
frequently used is 1 : 62,500, which is about 1 inch to the mile. 

The mapping of large areas presents peculiar difficulties because 
of the curvature of the surface, which is negligible for a small 
area, and the converging meridians. These difficulties are met, in 
part, by map projections. 

Map projections. A projection is a flat picture, or representa- 
tion, of what the observer sees in three dimensions, as far as that 
is possible. If light were made to shine on an object of which 
certain points only were opaque, the shadows of these points on 
a sheet of paper would be a projection. Some projections represent 
the shapes of different parts, others the areas, but none can ac- 
complish both of these things at the same time. If the areas are 
equivalent, the shapes must be distorted. 

The orthographic projection. In this type of map, points are 
determined by drawing lines from the object at right angles to 
the paper. These lines are parallel to each other, just as rays of 
sunlight would be (Fig. 334). An orthographic projection of the 
block 123456 is to be made on the sheet A BCD. The parallel 
dotted lines are the projection lines and 1'2'3'4' is the projection. 




It is a view of the upper surface of the block as seen from a point 
far enough above, so that the lines of sight would be parallel to 
each other. This makes the projection the exact size of the upper 
surface of the block. 

In similar manner, a side view may be projected on a vertical 
surface, like a wall, ADEF. By 
using horizontal projection lines 
we get figure 3"4"5"6", which 
is the orthographic vertical pro- 
jection of the block. 

Figure 335 shows how to 
draw polar and equatorial pro- 
jections of the earth by the or- 
thographic method. In each of 
these projections we notice that 
distances are foreshortened near 
the edges. No scale of distances 
can apply to all parts of the 
map, but the projection is use- 

A B 

FIG. 334. Orthographic Projection 

FIG. 335. Orthographic Projec- 
tions of the Earth 

ful because it shows the appearance of a hemisphere just as a 
globe does. 

Figure 336 shows an orthographic projection of the western 
hemisphere. Notice the contraction of areas around the edges. 

Mercator's projection. Figure 337 is a Mercator's projection of 
the entire earth. We begin its construction with a horizontal line 
that represents the length of the circumference of the earth ac- 





FIG. 336. Orthographic Projection 
of the Western Hemisphere 

cording to a given scale. If we divide this line into eighteen equal 
parts, each part will represent 20 of longitude and vertical lines 
through these points will represent meridians. 

The meridians of the earth converge toward the poles, so that 
1 at the equator is about 70 miles, whereas, at the poles, 1 is 
zero. At latitude 60, 1 of longitude is about 35 miles. 

On Mercator's projection, the 
meridians are the same distance 
apart all over. Hence if 1 rep- 
resents 70 miles at the equator, 
it should be only half that at 
latitude 60, and much less as 
we approach the poles. Hence it 
is apparent there is great exag- 
geration of areas in high lati- 

Mercator corrected the dis- 
tortion that would occur in the 
shapes of land areas by making 
a degree of latitude twice as long 

at 60 as it is at the equator. This makes areas at 60 four times 
too large, but it preserves the true shape of the land, and direc- 
tions are correctly shown. Because of this, Mercator's projection is 
much used for mariners' charts. 

Degrees of latitude are correctly shown at the equator, but at 
other points the length of the degree is increased in exact propor- 
tion that a degree of longitude is decreased on the earth at the 
point in question. Note, in Fig. 337, the gradually increasing dis- 
tance used to represent 20 of latitude. For example, the distance 
from 60 to 80 is twice as great as from 40 to 60. 

Compare the size and shape of Greenland in Fig. 337 with its 
shape in Fig. 338, in which the Mercator plan is not used. 

Mollweide projection. In the Mollweide projection (Fig. 338) 
the equator is represented twice as long as the earth's axis, which 
will give correct areas. Meridians and parallels are equally spaced; 
the meridians are ellipses, and the parallels are straight parallel lines. 

Shapes of land are distorted especially near margins and in 
high latitudes; but the map is pleasing to the eye and shows the 
entire earth. 





A modification of the Mollweide projection is Aitoff's projection 
(Fig. 339). The meridians are farther apart at the center than at 
the margins and the parallels are slightly curved. This maintains 
equality of area while improving the shapes somewhat. 

FIG. 338. Mollweide Projection 

FIG. 339. Aitoff's Projection 

Globes. Globes represent the whole earth. They show relative 
sizes of the land masses and their relative positions correctly, and 
directions are correctly indicated on them. It is not possible to 
make them large enough to show the amount of detail necessary 
for many purposes, although sections of globes have been made 
that show limited areas with sufficient detail for most purposes. 



Relief. The elevations and depressions of the earth's surface 
constitute what is technically known as the relief of the earth. It 
is best represented by models in which a greater scale is used for 
the elevations and depressions than for horizontal distances. 
Models are expensive, cumbersome to handle, and limited to the 
representation of small areas. 

Figure 340 shows one way of representing the relief of a region 

on a flat surface. Lines called 
hachures are drawn to indicate 
. the paths that water would fol- 

FIG. 340. Hachure Map of a Por- 
tion of the Austrian Alps 


341. A Contour Map of the 
Land Shown Above 

low in flowing down the slopes. Sometimes steep slopes are indi- 
cated by short wide lines and gentle slopes by narrow lines farther 

Hachure maps give us a general idea of the relief of a region, 
but we cannot tell whether the ridges are a hundred feet high or 
thousands of feet high. This method was formerly used by the 
United States Coast Survey, but has been abandoned in favor of 
contour lines. 

Contours. Lines connecting all points of equal elevation above 
sea level are called contour lines (Fig. 341). Each contour line 
represents what the shore line would look like if the sea rose to 
that elevation. 

In a flat region there are few contours, while in a hilly region 
there will be many contours close together. When these become 



too close to distinguish, a greater contour interval is used. A con- 
tour interval of five feet may be used for a flat region, because 
there are no great differences of elevation and contour lines will 
be far apart. In a hilly region, a five-foot interval may bring lines 
so close together that they cannot be distinguished ; but a twenty- 
foot interval would require only one quarter the number of lines. 

While contour maps do not 
represent relief to the eye as well 
as hachures, they are of far 
greater value, and by practice 
one soon learns to see the relief 
and to interpret the features 
shown in terms of physiography. 

Interpretation of topographic 
maps. United States topographic 
maps are printed in colors to 
represent various features : black 
for everything built by man, like 

FIG. 342. Where contours form 
angles pointed uphill, we find a river 

roads, houses, and bridges; blue 

for water, including man-made canals; and brown for contour 


If a map has a small contour interval, like five feet, and its 
general color is white, because the contour lines are far apart, 
the slope of the land is very gentle and the region is flat. If the 
contour interval is large and the map is rather brown in color, 
owing to a large number of contour lines, the region is elevated, 
probably mountainous, and slopes are steep. Whenever the con- 
tour lines are close together, the slope is steep. The top of a hill 
will be shown by closed contours. 

Where contours bend convex to the highland, we find a river 
valley (Fig. 342). Where contours bend convex to the lowlands, 
we find a ridge. A young region will have young rivers with the 
contours massed along the courses of the rivers, whereas an old 
region will have few contours. 

Details of topography are best studied on the maps themselves 
in the laboratory. 



10 E- 

10 W 

Terrestrial Magnetism and the Compass 

The earth acts as if a huge magnet were buried in it. One 
pole of this magnet is found north of Hudson Bay, at latitude 
70 N and longitude 97 W. The south magnetic pole is at latitude 
72 S and longitude 150 E. 

The mariner's compass consists of a light magnetized needle 
or rod, suspended on a pivot, so that it can swing in a horizontal 

plane. One end of this needle 
points toward the north magnetic 
pole of the earth, and it is this 
property which makes the com- 
pass a valuable instrument. 

The direction of the compass 
needle is nearly north and south. 
In some places, however, this di- 
rection varies considerably from 
true north and south. This vari- 
ation is called magnetic declina- 
tion. If, for example, the compass 
needle points 10 west of the 
north-south line at a given place, that place is said to have a 
10 W declination. 

Lines connecting places with the same declination are called 
isogonic lines; and lines connecting places of no declination are 
called agonic lines (Fig. 343). In attempting to determine direc- 
tion by the compass, it is therefore necessary to know the mag- 
netic declination for the particular place. One agonic line crosses 
the United States from Lake Superior through Ohio and Kentucky 
to South Carolina. On this line the compass needle points true 
north. At all places in the United States east of this line, the needle 
points west of north; that is, there is a west declination. At all 
places west of the agonic line there is an east declination. In Maine 
the declination is about 20 W, and in the state of Washington 
it is about 20 E. 

If the magnetic declination were constant, the compass would 
be a more dependable instrument than it is. In 1500, when the 
great explorations were being made, an agonic line lay in the 

FIG. 343. Isogonic Lines 


middle of the Atlantic Ocean. In 1600, it ran from Finland to 
Egypt. In 1700, it returned to the Atlantic and now it runs 
through the United States. 

The compass needle is abnormally deflected by masses of iron 
ore buried in the earth, by steel buildings, electric generators and 
other electrical machines, and by sun spots, which produce mag- 
netic storms that completely upset compass readings. 

On a steel boat where compass deflections are abnormal, the 
gyrocompass is used. The gyrocompass points true north, since its 
pointer is in no way influenced by the earth's magnetism, but 
rather by the earth's rotation.* 

The earth inductor compass depends upon the earth's magnetism, 
but is unaffected by magnetic declination. It consists essentially 
of a small electric generator turned by a wind motor, since it is 
used chiefly on airplanes. The armature of the generator cuts the 
earth's magnetic lines of force and generates a small current which 
is delivered to a sensitive galvanometer on the dashboard. 

If the pilot wishes to go due west, he turns an indicator on the 
dashboard until it reads West. This moves the brushes of the 
generator so that no current is delivered as long as the plane is 
going west. The galvanometer will therefore register zero as long 
as the plane is headed west. But as soon as the direction is changed, 
the earth's magnetic lines of force will be cut at a different angle 
and the generator will deliver a current to the galvanometer. As 
soon as he is aware of the deviation of his galvanometer needle, 
the pilot changes his direction until the needle again reads zero.f 

* For a simple explanation of the action of the gyrocompass, see p. 364 of 
Unified Physics by Fletcher, Mosbacher and Lehman. McGraw-Hill. 

t For a more complete explanation of the earth inductor compass, see 
p. 474 of Unified Physics. 


1. While streams are lowering their beds toward sea level, areas between 
streams are also being lowered, (a) Distinguish between weathering and 
erosion. (6) Name three agents of weathering that aid in lowering inter- 
stream areas, (c) Why are some interstream areas lowered more rapidly 
than others? (d) Would weathering be more rapid in mountain or plain 
areas? (e) What physiographic feature would finally be produced if this 
lowering of the surface continued long enough? (/) Explain why this physi- 
ographic feature is seldom produced. 

2. In May, 1934, there was a heavy "dust storm" in southern 
New York State, (a) From what part of the United States did the dust 
come? (6) How was the dust transported? (c) How was the place of origin 
affected by the removal of this dust material? (d) State two possible reasons 
why such dust storms were not common in the past, (e) How does the 
origin of the loess in China compare with that of these dust deposits? 
(/) Name an area not previously mentioned where dust storms are frequent. 

3. Transatlantic cables frequently have to be mended after earthquakes 
originating in the Atlantic. By experience the cable companies know where 
to look for the breaks, (a) What causes the earthquakes? (6) Why do the 
breaks in the cable usually occur in the same locality? (c) Name the instru- 
ment by which people in New York know when an earthquake is taking 
place in the Atlantic, (d) What ocean phenomenon frequently accompanies 
a submarine earthquake? (e) Name two great earthquake belts of the earth. 
(/) Name a locality in the United States, not included in the major earth- 
quake belts, where a destructive earthquake has occurred, (g) Why are 
there many volcanoes in the major earthquake belts? 

4. Give a physiographic reason for each of the following: (a) Fogs occur 
off the coast of Newfoundland. (b) Coal has been found in the Antarctic, 
(c) The return of Halley's comet, unlike that of most comets, can be pre- 
dicted, (d) Deltas are more common in lakes than along the seacoast. 
(e) Relative humidity usually decreases as the air becomes warmer. 
(/) Limestone frequently forms ridges in arid regions, although it forms 
valleys in humid regions, (g) The port of Hammerfest, Norway, although 
situated within the Arctic Circle, is open throughout the year. 

5. (a) How are igneous rocks formed? (b) Name an igneous rock, 
(c) Give a definite locality where the igneous rock named in answer to b 

* Adapted from New York State Regents Examinations. 


may be found, (d) Name (1) a sedimentary rock of chemical origin, (2) a 
sedimentary rock of mechanical origin, (3) a sedimentary rock of organic 
origin, (e) Explain how one of the sedimentary rocks named in answer to 
d was formed. (/) To what class of rocks does marble belong? 

6. The elements of which the earth is composed are also found in the 
sun. (a) Name a theory that attempts to prove that the material of the 
earth came from the sun. (6) Who was the author of the theory named in 
answer to a? (c) What is meant by sun spots? (d) Name three types of 
bodies other than planets in the solar system, (e) State two important 
characteristics of the planet Saturn. 

7. Some of the statements below are true and some are false. Copy the 
letters a, 6, c, etc., and write the letter T next to each true statement. If 
the statement is false, write the word or phrase that should be substituted 
for the italicized word or phrase to make the statement correct, (a) Longi- 
tude is measured from the international date line. (6) Neap tides occur at 
the time of new and full moon, (c) If the angular elevation of the North 
Star is 60 degrees, the latitude of the place is 43 degrees N. (d) The steeper 
the slope, the faster the rain will sink in. (e) Vegetation helps to prevent 
wind erosion. (/) If the velocity of a stream is doubled, it can carry more 
sediment, (g) The ocean currents in the South Atlantic move in a clockwise 
direction, (h) The valley of a stream is lengthened by headward erosion, 
(i) The Delaware River at the water gap is narrow because the rock is 
resistant, (j) Ocean waves are caused by the wind. 

8. Which of the words or expressions in parentheses makes each of the 
following statements true? (a) An iceberg breaks off from (floe ice, neve", 
glacier front). (6) A healthful relative humidity for indoor work is (80%, 
20%, 50%, 35%). (c) The stars around the North Star (appear to revolve, 
actually revolve, appear to remain fixed), (d) The largest planet is (Venus, 
Mercury, Jupiter, Saturn), (e) On March 21, in our latitude the sun ap- 
pears to rise in (the southeast, the northeast, the east, the northwest). 
(/) Some sandstones are coarser than others because (they were deposited 
by swifter streams, they have not been weathered, they were deposited 
far from land). 

9. Which of the following statements is true? (a) The rotation of the 
earth causes the tidal bulge to pass around the earth. (6) Different con- 
stellations of stars are visible at different seasons because of the revolution 
of the earth, (c) If the earth's axis were inclined 30 degrees, New York 
State would have more hours of sunlight on June 21. (d) On June 21, at the 
Tropic of Capricorn the noon sun casts no shadow, (e) The rotation of the 
moon causes its phases. (/) All places on the same meridian see the noon 
sun at the same elevation, (g} A mineral common in granite is feldspar. 
(h) Elevations on a topographic map are shown by lines called isobars. 


10. Copy the following, filling the blanks with the correct words or ex- 

As the center of a "low" approaches Albany, N. Y., the winds shift 

to a quarter, the barometer moves , and the mercury in 

the thermometer moves . These things indicate that (fair, stormy) 

weather is approaching. If the center of the low passes to the 

south, the wind will shift to a quarter. After the low has passed, the 

barometer moves , the thermometer - , and we have 

weather. Weather conditions may be read on weather maps from lines 
called - , which connect places having equal air pressure, and from 
lines called , which connect places having equal temperature. 

11. Give a physiographic reason for each of the following statements: 
(a) A corked bottle thrown into the Pacific Ocean off the coast of Japan 
was picked up near San Francisco. (&) Early on a clear morning the grass 
in a field was found to be very wet although no rain had fallen for several 
days, (c) During a tornado the walls of a building fell outward, (d) In 
some sections of the tropics people frequently make appointments by say- 
ing, "I will meet you after the rain next Tuesday." (e) An eclipse of the 
sun which was visible on the Pacific Ocean began on Wednesday, Febru- 
ary 14, 1934, and ended on Tuesday, February 13, 1934. (/) The ocean 
floor is smoother than the land areas. (0) The shape of the earth is an 
oblate spheroid. 

12. Account for the rainfall or the lack of rainfall in each of the follow- 
ing places: (a) eastern coast of Central America, (6) Central Australia, 

(c) northwest coast of the United States, (d) Amazon valley of Brazil, 
(e) Great Basin of the United States, (/) British Isles, (g) New York City. 

13. The plains regions of the United States, Europe, and Asia are better 
adapted to living conditions than the level areas of plateaus, (a) Explain 
why this is true with regard to (1) climate, (2) ground-water conditions, 
(3) soil conditions, (4) location of roads and railroads, (b) What is the es- 
sential difference between a plain and a plateau? (c) How do the river 
valleys of plateaus differ from those of plains? 

14. The Grand Canyon in Arizona is noted for its size and grandeur. 
It is over 200 miles long and 6,000 feet deep. ^ (a) Is the Grand Canyon 
located in a plain, a plateau, or a mountain area? (b} What is the rock 
structure of this region? (c) How did the Canyon reach its great depth? 

(d) Why is the Canyon wider at the top than at the bottom? (e) Why do 
the walls of the Canyon show alternate steep and gentle slopes? (/) Ac- 
count for the fact that rocks of marine origin are found near the top of the 
Grand Canyon. 

15. The ocean waters cover nearly three fourths of the earth's surface, 
(a) What is the average depth of the ocean? (6) Mention one mineral other 


than common salt that is found in sea water, (c) Mention two gases found 
in sea water, (d) Explain why sea water is salty, (e) Explain why some 
parts of the sea are more salty or less salty than others. (/) How does the 
topography of the ocean floor compare with the topography of land areas? 

16. Venus is observed through a telescope to have phases like those of 
the moon, (a) To what class of heavenly bodies does Venus belong? (b) To 
what class does the moon belong? (c) Why is Venus visible? (d) Compare 
Venus with the earth as to size and relative distance from the sun. (e) Name 
in order four phases of the moon. 

17. Rearrange the words in column A so that each one is on the same 
line with the word or expression in B that is most closely related to it. 

Column A Column B 

1 seismograph elements in the sun 

2 barograph wind gauge 

3 spectroscope altitude of the sun 

4 anemometer continuous temperature record 

5 rain gauge humidity 

6 planetarium longitude 

7 thermograph earth tremors 

8 sextant movements of heavenly bodies 

9 hygrometer atmospheric pressure 
10 chronometer precipitation 

18. Below are the names of several cities with the meridians from which 
their standard time is determined. Suppose that it is 10 A.M. Saturday in 
our eastern standard time belt; what is the time and the day in each of 
the following cities? (d) Philadelphia, Pa., 75 W, (6) London, England, 0, 
(c) Sydney, Australia, 150 E, (d) Manila, Philippine Islands, 120 E, 
(e) Calcutta, India, 90 E. 

19. Some of the statements below are true and some are false. If the 
statement is true, write nothing; but if it is false, write the word or phrase 
that should be substituted for the italicized word or phrase to make the 
statement true, (d) Rain occurs principally in the northeast quadrant of a 
low-pressure area. (6) The summer monsoon over India blows from the 
southwest, (c) Tornadoes occur more frequently over tracts of level land 
than in mountains, (d) Tidal range in the Bay of Fundy is 50 feet. 

20. Copy the following, filling each of the blanks with the correct word 
or expression. 

As the earth moves in its orbit during its period of - , the axis is 
inclined degrees away from a line perpendicular to the orbit. Since 

the pole of the axis always points to , every position of the 

axis is always to every other position of the axis. This causes the 
shifting of the ray of the sun over a belt degrees wide, 


between the dates of the solstices on and . The northern 

limit of this belt is the . 

21. Copy the following table and fill in columns A and B. 

Column A Column B 

Cause Example 


Irregular shore line 

Block mountains 

Lacustrine plain 

Valley glaciers 


22. (a) Make a labeled diagram to show the umbra and penumbra of 
the moon's shadow during an eclipse of the sun. (6) What phase of the 
moon is indicated in this diagram? (c) Name the tide that will occur when 
the moon is in this position, (d) What part of the shadow produces a total 

23. (a) Explain the process by which residual granite boulders are 
rounded by exfoliation (the peeling off of the outer surface). (6) Dis- 
tinguish between mechanical and chemical weathering, (c) Mention one 
factor that determines the rate of weathering, (d) What is meant by 
residual mantle rock? (e) Name three agents that form residual mantle rock. 

24. Explain the difference in the meaning of the terms in each of the 
following pairs : (a) weathering erosion, (6) ocean creep ocean cur- 
rent, (c) river system river basin, (d) pothole sinkhole, (e) denuda- 
tion diastrophism, (/) igneous metamorphic, (g) fringing reef spit. 

25. After a rain some of the water sinks into the ground, forming 
ground water, (a) Explain what is meant by the water table. (6) Show by 
a labeled diagram the relation of the water table to (1) a permanent well, 
(2) a temporary well, (c) Give three conditions necessary to form an ar- 
tesian well, (d) Mention one way in which ground water is destructive. 
(e) Mention one way in which ground water is constructive. 

26. Rearrange the items of column A so that each one is on the same 
line with the word or expression in B that is most closely related to it. 

Column A Column B 

1 anticline gravity deposit 

2 pervious eclipses 

3 igneous equatorial diameter 

4 zenith lets water seep through 

5 7,926 miles condensation 

6 offshore bar point overhead 

7 dew point river deposit 

8 talus lava flow 


9 umbra shadow wave deposit 

10 natural levee upfold of rock 

27. Rearrange the items of column A so that each one is on the same 
line with the word or expression in B that is most closely related to it. 

Column A Column B 

1 Mars largest planet 

2 Neptune a satellite 

3 Venus eighth in distance from the sun 

4 Earth most distant planet 

5 Saturn orbits between Mars and Jupiter 

6 Jupiter nearest the sun 

7 Asteroids shows " canals" and white polar caps 

8 Moon size most like earth 

9 Pluto average period of rotation 24 hours 
10 Mercury rings around equatorial region 

28. Account for the formation of each of the following physiographic 
features in New York State : (a) the ridge of hills along the north shore of 
Long Island, (6) the Finger Lakes, (c) the plain bordering the southern 
shore of Lake Ontario, (d) the Palisades of the Hudson, (e) the Niagara 
gorge, (/) Rockaway beach, (0) New York Harbor. 

29. Give the difference in the meaning of the terms in each of five 
of the following pah's : (a) perihelion aphelion, (6) weather climate, 
(c) lava volcanic ash, (d) planets planetoids, (e) civil day conven- 
tional day, (/) tributaries distributaries, (g) alluvial lacustrine. 

30. Give a physiographic explanation for each of the following state- 
ments : (a) Drills used in cutting into certain types of rock are often tipped 
with pieces of diamond. (6) A thermogram for a period of 24 hours showed 
the lowest temperature shortly before sunrise, (c) During a certain summer 
the farmers near a town had to get water from the town well because their 
own wells were dry. (d) Offshore bars may become bay-mouth bars. 
(e) The Mohawk-Hudson Valley was once an outlet for the Great Lakes. 

31. The bed of Great Salt Lake is covered with a layer of pure salt, 
(a) What caused the layer of salt to form? (6) What is the source of the 
salt in the water of the lake? (c) Give a brief history of Great Salt Lake, 
including one evidence which scientists have found to indicate that the 
lake has not always been the same, (d) Explain two ways in which lakes 
in the eastern part of the United States may be destroyed or disappear, 
(e) Name an extinct lake. 

32. (a) State four conditions that control the climate of a region. 
(6) For each of the following, state how a climatic control has caused the 
climate of the two places to differ: (1) St. Paul, Minn., and Boston, Mass., 


(2) San Francisco and New York City, (3) Mexico City and the eastern 
coast of Mexico, (4) Panama and Chicago, (c) Explain why winters in the 
northern hemisphere are not so cold as winters in the southern hemisphere. 

33. Give a physiographic explanation for each of five of the following 
statements: (a) A layer of shale was changed to slate near a granite in- 
trusion. (6) It is calculated that the reservoir of Boulder Dam will be filled 
with silt in 200 years, (c) Western Oregon has more rainfall than North 
Dakota, (d) A cloudy sky hinders the formation of dew or frost, (e) It is 
often impossible to read the inscriptions on very old tombstones. (/) The 
great deserts of Africa and Australia are situated in the trade-wind belts. 

34. The effect of erosion is opposed by forces in the earth's interior, 
(a) State two methods by which forces in the earth's interior change the 
earth's surface. (6) Name two surface features formed by forces from the 
earth's interior, (c) Name two agents of erosion, (d) State two evidences to 
show that the work of erosion is opposed to the forces of the earth's in- 
terior, (e) How does weathering aid the work of erosion? 

35. During the period when most of the northern part of North America 
was covered by glaciation, the Rocky Mountains were being eroded by ex- 
tensive valley glaciers. 

Compare glaciation in these two regions, using the following topics: 
(a) The place where the ice originated in each region. (6) The general 
direction of the movement of the ice in each region, (c) The names of two 
types of deposits common to each region, (d) The name of one feature due 
to destructive action of the valley glacier but not produced by the con- 
tinental glacier. 

36. Give a physiographic reason for each of five of the following: 
(a) Beach grass has been planted along the boulevard at the Jones Beach 
State Park on the southern shore of Long Island. (6) During a windstorm 
the windshield of an automobile became etched so that it was no longer 
transparent, (c) Large forested areas along the headwaters of streams are 
preserved by state governments, (d) The water is deepest at the outside 
of the curves of the lower Mississippi, (e) A man in St. Louis hears on the 
radio at 8 o'clock a program broadcast from New York City at 9 o'clock. 
(/) Astronomers have seen only one side of the moon. 

37. In August, 1933, a destructive hurricane affected Norfolk, Va. 
(a) What causes the wind to reach hurricane force? (&) What was the ap- 
proximate barometer reading in Norfolk when the center of the hurricane 
was over the city? (c) Why does the direction of the wind change as the 
hurricane passes? (d) Where is the probable origin of hurricanes that affect 
the Atlantic coast? (e) What is the usual path of these Atlantic hurricanes? 
(/) What caused clear weather in Canada at the time when the Atlantic 
coast was swept with heavy rains? 



38. Copy the following table and classify the following terms by placing 
each in the proper space in the table under the agent to which it is chiefly 
due, and indicating whether it is formed by the constructive (building-up) 
action or the destructive (wearing-away) action of that agent: sinkholes, 
drumlins, sand dunes, hanging valleys, flood plain, loess of China, wind 
gap, stalagmite, peneplain, Carlsbad caverns. 





Constructive action 

Destructive action 

39. Show by means of a labeled diagram the nature of each of the 
following: synclinal ridge, young river valley, geologic fault, stalactite, 
eroded plateau, dome mountain, oxbow lake. 

40. Copy column B and after each item write the number of the word 
or phrase in A that is most closely related to it. 

Column A 

1 dust 

2 constellation 

3 young river 

4 parallel scratches on rock 

5 mineral 

6 angle of elevation of North Star 

7 igneous rock 

8 diastrophism 

9 hygrometer 

10 planet 

11 old river 

12 new moon 

Column B 


oxbow lake 

folded mountains 





color of sunset 

spring tide 


41. Make a drawing indicating the relation of the earth to the sun on 
June 21. Indicate by lines and labels the following: the earth's axis, the 
daylight circle, the equator, the Arctic Circle, the Antarctic Circle, the 
tropics of Cancer and Capricorn, the point of vertical ray (direct ray), 
the number of degrees of inclination of the axis. In your drawing indicate 
by shading the half of the earth that is out of the sunlight. 

42. (a) What is meant by dew point? (6) Describe an experiment by 
means of which dew point may be found. 


43. (a) Name five weather conditions indicated on a weather map. 
(6) Give for each of three of the conditions mentioned the instrument used 
in recording it. (c) State two ways in which weather information is im- 
portant in aviation. , 

44. Give a brief explanation of each of the following situations : (a) A 
person digging into a sand dune discovers the remains of a building. 
(6) The valley of the Hudson River continues out under the Atlantic Ocean 
for many miles, (c) A huge boulder of granite weighing about 50 tons is 
found resting on a hill of sandstone in New York State, (d) The shells of 
marine animals are found embedded in solid rock near a mountain top. 
(e) Winter in the northern hemisphere occurs when the earth is closest 
to the sun. (/) Although the British Isles are at about the same distance 
north of the equator as Labrador, they have a much milder climate. 
(g) Although the ice in a glacier moves slowly forward, sometimes the 
front of the glacier recedes. 

45. Give four evidences that a great ice sheet once covered nearly all 
of what is now New York State. Explain how the direction of the move- 
ment of this continental glacier is determined. How may the soil left be- 
hind when a glacier has receded be distinguished from ordinary residual 
soil? Why do regions that have been glaciated usually contain many more 
lakes than nonglaciated regions? 

46. Give a reason why each of the following statements is true or 
not true: (a) Coal is an igneous rock because it burns. (6) Streams 
deposit material where the current flows most swiftly, (c) A degree of 
longitude is shorter near the north pole than near the equator, (d) People 
and objects are kept on the earth by the force of the earth's magnetism. 
(e) Large caves are frequently found in limestone. (/) The tides are highest 
when the moon is in first or last quarter. 

47. The Panama Canal Zone has a wet and a dry season caused by the 
shifting of the planetary winds, (a) Why do the wind belts shift? (6) Give 
three conditions that produce the wet season at Panama, (c) Name the 
planetary winds that cause the dry season, (d) Explain why the winds 
mentioned in answer to c are drying winds, (e) Name another region that 
has a climate similar to that of Panama. 

48. Copy the statements in each of the following groups, arranging the 
events in the order of their logical sequence: 

(a) The Palisades of the Hudson 

The igneous rock solidified with columnar structure. 

Talus slopes were formed. 

The igneous rock was affected by weathering. 

Molten igneous rock flowed between sedimentary layers. 

Erosion removed the overlying rock. 


(6) The Yellowstone geysers 

The geyser shoots into the air. 

Ground water dissolves part of the surrounding rock. 

The overflow deposits geyserite (mineral matter) in the geyser 


Decreased pressure suddenly changes the heated water to steam. 
Rain water sinks into the ground and becomes heated. 

49. Copy the following, filling the blanks with the correct word or ex- 

Rain water seeps into the ground and becomes what is known as 
water. As this water moves through decaying organic matter in 
the soil, it takes into solution the gas - . In limestone regions this 
solution reacts chemically on the calcium carbonate or limestone and re- 
moves it as calcium bicarbonate. This causes cracks in the rocks to 

and leaves in the limestone. Changes of temperature or 

pressure or sudden shock cause the soluble calcium bicarbonate to give 

off and . The residue builds formations known as 

and . Two states famous for these limestone formations are 

and . 

50. Which of the following statements are true? Explain, (a) Ocean 
currents have a clockwise motion in the northern hemisphere. (6) Sea 
water freezes at the same temperature as fresh water, (c) Tidal range is the 
same for every day in the year, (d) Currents are formed in the Indian Ocean 
by the monsoon winds, (e) The horse latitudes are regions of abundant 
rainfall. (/) All places on the same parallel of latitude have the same cli- 
mate, (g) Climate in the past was always the same as climate of today. 
(h) In the temperate zones, daily weather changes are due to pressure 
changes, (i) Rising air currents during a thunderstorm are caused by local 
high pressure, (j) The rotation period of the moon is 27| days. 

51. Copy column B and at the left of each item write the number of 
the word or expression in A that is most closely related to it. 

Column A Column B 

1 halo asteroids 

2 186,000,000 rainfall 

3 time belt diameter of earth's orbit 

4 freezing point lunar eclipse 

5 windward slopes proof of earth's rotation 

6 no longitude refraction of light 

7 full moon solar time 

8 sundial 15 degrees wide 

9 star trails north and south poles 
10 planetoids Centigrade 



52. Which of the words or expressions in parentheses makes each state- 
ment true? (a) A fleecy mass of ice particles at a high altitude is called a 
(cirrus, cumulus, stratus) cloud. (6) If one is traveling westward and it is 
Tuesday just east of the international date line, it is (Monday, Tuesday, 
Wednesday) just west of the line, (c) Morainal material tends to accumu- 
late on a valley glacier chiefly (at the head, evenly over the surface, near 
the sides), (d) On March 21, an Eskimo living in northern Alaska would 
have (24 hours, 12 hours, 6 hours, hours) of sunlight, (e) A mineral 
which has a shell-like fracture is (quartz, mica, calcite). (/) A violent cir- 
cular windstorm of small area is a (chinook, tornado, anticyclone) . (g) An 
instrument used in determining latitude is the (sextant, spectroscope, 

53. Select Jive of the following names and write a statement that will 
show that you are familiar with the work of each man selected: Cham- 
berlin, Pope Gregory, Fahrenheit, Foucault, Laplace, Mercator, Byrd, 
Ferrel, Piccard. 

54. (a) What is meant by the geographic cycle of a mountain? 
(6) Copy the table below and complete it by writing the following 
characteristics in the proper columns : mesas, slightly uplifted sea bottom, 
low relief and deep residual soil, avalanches and landslides, high flat- 
topped divides, rounded peaks, ruggedly dissected surface in horizontal 
rock, monadnocks. 







55. Give a physiographic reason for each of five of the following state- 
ments: (a) Anthracite coal is found only in regions of folded and disturbed 
strata. (6) There are no valley glaciers in Australia, (c) If time belts and 
stops are disregarded, the flying time of a westbound plane across the 
United States is longer than that of an eastbound plane, (d) Earthquakes 
occur frequently in California, (e) There are fossil coral reefs in New York 
State. (/) Isotherms follow the parallels of latitude more closely over the 
ocean than over the land. 

56. Every stream is engaged in the three processes of (1) vertical 
downcutting or degrading, (2) upbuilding or aggrading, and (3) lateral 
cutting or planation. (a) Explain one effect of each of these processes on 
the stream valley. (6) State one cause of upbuilding by rivers, (c) Make a 


labeled diagram of a mature stream to show a condition under which up- 
building and cutting may take place at the same time. 

57. Three regions in the United States having swampy areas are 
(1) Minnesota, (2) Florida, and (3) the Mississippi Valley section of 
Louisiana, (a) Give the origin of the swampy regions in each of these three 
localities. (6) What is the relationship of the water table to swamps? 
(c) Explain how a lake may become a swamp, (d) What is the economic 
importance of the swamps that existed millions of years ago in Pennsyl- 
vania? < 

58. Assume that you are guiding a group of people on an automobile 
trip through your state. Mention four physiographic features, no two of 
the same origin, that you would point out to them and give a brief ex- 
planation of the formation of each feature. 

59. In each of the following groups there is one word or phrase that in- 
cludes all the others in the group. What is this word or phrase in each case? 

(a) mesa, butte, monadnock, resistant rock, tableland 
(6) monsoons, polar whirls, southeast trades, terrestrial winds, prevail- 
ing northwesterlies 

(c) soil creep, action of gravitation, air pressure, glacial movement, 

(d) intrusion, vulcanism, eruption, lava flow, igneous rock 

(e) natural levees, alluvium, fans and cones, flood plains, deltas 
(/) barrier reef, atoll, Bermuda coral, fringing reef 

(g) sandstone beds, lake deposits, strata, plateau structure, deposits 

on continental shelf 

(K) solar eclipse, moon's shadow, penumbra, new-moon phase, corona 
(i) iceberg, rock flour, neve, crevasse, glacier 
0') quartz, mica, granite, feldspar, hornblende 

60. Which of the following statements is true? (a) The moon has an 
atmosphere. (6) Earthquakes are characteristic of old mountains, (c) The 
line representing the earth's axis in December is parallel to the line repre- 
senting the earth's axis in June, (d) If the inclination of the earth's axis 
were increased to 30, winters in the northern hemisphere would be colder. 
(e) Bodies weigh slightly more in the equatorial region than at the poles. 
(/) Observation of sun spots proves that the sun rotates, (g) Places west 
of a given meridian have later time than places on the meridian, (h) A 
region of high relief should be mapped with a large contour interval. 
(i) Streams deposit material where the current flows swiftly, (j) Wind 
erosion is active in humid regions. 



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Tyrrell. Volcanoes. Butterworth. London. 1931. 
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Agassiz, Lake, 124. 

Agate, 37. 

Agonic line, 542. 

Agriculture, on flood plains, 88; on 
mountains, 222; on plains, 174, 
178, 183; and soil erosion, 56; 
and water table, 133. 

Air, 349; composition of, 351; condi- 
tioning of, 352; functions of, 352; 
height of, 349; origin of, 355; 
pressure of, 375; properties of, 
350; saturated, 405; temperature 
of, 358; weathering due to, 41. 

Alkali, 148. 

Alluvial fans, 52, 66. 

Alluvial plains, 174. 

Altitude of a star, 310. 

Aluminum production, 31. 

Amazon River, 70. 

Amethyst, 37. 

Anemometer, 400. 

Angle of repose, 46, 48. 

Animals of past ages, 262, 263, 264, 

Anthracite coal, 24. 

Anticline, 197. 

Anticyclone, 379, 391; velocity of, 

Aphelion, 299. 

Appalachian Revolution, 276. 

Aquifer, 140. 

Archeopteryx, 280. 

Archeozoic Era, 261. 

Arctic Circle, 303. 

Arid lands, 136, 462. 

Artesian wells, 140; on coastal plains, 

Asteroid, 339. 

Atlantic Coastal Plain, 521. 

Atmosphere, height of, 349; how 
heated, 361; origin of, 355. See also 

Atoll, 521. 

Aurora, 430. 

Ausable Chasm, 80. 
Avalanche, 46, 51. 

Bad Lands, 183. 

Bar, bay, 516; offshore, 514. 

Barograph, 379. 

Barometer, aneroid, 377; mercury, 

376, 377. 

Barrier reef, 521; beach, 514. 
Basalt, 5, 6, 14. 
Base level, 65, 168. 
Batholith, 253. 
Bayou Lafourche, 103, 104. 
Beach, 490; barrier, 491, 514. 
Beryl, 36. 

Bituminous coal, 24. 
Bjerknes theory of cyclones, 393. 
Blizzard, 400, 438, 464. 
Bloodstone, 37. 
Bog, floating, 159. 
Bonneville, Lake, 159, 179. 
Bortz, 35. 

Boulder, 53; perched, 122. 
Boulder Dam, 86, 162. 
Breakers, 488. 
Breeze, land, 389; mountain, 390; 

sea, 389; valley, 390. 
Brontosaurus, 278. 
Building stones, 32. 
Butte, 189. 

Calcareous tufa, 147. 

Calendar, 318; Gregorian, 319; 

Julian, 318. 
Cambrian Period, 269. 
Canyon, Grand, 195, 262. 
Carbonados, 35. 
Carbon dioxide in air, 353, 363. 
Cascades, 82. 

Cascadian Revolution, 282. 
Caverns, 41, 146. 
Cenozoic Era, 282. 
Chalcedony, 37. 
Chalcopyrite, 31. 




Chalk, 17. 

Chesapeake Bay, 522. 

Chimney rocks, 490. 

Chinook winds, 399, 463. 

Chromosphere, 335. 

Cirque, 114, 154. 

Clay in soil, 54. 

Clay products, 35. 

Cleavage, 20. 

Cliff, sea, 490. 

Climate, continental, 451; dry, 446, 
446; equatorial desert, 448; glacial, 
453; humid continental, 464; 
humid subtropical, 450, 463; ma- 
rine west-coast, 451; Mediter- 
ranean, 450; moist temperate, 450; 
monsoon savanna, 446; mountain, 
452; of the Pacific coast, 461; of 
the past, 453, 454; polar, 453; 
tropical rainy, 445; tropical sa- 
vanna, 445; warm, 453. 

Climatic, controls, 442; regions, 460; 
zones, 443. 

Cloud, 411; cirrus, 411, 412; cumulus, 
412, 413; formation, 407, 410; 
nimbus, 413; stratus, 414. 

Coal, anthracite, 24; bituminous, 
24; origin of, 164; production of, 
28; sedimentary, 19. 

Coast Range, 204, 278, 282. 

Coasts. See Shore lines 

Cold wave, 465. 

Colorado River, 86, 188, 262. 

Columbia River, 86; lava flow, 251. 

Columnar structure, 34, 199. 

Comet, 340. 

Compass, 542; earth inductor, 543; 
gyro-, 543. 

Condensation, 407. 

Conglomerate, 16. 

Continental shelf, 7, 480; emerging 
of, 513. 

Continents, origin of, 5. 

Contours, 540. 

Copper ores, Lake Superior, 268; 
production, 30. 

Coquina, 16. 

Coral reef, 520. 

Cordillera, 203. 

Corona, 335. 

Crater, 238. 

Crater Lake, 154, 244. 

Creep, 47, 504. 
Cretaceous Period, 280. 
Crevasse, 69; in glacier, 109. 
Currents, cause of, 500, 502; cir- 

cumpolar, 505; and climate, 506; 

equatorial, 505; equatorward, 503; 

longshore, 489; monsoon, 504; 

ocean, 499, 501; poleward, 502. 
Cut-bank, 90. 
Cutoff, 90. 

Cycle of stream erosion, 91. 
Cyclone, 379, 391; cellar, 396; 

movements of, 394; origin of, 393; 

velocity of, 395; weather in the, 


Danube River, 70. 

Date line, international, 317. 

Day, civil, 316; conventional, 318; 

and night, 295, 301. 
Dead reckoning, 314. 
Delaware Bay, 522. 
Delta, 69. 

Dendritic drainage, 66. 
Deposition, by glaciers, 50, 116; by 

ground water, 147-150; by rivers, 

67; in the sea, 481; by waves, 50; 

by wind, 47-50. 
Deserts, 448. 
Devonian Period, 271. 
Dew, 409; point, 406. 
Diabase, 15. 
Diamonds, 35. 
Diastrophism, 195. 
Dike, 252. 
Dinosaurs, 280. 
Diorite, 15. 
Dipper, 311. 
Distributaries, 69. 
Divide, 78. 

Doldrums, 381, 382, 388. 
Drumlin, 118. 
Dry farming, 49, 137, 462. 
Dune, sand, 47. 
Dust, in air, 355, 363; mulch, 137; 

storms, 49, 58. 

Earth, 42; axis of, 294, 297; composi- 
tion of, 11; curvature, 292; di- 
ameter, 291; equator, 294; in 
space, 290; origin of, 4; rotation, 
294; the planet, 336. 



Earthquake, 228; belts, 232; cause 
of, 232; Charleston, 228; distribu- 
tion of, 231; Ischian, 228; Messina, 
231; precautions against, 234; 
San Francisco, 229; Tokyo, 235. 

Earthshine, 325. 

Eclipse, annular, 328; of moon, 326; 
partial, 328; of sun, 326. 

Ecliptic, 4. 

Emerald, 36. 

Equinox, autumnal, 302; vernal, 301. 

Eratosthenes, 291. 

Erosion, 7; by ground water, 144- 
147; headward, 78; interval, 258; 
by river, 65, 91; by waves, 489- 

Erratic, 117. 

Esker, 118. 

Establishment of the port, 498. 

Estuary, 92, 516, 523. 

Etna, 244. 

Evaporation, 407. 

Exfoliation, 42. 

Faceted boulder, 113. 

Faceted spur, 115. 

Fall line, 84, 171. 

Falls, 82-84; Glens, 87; Niagara, 83; 

Trollhatte, 85. 
Farming, dry, 49, 137, 462. 
Fathometer, 314. 
Fault, 199; scarp, 200. 
Feldspar, 14. 
Felsite, 15. 
FerrePs Law, 387. 
Finger Lakes, 82. 
Fiord, 519. 
Fire damp, 19. 
Fissure, 199. 
Flood control, 101. 
Flood plain, 68, 87; importance of, 


Floods, 99; and soil, 100. 
Foehn, 400. 
Fog, 410. 
Fold, 197. 
Fool's gold, 32. 
Forest reserves, 224. 
Formations, 260. 
Fossil, 257. 
Foucault, 294. 
Freestone, 34. 

Frost, 406, 409; action, 40, 114; 

killing, 468. 
Fujiyama, 239. 

Gabbro, 14. 

Ganges River, 173. 

Geanticline, 198. 

Gems, 35. 

Geological history, 257. 

Geosyncline, 198. 

Geyser, 143. 

Geyserite, 149. 

Giant's Causeway, 15, 199. 

Glacial, drift, 116, 126; milk, 117; 

period, causes of, 120; striae, 113; 

till, 51, 116. 
Glaciated valley, 115. 
Glacier, 107; Alpine, 108; con- 
tinental, 108; piedmont, 108; 

transportation by, 50; valley, 108. 
Gneiss, 22. 
Gorge, 81. 
Gradient, pressure, 380; stream, 65; 

temperature, 371. 
Grand Canyon, 188, 195, 207, 262. 
Grande Coulee, 86. 
Granite, 5, 6, 12, 14; production, 33. 
Graphite, 261. 
Gravel, 53. 

Great Barrier Reef, 521. 
Great Basin, 179, 181, 207. 
Great Lakes, 179, 285; origin of, 124. 
Great Salt Lake, 179, 285. 
Greenhouse effect, 363. 
Greenwich, 312. 
Groundswell, 488. 
Ground water, 131; and agriculture, 

133; destructive work of, 144; 

deposits from, 147-149. 
Gulf Stream, 506, 507. 
Gullying, 59. 
Gypsum, 18. 

Hachures, 540. 

Hail, 415, 416. 

Halo, 423. 

Hanging valley, 82. 

Harbor, 523; artificial, 532; crater, 

530; delta, 530; fiord, 530; good, 

524; island, 531; lagoon, 531; 

New York, 526; submerged valley, 




Hardness, table of, 35. 

Hard water, 41. 

Hawaiian Islands, 248. 

Heat, conduction of, 360; convection 

of, 360; equator, 368; radiation 

of, 360; transfer of, 360. 
Hematite, 18, 30. 
Henry Mountains, 206. 
High, a, 391. 

History of the earth, 257. 
Hood, Mt., 250. 
Hornblende schist, 23. 
Horse latitudes, 381, 382, 386, 388. 
Hot wave, 398, 465. 
Hudson River, 93, 124, 520, 523. 
Humidity, 405, 471; absolute, 405; 

relative, 405, 408, 409. 
Humus, 44, 53, 55. 
Hurricane, 395. 
Hygrometer, 408. 

Ice Age, 120. 

Iceberg, 111. 

Ice, sheet, 416. 

Igneous rocks,. 13-16; texture, 253. 

Insolation, 358, 366, 367; absorption 

by air, 367. 

Interglacial Period, 121. 
International date line, 317. 
Intrusions, igneous, 252. 
Iron ores, 18, 268. 
Iron, production of, 30. 
Irrigation, 134, 222. 
Isobar, 379, 380. 
Isogonic lines, 542. 
Isostasy, 7, 8, 196. 
Isotherm, 371, 466. 
Isothermal Layer, 362. 

Jade, 37. 

Jetties, 69. 

Joint, 199. 

Jupiter, 337. 

Jupiter Serapis, Temple of, 193. 

Jura Mountains, 208-209. 

Jurassic Period, 278. 

Kame, 118. 

Kaolin, 35. 

Karst topography, 146. 

Katmai, Mt., 249. 

Kettle, 119, 155. 

Knob and basin topography, 119. 
Krakatoa, 247. 

Laccolith, 252. 

Lagoon, 156, 491, 514, 517. 

Lake, Agassiz, 179; Algonquin, 125; 

artificial, 161; Bonneville, 179, 

285; Champlain, 158; delta, 156; 

glacial, 154; Great Salt, 179; 

in limestone, 158; kettle, 154; 

Nipissing, 125; Ontario, 180; ox- 
bow, 155; playa, 158; Pontchar- 

train, 157. 
Lakes, alkaline, 158; destruction of, 

159; Finger, 126; Great, 179, 285; 

origin of, 153. 
Landslide, 46. 
Lassen Peak, 249. 
Latitude, finding, 310, 312; parallels 

of, 309. 
Lava, Columbia River, 251; surface 

flow, 252. 

Lead, production, 31. 
Levees, artificial, 69; natural, 69. 
Light, white, dispersion of, 419, 422; 

refraction of, 420. 
Lightning, 425; dangers from, 426; 

protection from, 428; rod, 429. 
Light year, 333. 
Limestone, 17; production, 33. 
Limonite, 18. 
Lipalian Interval, 269. 
Loam, clay, 56. 
Locks, 85. 
Loess, 50. 
Long waves, 234. 
Longitude, finding, 313; meridians 

of, 309, 312. 
Looming, 421. 
Low, a, 391. See Cyclone 

Magma, 15. 

Magnetic declination, 542. 

Magnetism, terrestrial, 542. 

Magnetite, 30. 

Maine, coast of, 523. 

Mammals, age of, 282. 

Mantle, rock, 42; transportation of, 

Maps, 535; contour, 540; hachure, 

540; topographic, 541; weather, 




Marble, 21, 24, 33. 

Marine zones, 480. 

Mars, 336. 

Maturity of stream, 65. 

Mauna Loa, 239. 

Meander, 89, 90; entrenched, 94; 

straightening of a, 103. 
Mercury, 336. 
Meridian, ante, 316; post, 316; 

prime, 312. 
Mesa, 189. 
Mesozoic Era, 276. 
Metals in the earth, 11, 29. 
Metamorphic rocks, 19; kinds of, 22. 
Meteors, 331, 341. 
Mica, 14. 
Mica schist, 23. 
Midnight sun, 305. 
Mineral, 12; production of, 27; 

veins, 149. 
Mining, 219. 
Miocene Period, 284. 
Mirage, 422. 
Mississippi River, 70, 77, 101, 103, 

166, 173. 

Mississippian Period, 271. 
Moisture in air, 405; condensation, 

407; distribution, 409. 
Monadnock, 168. 
Monsoons, 390. 
Moon, crescent, 324; eclipse of, 325, 

326; full, 325; motion of, 322; 

new, 324; phases of, 324; surface 

of, 323; waxing and waning, 325. 
Moraine, 111; ground, 112; kettle, 

123; lateral, 112; terminal, 112, 

Mountains, and agriculture, 222; and 

history, 215; Appalachian, 206, 

209, 282; as barriers, 216; block, 

200; Catskill, 206; climate of, 

214; Coast Range, 204, 278, 282; 

dome, 206; Elk, 206; folded, 208; 

Henry, 206; industries in, 219; 

Jura, 208; mature, 213; old, 213; 

Rocky, 212, 282; Sierra Nevada, 

204, 278, 282; synclinal, 210, 211; 

young, 212. 

Mountain ranges, origin of, 8. 
Movements of earth's crust, 192. 
Mudstone, 17. 
Muscle Shoals, 86. 

Nadir, 302. 
Nares Deep, 479. 
Natural bridge, 146. 
Navigation, 314. 
Nebula, spiral, 333, 344. 
Nebular hypothesis, 341. 
Neptune, 338. 
Nevadian disturbance, 278. 
Neve, 108. 
New York Bay, 522. 
New York Harbor, 526. 
Niagara Falls, 83. 
Nile River, 70, 89. 
Nitrogen in the air, 354. 
North Pole, celestial, 296. 
North Star, 296, 310. 
Norther, 400. 
Northern lights, 430. 

Obsidian, 16. 

Ocean, 477; currents, 499; deeps, 

479; origin of, 6. 
Ohio River, 75, 99. 
Old age of stream, 65. 
Oligocene Period, 284. 
Ooze, 482. 
Opal, 37. 

Ordovician Period, 269. 
Ore, definition, 29; origin of, 29; 

production of, 29-32. 
Oxbow lake, 90. 
Oxygen, in air, 353. 
Ozone, 363. 

Paleozoic Era, 269. 

Palisade disturbance, 278. 

Palisades, 15, 199, 252. 

Panama Canal locks, 85. 

Parallels, 309. 

Passaic Falls, 82. 

Peat, formation, 164. 

Pegmatite, 15. 

Pelee, Mt., 246. 

Peneplains, 167. 

Pennsylvanian Period, 271. 

Perihelion, 299. 

Period, 261. 

Permian Period, 276; salt deposits, 


Petrified moss, 147. 
Petrified wood, 149. 
Petroleum, production, 28. 



Physiographic regions, 181. 

Physiography, 3. 

Piedmont plateau, 172, 188. 

Piracy, stream, 79. 

Plain, flood, 68, 87; lacustrine, 178; 

lake, 178, 180; outwash, 117, 178; 

piedmont, 174. 
Plains, Great, 180. 
Planet, 4, 330, 338. 
Planetesimal hypothesis, 343. 
Planetoids, 331, 339. 
Plateau, 187; Appalachian, 189; 

climate of, 189; Colorado, 187; 

dissected, 206; erosion of, 188; 

Tibetan, 188. 
Playa, 158. 

Pleistocene Period, 285. 
Pliocene Period, 285. 
Pluto, 331, 338. 
Polaris, 310, 313. 
Poles, 291, 304, 309, 310. 
Pompeii, 246. 
Pontchartrain, Lake, 157. 
Porphyry, 15. 
Potholes, 84, 115. 
Potomac River, 93. 
Precious stones, 35. 
Pressure belts, 382; barometric, 


Pressure gradient, 380. 
Prevailing westerlies, 386, 387, 388, 

Projection, Aitoff's, 539; map, 535; 

Mercator's, 536; Mollweide, 537. 
Proterozoic Era, 266. 
Psychrometer, 408. 
Pterodactyls, 278. 
Pumice, 241. 

Quartz, 11, 37. 
Quartzite, 20, 22. 

Radiation, solar, 365. 

Radio position finder, 314. 

Radioactivity, 253, 254. 

Rain, 414. 

Rainbow, 423. 

Rainfall, 63; map of, 447; in U. S., 


Rainier, Mt., 250. 
Rapids, 81, 84, 87, 91. 
Red River, 179. 

Reforestation, 104. 

Refraction, 420-422. 

Regelation, 108. 

Rejuvenation of region, 92. 

Residual mantle, 44. 

Retarding basins, 104. 

Revolution of earth about sun, 297. 

Rhine River, 89, 173, 174. 

Rhinestone, 37. 

Rhone River, 70. 

Rio Grande River, 77. 

River, antecedent, 92; rejuvenated, 
92; terraces, 94. 

Rivers, Amazon, 70; Colorado, 65, 
86, 188; Columbia, 86; Danube, 
70; Delaware, 93; Ganges, 173; 
Green, 93; Hudson, 93, 520, 523; 
Hwang Ho, 173; Mississippi, 70, 
77, 101, 103, 156, 173; Missouri, 
77; Nile, 70, 89; Ohio, 75, 99; 
Po, 173; Potomac, 93; Red, 179; 
Rhine, 89, 173, 174; Rhone, 70; 
Rio Grande, 77; St. Lawrence, 70; 
Susquehanna, 70, 86, 93; Tennes- 
see, 86; Yellow, 89. 

Rivers, and commerce, 74; as high- 
ways, 73; in maturity, 87; as 
national boundaries, 77; and water 
supply, 76; in youth, 80. 

Roche moutonne, 113-114. 

Rock glacier, 47. 

Rock mantle, 42, 44; movements of, 

Rocking stone, 122. 

Rocks, 12; classification of, 23; 
igneous, 12; metamorphic, 12; 
sedimentary, 12, 16. 

Rocky Mountains, origin of, 282. 

Roosevelt Dam, 135. 

Rubies, 36. 

Running water, 63; deposition, 66, 
68; transportation by, 63; velocity, 

Salt, 18; lakes, 158. 

San Andreas Rift, 230, 282. 

San Francisco, Mt., 250. 

Sand dune, 47. 

Sandstone, 16; production, 34. 

Sandy Hook, 516. 

Sandy loam, 56. 

Sapphire, 36. 



Sargasso sea, 508. 

Satellite, 339. 

Saturn, 337. 

Savanna climate, 445. 

Schist, 22. 

Schistosity, 20. 

Scoria, 241. 

Sea, 476; basins, 478; cave, 193, 
491; cliff, 490; divisions of, 477; 
ice in the, 483; movements of, 487; 
temperature of, 482; water, 480. 

Seasons, 297. 

Sedimentary rocks, 16; deposits, 481. 

Seismic sea waves, 233. 

Seismograph, 233. 

Sextant, 312. 

Shale, 16. 

Shasta, Mt., 244, 251. 

Sheet wash, 59. 

Shore, fault-plane, 517; mountain- 
ous, 518. 

Shore lines, 513; drowned, 517; 
emergent, 513; fiord, 519; ir- 
regular, 515; regular, 514. 

Sierra Nevada Mts., 278. 

Sill, 252. 

Silt, 53. 

Silurian Period, 271. 

Simoom, 398. 

Sinkhole, 145. 

Sirocco, 398. 

Sky, color of, 420. 

Slack water, 493. 

Slate, 20, 22; production, 34. 

Sleet, 415. 

Slip-off slope, 90. 

Smudges, 364, 410. 

Snow, 414; line, 107; in U. S., 470. 

Soil, 52; black, 44; classification, 55; 
fertility, 54; mineral, 55; red color 
of, 55; residual, 53; sand in, 53. 

Soil erosion, 56; causes of, 57; con- 
trol of, 60; and flood control, 60. 

Solar prominences, 335. 

Solar system, 4, 330; size of, 331. 

Solstice, summer, 302; winter, 302 

Solution of rock, 63, 144-146. 

Soo Canal, 76. 

Sounding, echo, 314. 

Spectrum colors, 419, 422. 

Spillways, 103. 

Spit, 492, 516, 517. 

Springs, 141; fissure, 142; hot, 143; 

mineral, 142. 

St. Lawrence River, 70, 124-125. 
St. Mary's Canal, 85. 
Stacks,- 490. 
Stalactites, 147. 
Stalagmites, 147. 
Standard time, 315-316. 
Star trails, 2%. 
Steppes, 446, 449. 
Stone quarrying, 33. 
Storms, 391-395. 
Strata, 17. 
Stratification, 67. 
Stratosphere, 358, 361. 
Stream capture, 79. 
Stream deposits, 68. 
Stream, erosion, cycle, 91; effect of 

depression, 92; effect of elevation, 

92. See also River 
Streams, transportation by, 51. 
Striae, glacial, 113. 
Stromboli, 246. 
Submerged coast, 516, 521. 
Sulphur, production, 32. 
Sun, 333; elements in, 334; heat of, 


Sunrise, 302, 304. 
Sunset, 302, 304. 
Sunspots, 334, 363, 365. 
Superior, Lake, 124. 
Surf, 488. 

Susquehanna River, 70, 86, 93. 
Swamps, 90, 163. 
Syenite, 15. 
Syncline, 197. 

Talc schist, 23. 

Talus, 41. 

Tarn, 114, 154. 

Taylor, Mt., 251. 

Temperature gradient, 371. 

Terrace, wave-built, 490. 

Terracing, 223. 

Thermograph, 364. 

Thermometer, 364; centigrade, 365; 

Fahrenheit, 365. 
Thunder, 429. 
Thunderhead, 411. 
Thunderstorm, 395, 396. 
Tidal bore, 494. 
Tidal disruption theory, 345. 



Tidal races, 493. 

Tides, cause of, 495; ebb, 493; effects 
of, 499; flood, 493; neap, 498; 
range of, 493; in rivers, 494; run- 
ning of, 493; solar and lunar, 498; 
spring, 498. 

Till, glacial, 116. 

Till plains, 176. 

Time, daylight-saving, 315; geo- 
logic, 258; local, 314; solar, 315; 
standard, 315; sun, 314. 

Topaz, 36. 

Topography, karst, 146. 

Tornado, 395, 396, 397. 

Tourmaline, 36. 

Trap, 15. 

Travertine, 147. 

Tremor, primary, 234; secondary, 

Triassic Period, 276. 

Trilobite, 269, 270. 

Tropic of Cancer, 303. 

Tropic of Capricorn, 303. 

Tropics, 434. 

Troposphere, 362. 

Tsunami, 233. 

Turquoise, 37. 

Typhoon, 395. 

Ultraviolet rays, 367. 
Unconformity, 259. 
Undertow, 488. 
Uranus, 338. 

Vadose water, 131. 

Valley, glacier, 108, 519; hanging, 
116; river, 78; train, 117; U-- 
shaped, 115; V-shaped, 91. 

Varves, 117, 259. 

Veins of mineral, 149. 

Velocity of river, 67, 68. 

Venus, 336. 

Vesuvius, 240; eruption of, 245. 

Volcanic cones, 205; life history of, 
242; young, 243. 

Volcanoes, 238, 266; active, 249; 
craters of, 238; distribution of, 242; 

explosive, 238; extinct, 250; Ha- 
waiian, 205; origin of, 253; quiet, 

Warp, broad, 198. 

Watchung surface flow, 252. 

Water, hard, 145. 

Waterfalls, 82; recession of, 84. 

Water gap, 93. 

Water power, 85, 220. 

Water table, 131; perched, 133. 

Water vapor in air, 354. 

Wave-built terrace, 490. 

Wave-cut bench, 490. 

Wave erosion, 489. 

Waves, 487; pounding of, 488; 
seismic, 233. 

Weather, 433; in the anticyclone, 
438; in the cyclone, 437; in the 
tropics, 434; maps, 392, 440; out- 
side the tropics, 435; prediction, 
436, 441; signs and proverbs, 441. 

Weather Bureau, 440. 

Weathering, 40; chemical, 40; ex- 
foliation, 42; frost action, 40; 
gravity, 46; mechanical, 40; oxy- 
gen, 41; plants and animals, 43; 
temperature, 42; water, 41; wind, 

Wells, 137; artesian, 140; construc- 
tion, 139; location, 138. 

Whetstone, 34. 

Wilson Dam, 105. 

Wind abrasion, 47. 

Wind belts, 387; shifting of, 389. 

Wind erosion, 47. 

Wind gap, 93. 

Winds, 384-401, 471; cause of, 384; 
continental, 390; cyclonic, 391, 
395; deflection of, 385; monsoon, 
390; shifting of, 398, 399; special, 
399; velocity, 400. 

Youth of stream, 65. 

Zenith, 302. 

Zones, climatic, 443, 444.