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Candidate Ph. D., University of California. 


Issued February 4, 1897. 








P. 42, line ii from the bottom, for "of," read "on." 

P. 53, line 9, for "Plate III, fig. i," read "Plate II, fig. i." 

P. 69, line 6 from the bottom, for "Plate III, fig. i," read 

"Plate III, fig. 2." 

P. 71, line 10, for "steam," read "stream." 
P. 54, line 10. The statement that certain areas appear on 
the map without definite boundaries is not in accord- 
ance with the fact. Owing to a misunderstanding on 
the part of the lithographer, the areas in question are 
delimited by the usual dotted line. 





VOL. I, No. i 





Candidate Ph. D., University of California. 


Issued February 4, 1897. 






Candidate Ph. Z>., University of California. 









Main Ridge 4 

Types of Topography 5 

Slope of Summits 6 

Two Types of Drainage 7 


Echo Lake 8 

Sea Cliffs 9 

Bays 10 

Beaches 10 

Terraces n 



1. DIORITE 14 

Macroscopic Characters 14 

Microscopic Characters 15 


Occurrence 19 

Character 19 

Macroscopic Characters 20 

Microscopic Characters 20 

Analysis 25 

Inliers of Basement Rocks 25 

Porphyrite Dykes 26 


Occtirrence 28 

Macroscopic Characters 28 

Microscopic Characters , 29 


Occurrence 30 

Macroscopic Characters 31 

Microscopic Characters 32 

Glassy Fades 35 

Basaltic Fades 37 

Analysis 41 

[ i | January 12, 1897. 







2. TUFF 43 

3. SHALE 43 

Microscopic Characters 44 

Character of the Organic Remains 45 

Chemical Characters 48 

Origin of the Shale 49 

Analysis of Limestone 50 





Macroscopic Characters 55 

Microscopic Characters 57 











THE existing literature bearing upon the geology of Santa 
Catalina is very limited, consisting of a short note in Whit- 
ney's Geology, 1 a brief report 2 and other scattered notes in 
the various Annual Reports of the State Mining Bureau, 
and a recent account of the topography of the island, by 
Prof. Lawson. 3 The report of the State Mining Bureau 
is not only superficial, but very inaccurate. In Whitney's 
report the absence of terraces is noted, contrasting with the 
neighboring islands, and the suggestion is made that this 
island may be sinking. Both of these points are elaborated 

1 Geol. Surv. of Cal., Geol., Vol. I, pp. 182-186. 
2 Tenth An. Rept., State Mineralogist, pp. 277-281. 

8 "The Post-Pliocene Diastrophism of the Coast of Southern California," by Andrew 
C. Lawson. Bull. Dept. Geol., Univ. Cal., Vol. I, No. 4, pp. 135-139' 


by Prof. Lawson, who further calls attention to the older 
topography of this island. 


Santa Catalina Island, one of the group known as the 
Channel Islands, off the coast of southern California, lies 
about 20 miles south of San Pedro Hill, the nearest point on 
the mainland. At about the same distance south of Santa 
Catalina lies the island of San Clemente, the three eleva- 
tions being nearly in a straight line. 

The general trend of the island is northwest by west. 
Its length is approximately twenty-one miles, with an aver- 
age width of three miles, varying from half a mile at the 
isthmus to about eight miles in the widest part. The pre- 
vailing winds are from west to southwest, and the waves 
exert their greatest force on the southwest face of the island. 
They are, however, by no means inactive on the landward 
side, as is shown by the rapidly retreating shore-line. 

The only settlements on the island are the summer resort 
at Avalon, and a small community at the isthmus. Besides 
these, a few solitary houses are located at different points on 
the coast. The island was once occupied by Indians, and evi- 
dences of their camps occur frequently in the form of shell 
fragments, rounded stone implements, and earth blackened 
by the camp fires. Owing to its ruggedness and the scarcity 
of water, the island is habitable in only a few places. There 
are half a dozen or more springs and creeks which do not 
dry up during the summer, and a few wells supply the other 
points. All the water is decidedly alkaline. 

The vegetation consists chiefly of herbage and shrub- 
bery or underbrush, cactus forming an important part. The 
larger trees, except for a few dwarf oaks, are confined to 
the bottoms of the canons. The summits, in general, are 
bare of everything except grass and cactus, but the majority 
of the slopes are thickly covered with an often impenetrable 
growth of scrub-oak, greasewood (Adenostoma fascicula- 
tumf), and elder, intermingled with cactus. It is note- 


worthy that, in spite of the oftentimes luxuriant vegetation, 
the soil-covering is generally very thin, and the underlying 
formations are constantly exposed. 



Main Ridge. The island is traversed from end to end by 
a single main ridge, with branch ridges running out on either 

FIGURE i The Isthmus, looking south. 

side. Beginning about a mile from the southeastern extrem- 
ity of the island, this ridge makes a bold sweep around the 
head of Avalon Canon to a point nearly west of Avalon. 
There it makes an abrupt turn, almost at right angles, and 
then follows very nearly the line of the northern coast, at an 
average distance of about a mile from the shore, till it reaches 
the isthmus. (See fig. i.) This is alow divide, in the form 
of a saddle, with very gentle slopes. It has a length, 
between the bounding hills, of less than a quarter of a mile, 


and its greatest elevation is about twenty feet. At either 
end of the isthmus the hills rise very abruptly to the main 
ridge, which is here from 800 to 900 feet in height. West 
of this point the ridge has two divisions, which unite, less 
than a mile and a half beyond, to form again a single ridge, 
continuing to the end of the island. On this end the ridge 
lies nearer the south than the north shore. One noticeable 
feature of this main watershed is the general uniformity of 
its height. For the greater part of its length the variations 
in altitude are not more than two or three hundred feet, the 
average elevation being about 1,400 feet. The two greatest 
elevations are near the center of the island, the peak known 
as " Orizaba" (or " Brush Mountain "), marked 2,109 feet 
on the map, and " Black Jack," the peak about a mile to 
the northeast of this, about a hundred feet lower. 

Types of Topography. The general character of the 
topography is very bold and rugged, and shows an advanced 
stage of development. A general view of the island from 
almost any point gives an impression of a close succession 
of sharp, steep ridges and V-shaped canons. One of the 
most marked examples of this effect is in the slopes of 
Avalon Canon, particularly on the west side, when seen 
from the opposite summits. 

Viewed in detail, the island shows two prevailing forms 
of topographic relief: (i) the sharp ridges and V-shaped 
canons just referred to, and (2) the rounded and level forms 
belonging to an older topography. The slope of the canon 
walls, in the first type, is usually steep, occasionally having 
an angle of 40 or over. The first form is the prevailing 
one in most parts of the island, masking the remnants of the 

The second type of topography is strongly contrasted 
with the first. It is found in the higher parts of the island, 
best developed in the eastern end. It is shown in the level 
character of the main ridge, and of several of the minor 
ridges which approximate it in altitude. These latter are 
(i) the principal ridges between Middle Ranch Canon and 


the main ridge bounding Avalon Canon on the west; (2) 
the ridge connecting the main ridge with the point north of 
Whitley's Cove; and (3) a portion of each of the ridges 
running from the main ridge into the Little Harbor region 
(which comprises the semicircular area within a general ra- 
dius of about three miles from Little Harbor). 

In the lower portions of the Little Harbor region this sec- 
ond type of topography again appears. Within this area 
the tributary ridges, radiating from a central point not far 
from Little Harbor, rise to the higher slopes by a long, 
moderate incline. Beginning at the shore-line, with a cliff 
of from 200 to 300 feet, the rise above this is very gradual, till, 
at an average distance of a mile and a half from the water, a 
height of about 600 or 700 feet is reached. Beyond this 
the grade increases, and an altitude equal to that of the main 
ridge is soon reached, usually some little distance from the 
main ridge itself. Standing on the lower and more level 
portion of this area, and looking either toward the isthmus 
or in the opposite direction, one sees a great amphitheater, 
the distant ridges rising one above another, like gigantic 
tiers of seats, up to the main ridge. Were it not for the 
recent stream erosion we should thus have in the immediate 
neighborhood of Little Harbor an almost even surface, with 
a gentle seaward slope. The present drainage, however, 
has dissected this surface, cutting channels some of which, 
in their lower stretches, have a width of 100 yards or more, 
and a depth of perhaps 200 feet. In places the streams 
have made considerable deposits, and at a number of points 
these have been cut through, in very recent times, to a max- 
imum depth of about twenty-five feet, at some distance from 
the shore. 

Slope of Summits. It has been seen that the main ridge 
and certain of the branch ridges are, in a general way, level 
in the direction of the length of the island. In those por- 
tions of the main ridge on either side of Avalon Canon 
which are oblique to the trend of the island, the generally 
level summits are seen to slope at an angle of a little over 


one degree toward the northern shore. (See figs. 2 and 
3.) In fig. 2, Black Jack and Orizaba and a portion of 
the ridge between Silver and Middle RancrT Canons are 
seen above the main ridge. At the " west end " (that por- 



FIGURE 2 Outline of the summit of the main ridge west of Avalon Canon, 
as seen from the summit of the ridge on the opposite side of the canon. 

tion of the island west of the isthmus), the more northerly of 
the two branches of the main ridge has an average height 
throughout its length, about 200 feet lower than the other. 
Thus it appears that in transverse section the island shows 
a general slope toward the mainland. 



FIGURE 3 Outline of the summit of the main ridge south of Avalon Canon, 
as seen from the summit of the ridge on the opposite side of the canon. 

Two Types of Drainage. The principal stream canons 
running from the main watershed are of two types, which 
may be readily distinguished on the map. Those canons 
which have their mouths on the northern or landward coast 


are broad and open stream valleys, while those running 
down to the opposite shore have a long and trough-like 
character which they preserve to the shore-line. These 
two types are quite pronounced over all the southeastern 
division of the island, while at the isthmus the two harbors 
which are merely submerged stream valleys still show 
the same contrast. In the western end this characteristic, 
though still evident, is not so marked. The narrow, trough- 
like canons are occasionally somewhat broader in their upper 
portions than near their mouths, where they are frequently 
mere rocky gorges. The most pronounced example of the 
narrow type is Silver Canon, whose walls near the mouth 
rise to a height of over 1,000 feet, while the distance between 
them at the base is in places not more than from twenty-five 
to a hundred feet. The length of this canon is about three 
miles. Avalon Canon, a good example of the other type, 
has a length of about two miles, with a mean width, from 
watershed to watershed, of somewhat more than that. 
From the main ridge on either side a great number of 
rather short and steep V-shaped canons are tributary to 
the main valley, these stream beds making the descent of 
1,200 or 1,400 feet within an average distance of about a 

All the forms of topography thus far described are largely 
independent of the material from which they are carved; 
that is, variation in the character of the rocks has but little 
connection with variation in topographic form. 


Echo Lake. There is one small lake on the island, sit- 
uated about a mile to the northeast of Black Jack, at an 
altitude of about 1,300 feet. This belongs to the class of 
ephemeral lakes. Visiting it two summers in succession, at 
the same season, the writer found it, the first time, a shal- 
low pond about 100 yards long, while the next year it was 
entirely dry. It is a small drainage lake, without outlet, 
probably shut in by faulting. 


Sea Cliffs. Except for the openings formed by the canon 
mouths, cliffs surround the island on all sides, running from 
one or two hundred feet to 1,400 feet or more in height. 
The boldest and highest cliffs are found at the west end, and 
between Silver Canon and the southeastern extremity of the 
island. The highest of all are just to the east of Silver 
Canon, where the waves have cut across the end of a minor 
ridge whose altitude equals that of the main ridge. These 
cliffs, although furnishing excellent geological sections, are 
wholly inaccessible at nearly all points, owing to their height, 
the angle at which they meet the water, and the absence of 

The cliffs are rapidly receding, in many cases more rapidly 
than the streams which trench their surfaces can cut down 
their channels. This is shown by the V-shaped openings 
on the face of the cliff, from 50 to 200 feet or more above 
the water. Such are the mouths of the canons draining the 
southern slopes of the main ridge at the head of Avalon 
Canon. These open on the southern coast, about two miles 
to the east of the entrance to Silver Canon. The rapidity 
of the cliff-cutting here will appear the more remarkable 
when it is known that these streams, though draining com- 
paratively small areas, are torrential in character. It must, 
however, be remembered that they are active only during 
the rainy season. 1 

In addition to these larger V-shaped openings, several 
smaller ones were seen along the higher parts of the cliff, 
less than a mile to the east of the entrance to Silver Canon. 
These, from the water side, present the appearance of a 
stream draining outward over the face of the cliff. From 
above, however, it is seen that the drainage is inland, toward 
Silver Canon. This phenomenon is due to the cutting back 

*A phenomenon similar to that above described has been observed by the writer at 
several points along the California coast between Port Harford and Santa Monica. Here 
the recent streams have carved narrow channels in the surface of the lowest terraces 
which border the shore, and have formed clear-cut V's on their upper edge. The cause 
here (unlike that in the case of Santa Catalina) is, no doubt, that an insufficient time 
has elapsed, since the elevation of the coast, for the streams to deepen their channels 


of the watershed so rapidly that the drainage has not had 
time to adjust itself to the changed conditions. 

Bays. The coast of the island, particularly on the land- 
ward side, is indented with numerous bays. On the north side, 
partly on account of less active cutting along the coast, and 
partly on account of the more open canons whose submerg- 
ence has produced the bays, they are wider, and generally 
furnish safe landing places. On the other side of the island, 
although there are numerous recesses in the shore-line (par- 
ticularly of the west end), these openings are generally 
surrounded by high cliffs, and there are only two bays, 
Catalina Harbor at the isthmus, and Little Harbor. 

Beaches. Several canons on the south side of the island, 
while not forming bays, have beaches at their mouths. In 
many cases, both here and on the northern coast, the beaches 
have been built up by wave action so as to form along the 
shore a barrier from five to ten feet higher than the area just 
behind. The beaches, in general, consist of coarse, well 
rounded, and flattened shingle, though one or two excep- 
tions were seen where the beach was largely composed of a 
rather fine sand. Apart from the beaches which mark the 
entrance to the larger canons, there are a few very narrow 
beaches for short stretches at the base of the cliffs, only 
on the landward side. These are in general accessible only 
at low water. 

The beaches as a rule are curved in outline, concave to- 
ward the ocean. A marked exception to this is seen in the 
projecting, tongue-like, Pebbly Beach. This has been built 
up by the opposing action of two series of waves, which, 
coming from either direction along the coast, meet at this 
point. Not only does the beach exhibit the barrier-like 
character mentioned above, but its outer surface shows a 
series of narrow terraces formed by the waves. As many 
as six were seen at one point. 

Another form is shown in the hook which marks the en- 
trance to Catalina Harbor, and is known as Ballast Point. 
This is built of coarse shingle, some of the material compos- 


ing it having a diameter of about a foot. High winds blow 
daily through the narrow pass at the isthmus, causing a strong 
inward current, which is gradually bringing about a shoaling 
of the harbor. Thus here, as at Pebbly Beach, the accu- 
mulation of shore-drift, through the action of waves and 
currents, has more than kept pace with the sinking of the 

Terraces. The pronounced contrast which Santa Cata- 
lina presents in its topography, not only to the adjacent land 
areas, but to the greater part of the coast of California, has 
already been shown by Prof. Lawson. 1 The most striking 
difference is in the marked absence, on this island, of the 
terraces which are so clear-cut and pronounced on the slopes 
of San Pedro Hill and San Clemente. With but two excep- 
tions, Santa Catalina is devoid of any evident terracing from 
one end to the other. The terrace-like character of the lower 
levels of the Little Harbor region (already described) forms 
one of these exceptions. That this is, in part, at least, of 
the nature of a true terrace, is shown by the nearly level 
character of the various ridges in their lower parts, their 
gentle seaward slope, the change in grade at the rear, at an 
altitude of 600 or 700 feet, the planing off of the upturned 
beds of the basement series, with rolled pebbles scattered 
over the lower slopes of the andesite, besides more or less 
sandstone and conglomerate on these slopes bordering Mid- 
dle Ranch Canon. All these point to a time when this re- 
gion contained a bay, into an arm of which a stream, doubt- 
less an older form of that which now drains Middle Ranch 
Canon, brought the deposits just mentioned (shown on the 
map). It is possible that there is, besides this, a series of 
such terraces within this area. If so they are not strongly 
marked, and the fact could only be established by a more 
detailed observation than the writer had time for. Ter- 
racing similar to that found here must at one time have ex- 

1 " The Post-Pliocene Diastrophism of the Coast of Southern California," by Andrew 
C. I,awson. Bull. Dept. Geol., Univ. Cal., Vol. I, No. 4, pp. 135-139. 



tended along the cliffs bordering the island, 1 but it has been 
since removed by a prolonged period of active cliff erosion. 
That the evidence is preserved here is due to the fact that 
the terracing extended so far inland. This belongs to an 
earlier period than the terraces of the main coast and of San 

The other terraced structure occurs in the canon back of 

FIGURE 4 Dissected alluvial fan, southeast side of Avalon Canon. 

Avalon (see fig. 4) , about half a mile from the shore-line, and 
is seen on both sides of the canon which is about a quarter 
of a mile wide at this point as a broad platform extending 
up some distance into the canon-like opening on either side 
of the main valley. Its front edge has a gentle seaward 
slope, while from front to rear it rises gradually toward the 
hills. A sharp ascent of about' forty or fifty feet marks the 

1 Rolled pebbles were found scattered over a small area near the southeastern end 
of the island, at an altitude of 1,000 feet; also on the main ridge south of Avalon Canon, 
at about 1,400 feet; but the remains of an Indian camp within a hundred feet, in each 
case, made the evidence doubtful. In neither case, however, were any pebbles found 
among the remains marking the camp. 


front of the platforms on either side of the canon. Streams 
have cut into the surface and along the sides, forming sev- 
eral comparatively broad watercourses. Some of these 
streams have not yet reached the level of the main canon 
where they debouch upon it, and have formed small, rather 
low and broad alluvial fans beyond their mouths. 

The material composing the platforms consists of both 
rounded and angular fragments. From the form of the 
structures and the form and arrangement of their material, 
it is evident that we have here, not a stream terrace, as its 
appearance might at first indicate, but undoubted alluvial 
fans. 1 These have been dissected and cut away in recent 
geological times, owing to the drowning of the stream valley 
at Avalon, with a consequent shortening of the stream 
courses, and a deepening of the channels. 


The basement series of Santa Catalina consists of crys- 
talline metamorphic rocks, principally quartzite. This series, 
with the hornblendic rocks, the talc-schist and the serpentine, 
covers in a general way the whole western half of the island. 
Besides the main occurrence, there are patches of these 
rocks along the main ridge to the west of Avalon Canon. 
The basement rocks are cut by occasional dikes, which are 
principally at the west end, and have a general northeasterly 

The eastern end of the island is occupied by diorite and 
porphyrite. Bordering this area on the north, and of later 
age, is an area consisting of numerous flows of andesite, of 
which there are several other smaller occurrences besides. 
A particularly interesting area of these rocks is found to the 
east of Isthmus Cove, where, interbedded with the volcanics, 
is seen a band of tuff and diatomaceous earth. 

The lower slopes of the andesites in the Little Harbor 

Alluvial fans are by no means uncommon on the island, but no other case presents 
any similar terracing. 


region are covered with rolled pebbles, with several patches 
of sedimentary deposits. In the eastern portion of the 
Little Harbor region is a small area of rhyolite, which was 
not found elsewhere. A narrow strip of quartzite breccia 
occurs at the southeastern extremity of the island. 


There are three observed occurrences of the diorite : one 
along the cliff bordering the shore just to the north of 
Avalon, the second near the head of the canon back of 
Pebbly Beach, and the third in the lower portion of Silver 
Canon. These are apparently dikes of considerable width. 

Macroscopic Characters. The diorite is coarse-grained, 
and of a light grayish color, more or less mottled. The 
specific gravity of a specimen from Silver Canon was found 
to be 2.777. In the coarser-grained specimens the separate 
minerals may easily be seen without recourse to a lens. 
With the lens the rock is seen to be composed of a ferro- 
magnesian mineral, feldspar and a varying amount of 

The feldspars range from a somewhat glassy condition to 
one in which they are whitish and more or less opaque. 
They constitute, in general, the principal mineral of the 
diorite. The ferromagnesian mineral is hornblende, dark 
green in color, and altered in part to chlorite. This mineral 
varies in amount from a little less than one-half to perhaps 
one-fifth of the surface area. Besides the hornblende an 
occasional leaflet of biotite was seen in the specimens from 
near Avalon, and also in one or two from Silver Canon. 
The quartz usually occurs in small areas scattered through- 
out the mass of the rock. In addition to these, magnetite 
is apparent in nearly all the hand-specimens of the coarser- 
grained varieties, being rather conspicuous in one specimen 
from the canon back of Pebbly Beach. 


Microscopic Characters. Under the microscope the dio- 
rites are found to have a nearly even-grained^ hplocrys- 
talline structure, and to be composed essentially of a lime- 
soda-feldspar, hornblende and occasional biotite, with free 
quartz always present in varying amounts. Augite is also 
present in some of the slides, and in nearly all magnetite 
is an important constituent. No apatite was observed in 
any of the sections. There is an occasional tendency to 
a porphyritic development among the feldspars. Mineral- 
ogically considered, the rock is a quartz-hornblende-diorite, 
with a tendency to lath-shaped forms among the feldspars. 

The feldspars are in general allotriomorphic, and tend to 
develop crystal faces only occasionally, where they come in 
contact with quartz. In some of the slides many of the 
feldspars are fairly clear and free from inclusions or de- 
composition products. Aside from these the majority are 
clouded by alteration products, which, in some cases, have 
partly or wholly obliterated the traces of the twinning lamel- 
lag, which are clearly shown in the fresher material. This 
cloudiness is apparently due in part to a kaolinization of the 
plagioclase, but also to calcite, which occurs in small, 
irregular patches and threads in many of the sections. This 
product is also found, in some instances, in lines along the 
twinning planes. Twinning takes place according to the 
Carlsbad, albite and pericline laws. Pericline twinning is 
the least frequent, and is usually seen under crossed nicols 
as a series of very fine lines. 

Excellent zonal structure is occasionally seen, but is in- 
frequent. The varying optical orientation in such cases 
shows that the mineral grows more acid from the center 
outward. Inclusions in the feldspars are not common, but 
rarely one of the largest crystals contains from one to a 
number of smaller feldspars which are without definite orien- 
tation toward their host, and without good crystal boun- 
daries. Inclusions of small ragged flakes of hornblende 
or chloritic material are of much more frequent occurrence. 
Several feldspars occur packed with small, irregularly 
bounded sections of what appears to be primary hornblende. 


The structure in this case is micropoikilitic. Occasionally 
sections contain numerous brightly polarizing, microscopic 
needles, doubtless of hornblende. 

Besides the decomposition products already mentioned, 
more or less epidote is usually present, generally in small 
irregular patches. 

All the diorites have doubtless been subject to stresses 
since they were consolidated, but only the rocks from Silver 
Canon give any marked microscopic evidence of it. In 
these rocks the feldspars seem to have been particularly 
affected, the hornblende and quartz showing little or no evi- 
dence of strain. The evidence here is of three kinds 
altered optical properties, bent crystals and fractures, which 
may or may not cause displacements. The extinction of 
the feldspars is frequently very indefinite and variable. 
Comparatively few of them show good extinction, and even 
these are sometimes considerably affected by cracks. In 
the others strain shadows take the place of the normal 
extinction. Bent crystals are not common, but a few of 
the sections show a distinct curvature, and a corresponding 
alteration in extinction. The fractures referred to are not 
the cracks frequently found in single individuals, but more 
extensive ones passing from crystal to crystal, simply as 
cracks, or forming veins which have been filled with sec- 
ondary matter. A few veins were found, the most pro- 
nounced one extending irregularly half through a slide, 
some of the feldspars on either side having suffered slight 
displacement. This vein is .05 mm. in width, and is 
filled principally with calcite, with more or less chlorite and 

From the extinction angles on either side of the albite 
lamellae, in favorable sections, the plagioclase appears to lie 
between a basic oligoclase and an acid labradorite. 

From its relations to the quartz and feldspars, the horn- 
blende seems to have been the first mineral in the order of 
the crystallization of the essential constituents of the diorite. 
The relations of quartz and feldspar show that a part of the 
feldspar was formed before, and part at the same time with 


the quartz. The simultaneous development with the quartz 
is evidenced by a frequent irregular intergrowth along the 
boundary line of the two minerals, and further by the occa- 
sional development of micropegmatitic structure. 

Quartz occurs in somewhat smaller individuals than do the 
feldspars. It is most frequently found in aggregations or in 
lines, as if, being the last mineral to form, it had filled the 
spaces between those previously existing. It is present in 
varying amount in all the slides, being fairly abundant in 
some, amounting to perhaps one-fourth of the total minerals 
of the slide. The sections vary in size from about .1 mm. 
to about 1.2 mm. It occurs in allotriomorphic forms, usu- 
ally with very irregular outlines, the sections being fre- 
quently somewhat intergrown on the margins. The sections 
are usually clear. Most of them contain liquid inclusions, 
occurring usually without any definite arrangement, though 
occasionally they are seen in lines extending through several 
sections. Besides these there may be seen with the higher 
powers occasional minute, greenish needles, and sections 
having the form of cross-sections of hornblende. They 
are without noticeable polarization. The quartz also con- 
tains occasional magnetite. 

The hornblende occurs in sections with very irregular 
boundaries, due to resorption. No approach to crystal 
forms was seen. The feldspar is always moulded on the 
hornblende, except in one case observed. In this section a 
small crystal of feldspar was seen apparently projecting into 
one side of the hornblende, the feldspar showing good 
crystal boundaries where surrounded by the hornblende. 
Aside from this instance the hornblendes contain no inclu- 
sions of feldspar, while the feldspars contain occasional 
inclusions of hornblendic material. The smaller feldspars 
are doubtless, in part, at least, contemporaneous with the 
hornblendes, though the feldspars in general are later. In 
size the former compare favorably with the feldspars. 
Twinning parallel to the orthopinacoid is common in the 
larger and fresher sections. The pleochroism is pronounced, 

( 2 ) January 9, 1897. 


c being green, 6 yellowish brown, and a pale yellow-green 
to almost colorless. The absorption formula is c > & > a. 
Inclusions are not common and are principally magnetite. 
There is an occasional intergrowth with biotite. The alter- 
ation of the hornblende is well advanced in many of the 
slides. It appears to be undergoing a uralitic change by 
which it is transformed into a dirty greenish, fibrous aggre- 
gate, much like " reedy hornblende," with a rather weak 
pleochroism. This secondary hornblende has usually a 
parallel arrangement of its fibres, and the terminals of the 
sections are generally more or less ragged. It also occurs 
in finely fibrous, irregular areas, with the fibres irregularly 
oriented. A further alteration of the hornblende is mainly 
into chlorite and calcite. 

Biotite is not common, though occasionally found. The 
sections are strongly pleochroic, always show irregular 
boundaries, and the mineral occurs either alone or inter- 
grown with hornblende. The biotite is in part altered to 
chlorite, and some of its sections are wholly surrounded by 
a chloritic margin. No inclusions occur, except occasional 
grains of magnetite. 

Augite is present in some of the slides, being variable in 
amount, but at times forming an important constituent. It 
generally presents very irregular boundaries, but several 
sections were seen showing roughly the crystal form charac- 
teristic of cross-sections of augite. It has a granular, much 
broken appearance, and a high refractive index. Its most 
characteristic feature is a clouding of the area by an opaque, 
dirty-brown decomposition product. Few of the sections 
were free from this product, and it marked the mineral 
wherever found. The augite is practically colorless, and 
is without any sensible pleochroism. No cleavage was 
observed anywhere, and only one case of twinning. Where- 
ever the augite comes in contact with the hornblende the 
boundary line is sharp and clear. When it occurs in isolated 
sections these are usually free from the uralitic product 
described in connection with the hornblende. This, 
together with the freedom of all the hornblende areas from 


the cloudy decomposition product of the augite, naturally 
leads to the conclusion that none of the fibrousjiqrnblende 
comes from the augite. 

Magnetite is found in all the slides, though not in any 
considerable amount. It occurs as inclusions in the other 
minerals, and is generally in the form of grains frequently 
showing partial crystal boundaries. In size they range up 
to .3 mm. Besides the grains, there are several very irregu- 
lar areas of considerable size up to 2 mm. or more in 
length in or near the areas of the ferromagnesian minerals. 


Following the usage of Iddings 1 the term "porphyrite " 
is here used to include those rocks which are characterized 
by a medium-grained porphyritic structure, and which con- 
tain among their essential constituents a lime-soda-feldspar. 
They constitute the connecting link, as it were, between the 
deep-seated diorites on the one hand, and the surficial 
andesitic rocks on the other, and pass by insensible grada- 
tions into either. The physical conditions attending and 
controlling its crystallization are the prime factor in deter- 
mining the position of the rock in the scheme of classifica- 

Occurrence. The porphyrite occurs in a single large area 
in the southeastern part of the island, and was not found 
elsewhere by the writer, except as smaller masses in the 
form of dikes. The main area has an average w r idth of 
about three miles, with an extreme length of about nine. 
It is cut by dikes of porphyrite and diorite, from two to 
thirty feet or more in width, which are shown on the cliffs 
at a number of places along the shore. 

Character. The rocks are very much weathered, and 
even those specimens which appeared to be fairly fresh 
were seen, when examined microscopically, to be consider- 
ably altered. In weathering the rocks first break into coarse, 

1 Twelfth An. Kept. U. S. Geol. Surv., Part I, pp. 582-584. 


irregular, block-like forms, looking, in some cases along the 
shore near Avalon, like square pillars projecting from the 
side of the cliff. These break up into smaller block-like 
masses, and the process is continued until the gravelly con- 
dition is reached. On the hill slopes the projecting masses 
frequently present similar forms with smooth surfaces, but 
it is not uncommon to see also small, boss-like projections, 
with rough, uneven surfaces. The cause of this difference 
in form is doubtless a variation in the grain of the rock. 
The soil formed from these rocks is generally of a dull, yel- 
lowish color. The porphyrite contains the same minerals 
that occur in the diorite, except biotite, which was not seen 
in any of the slides. It presents the same general charac- 
ters wherever found. 

Macroscopic Characters. The color of both the unaltered 
and weathered porphyrite is much the same as that of the 
diorite, the fresh hand-specimens varying from light to dark 
gray, most of them with a tinge of green. Little can be made 
out in the fresher specimens with the unaided eye, except an 
occasional feldspar, shown by the reflection from a cleavage 
surface. Hornblende crystals of some length up to 5 mm. 
or more are developed in one or two specimens. As the 
rock weathers the whitening of the feldspars usually brings 
out plainly the porphyritic structure. With the lens the por- 
phyritic feldspars may occasionally be distinguished from 
the medium-grained ground-mass, though these are usually 
masked more or less by the fracture of the rock, which 
leaves minute flakes or splinters clinging to the surface of 
the specimen. The rock frequently presents a slightly 
mottled surface, in the dark and light colors. Gleaming 
bits of pyrite may occasionally be seen in some of the 

Microscopic Characters. Microscopically the rock is holo- 
crystalline and porphyritic, with phenocrysts of a lime-soda- 
feldspar and of hornblende occasionally also of augite 
and a medium-grained granular ground-mass, composed es- 
sentially of feldspar and quartz. The phenocrysts vary con- 


siderably in number. In some slides there are comparatively 
few, while in others they constitute the larger portion of the 
slide. The ground-mass is never glassy, and does not show 
any flow structure, except to a slight extent in specimens 
from one or two dikes. Quartz is seen sometimes among 
the phenocrysts, though this is a very rare occurrence. 
Magnetite occurs as an accessory, but usually in very small 
amounts. No apatite was seen, and in the rock of which 
the analysis is given below no phosphorus was found. 
Most sections show little evidence of disturbance. 

The porphyritic feldspars occur in idiomorphic forms, 
which are somewhat tabular parallel to the brachypinacoid. 
The majority of the crystals show good boundaries, except 
where their growth was interfered with by the growth of 
other phenocrysts. Many of the sections, however, show 
boundaries which are more or less irregular or rounded, 
and due, in part, at least, to resorption. The sections vary 
in size from about .2 mm. to nearly 3 mm. Zoning is 
common. The twinning is in accordance with the albite 
and Carlsbad laws. Pericline twinning rarely occurs. As 
in the diorite, the feldspar lies between a basic oligoclase 
and an acid labradorite. Many of the sections are con- 
siderably cracked. Some of the feldspars are fairly fresh, 
but most of them are more or less clouded by decomposi- 
tion products. This cloudiness is due largely to a kaolini- 
zation of the mineral. Considerable areas are sometimes 
altered to calcite, with more or less epidote. The decom- 
position is such at times as to destroy, partly or wholly, 
the traces of twinning. Occasional inclusions of hornblende 
or chlorite are seen in the feldspars. One section was 
seen with a zone of chloritic material not far from the bound- 
ary, arranged in threads and fibres parallel to the longer 
direction of the feldspar. Small magnetite grains are rarely 
included in the feldspar. 

The hornblende is prismatic in habit, and usually either 
shows resorbed boundaries, or the original outlines are more 
or less obliterated by alteration products. No terminal planes 
were seen in any section. In two or three of the slides 


the characteristic lozenge-shaped cross-sections were seen, 
with a distinct prismatic cleavage. Twinning is common, 
parallel to the orthopinacoid. Comparatively few unaltered 
sections were found, most of the hornblende which was 
originally present in the slides having been altered to chlo- 
rite and calcite. In some of the slides the hornblende is 
wholly replaced by secondary minerals. The freshest sec- 
tions are frequently surrounded by a chloride border, or 
decomposition has begun along cracks and cleavage planes. 
The pleochroism of both chlorite and hornblende is the 
same as in the diorites. The hornblende in some cases has 
the fibrous character described under the diorites. In one 
slide in which this fibrous hornblende occurs, without good 
crystal boundaries, there is another secondary hornblende, 
with different optical properties, and having the form char- 
acteristic of augite. 

From what has been said it will be seen that hornblende 
is a primary constituent of the rock. It is also the dom- 
inant ferromagnesian mineral. Roughly estimated, it con- 
stitutes, with its decomposition products, about one-fourth 
or one-third of the total amount of minerals in the slides. 

Augite is found in varying amounts in nearly all the slides, 
usually in the form of irregular grains, or as irregular 
brownish or more or less opaque patches. It has the same 
general structure and habit as in the diorite and its decom- 
position products are similar. Usually it has a very insig- 
nificant position compared with the other minerals of the 
rock. In two or three of the slides, however, it compares 
in amount with the hornblende, and in one slide in particu- 
lar from a specimen from the canon back of Avalon the 
sections, though rather small, are quite numerous, constituting 
perhaps one-fifth or one-fourth of the total minerals of the 
slide. Here the mineral is fairly fresh and free from decom- 
position products. 

It is almost colorless or pale green, and without pleochro- 
ism. When any crystal boundaries are shown they are 
only partial. The form when developed shows the usual 
octagonal cross-section. The crystals occur as separate 


individuals. The feldspars are molded on the augites, 
and sometimes completely enclose the smaller sections. 
The augite shows no definite cleavage, but is usually more 
or less traversed by cracks. In this slide no hornblende 
was seen, though there are considerable areas of chlorite. 
No doubt hornblende was at one time present, being now 
represented by the chlorite, for the augite in the slide has 
no border of chlorite, nor is the latter seen along the cracks 
of that mineral. When near or touching chloritic areas the 
augites have as sharp boundaries and appear quite as fresh 
as those sections which have no chlorite near them. In 
weathering the augite alters to a granular, dirty brownish 
product, more or less opaque. In one slide, from the 
northern side of Avalon canon, what was originally augite 
with a short prismatic habit, is entirely altered to hornblende 
(see page 22) . The sections are idiomorphic, with the forms 
characteristic of augite, but with the cleavage of horn- 
blende . In vertical sections the cleavage is very pronounced , 
showing in part as open cracks. This hornblende is pleo- 
chroic, a being a very pale yellow, fc pale yellowish green, 
c greenish brown. The mineral is more or less dull in 
appearance, and the polarization colors are not clear. It is 
quite unlike the fibrous hornblende in character. One of 
the sections shows indistinct twinning lamellae parallel to 
the orthopinacoid. The augites contain as inclusions occa- 
sional magnetite grains. 

The magnetite varies greatly in amount, being almost 
entirely absent from some of the sections. In the rock 
containing abundant augite there is considerable magnetite 
in small, irregular patches or needle-like forms, at times in 
or cutting across the feldspars, or projecting into the augites. 
One slide from Pebbly Beach shows a few small grains with 
the crystal form of magnetite, but altered to limonite. 

The granular ground-mass of the porphyrite is composed 
of usually allotriomorphic feldspar and quartz, the larger 
proportion being of the former. At times, however, the 
feldspars tend to lath-shaped or rectangular forms. The 
borders of the grains in the ground-mass frequently inter- 


lock. Occasionally small, ragged flakes of hornblende or 
small patches of chlorite occur. The minerals of the 
ground-mass, in those slides showing the most pronounced 
porphyritic structure, have a diameter of from .03 mm. to 
.1 mm. As the size of the grains increases the rock 
assumes the structure of diorite-porphyrite, with occasional, 
though rare, porphyritic quartzes. All gradations were 
found between porphyrite and diorite. 

The quartz varies considerably in amount in the porphy- 
ritic rocks. Usually it is rather subordinate, but occasionally 
it is quite abundant, forming perhaps one-fourth of the 
minerals of the slide. These rocks, however, are not 
common, and are doubtless only local developments. One 
slide of the diorite-porphyrite is remarkable for the manner 
in which the quartz is developed with respect to the other 
minerals. This rock (from the slopes to the west of the 
entrance to Silver Canon) contains abundant phenocrysts 
of feldspar, with porphyritically developed quartz. Under 
crossed nicols the quartz appears as scattered and more or 
less rounded grains. Occasionally several of these are 
found near together, showing similar polarization colors and 
a common extinction. On revolving the stage it is seen that 
the rounded borders do not mark the limit of the sections 
of quartz; for the extinction of the mineral shows that it 
has an outer zone which has a pronounced micropoikilitic 
structure, being closely packed with finely polarizing feld- 
spars. The quartz occasionally shows a crystal form. The 
ground-mass of the rock is coarsely crystalline and consists 
largely of micropoikilitic quartz similar to the larger sections, 
but without clear centers. A very few of the larger sections 
also are wholly filled with feldspar aggregates. The clear 
centers together with the micropoikilitic margins indicate 
arrested development of the quartz, which began to form 
before the growth of the minute feldspars, the latter form- 
ing before the final crystallization of the quartz. The rock 
is much altered, and except in one or two instances the 
traces of twinning in the feldspar phenocrysts are wholly 


obliterated by decomposition products. Hornblende is 
wholly replaced by chloritic material. 

Analysis. The porphyrite was nowhere found in an en- 
tirely fresh condition. The following analysis was made 
from the freshest specimen obtained, as shown by its thin 


I. II. 

SiO 2 ............................ 63.82 65.71 

Ti O 2 . . . . ....................... trace 

A1 2 O 3 ........................... 16.53 i7- 8 

Fe 2 O 3 ........................... 1.28 2.84 

Fe O ............................ 2.93 1.79 

MnO ........................... trace 

Ca ............................ 5-57 5-24 

Mg O ............................ 1.99 2 -57 

Na 2 O ........................... 4.12 3.87 

K 2 O ............................. 77 1-02 

H 2 ............................ 1.82 

C O ............................ 1. 10 

99.93 100.12 

Sp. gr ............ .............. 2.689 

I. Porphyrite from Pebbly Beach, Santa Catalina. 
II. Quartz-diorite, Dognaska. (Banatite.) 

The analysis of the porphyrite differs but little from that 
of the banatite given above. In some of its aspects the 
diorite is not much unlike the microscopic character of 
some of the banatites. 

Inliers of Basement Rocks. At a number of points in the 
porphyrite area, following the general direction of the main 
ridge to the west of Avalon, and along a line extending 
from the coast northwest of Avalon to near the coast east of 
Silver Canon, there occur at intervals patches of the base- 
ment rocks. These are found not only along the main crest, 
but on several of the branch ridges to the west and north- 
west of Avalon, and on one of the branches running into 
Silver Canon. They also occur at irregular intervals at the 
base of the cliffs, from Avalon Harbor for a distance of 
about a mile to the northwest. The outcrops along the 
ridges vary from a few yards to nearly 200 feet in length. 


Only the largest have been indicated on the map (even 
those being necessarily made on a larger scale than that of 
the map), where they are shown as quartzite, though also 
including rocks from all the basement series. Those in the 
cliff sections occur as definite inclusions, and vary in length 
from a fraction of an inch to about 50 feet. The porphyrite 
in other places also contains inclusions, though nowhere are 
they so abundant as at the points mentioned. 

It is possible that some of the outcrops along the ridges 
are remnants of a former covering to the porphyrite, but 
undoubtedly some (if not all) of them exist as inclusions 
within the igneous rock, as do the occurrences along the 
shore ; for some of them occur in saddles along the main 
ridge, while those on the minor ridges are in great part 
hundreds of feet below the average altitude of the main 
ridge. To contain such large inclusions an intrusive of 
considerable size must be predicated. This, together with 
the size and form of the area of the porphyrite, points to 
its origin as a laccolite. 1 The microscopical character of 
the rock, together with its mode of occurrence, clearly 
indicates its intrusive nature. 

The mass of the porphyrite appears to be roughly dome- 
shaped, with a somewhat elliptical base, and though no 
remnant of a cover was found it cannot be doubted that one 
formerly existed, now removed by extensive and active ero- 
sion. The base of the mass was not seen at any point. 

Porphyrite Dikes. Occasional dikes penetrate both the 
porphyrite area and that of the quartzite, the latter at the west 
end particularly. At the mouth of Silver Canon the diorite, 
also, is cut by porphyrite dikes, one of which contains numer- 
ous inclusions of the diorite. Wherever the directions of the 
dikes could be determined they were found to be nearly ver- 
tical, or within 20 of the perpendicular, and approximately 

1 The term "laccolite" is here used in the sense of a somewhat dome-shaped mass 
which has been intruded into a yielding body of rock, not necessarily along the bedding 
planes. On this view the undisturbed condition of the beds previous to the intrusion is 
of minor importance, the main factors being the possession of basement and cover, and 
the dome-shaped form of the mass. 


parallel, the range being from N. 25 E. to N. 65 E. Prob- 
ably all of these dikes are of nearly the same age as the 
porphyrite, though many, or possibly most of them, are a 
little later, judging from the fact that the area of the por- 
phyrite itself is penetrated by them. Most of the specimens 
obtained are very much altered, and contain a comparatively 
large amount of calcite, but there is enough of the original 
structure left to show definitely that the rocks are porphyrite. 
In general nothing further than this could be determined, 
though most of the specimens appear to be not much unlike 
the rocks of the main mass. 

A somewhat different structure is shown in two or three 
of the slides, only one of which will be described. This is 
from a dike on the northern coast, at a point about a third 
of the distance from the isthmus to the extreme northwest- 
ern end of the island. The dike is nearly vertical, and has 
a width of eight feet. Specimens were taken from near the 
margin and central portion of this dike, the two being en- 
tirely different in appearance. That from near the center 
is a pale, even gray, while the other is a darker, mottled 
gray, with an intergrowth of light and dark areas. The 
darker parts appear to be compressed in a given plane, and, 
as seen with a lens, have usually a minute central cavity. 
The rock from the middle of the dike also contains here and 
there very minute thread-like cavities. 

Although the hand-specimens differ so much in general 
appearance the contrast is not so great under the microscope. 
Phenocrysts are not very numerous and are wholly of labra- 
dorite, which is quite fresh. They are considerably resorbed, 
and seldom show crystal boundaries. The ground-mass is 
almost wholly filled with lath-shaped feldspar microlites of 
various sizes, which show a pronounced flow structure in 
the slide of the marginal rock, this being less noticeable in 
the specimen from the center. A majority of these micro- 
lites have indented terminals. In the marginal rock slide 
there are two distinct types of areas in the ground-mass, in 
both of which microlites occur, one somewhat yellowish, and 
the other dark in color, from minute particles contained in 


it. In the first type the finer portion of the ground-mass has 
a microcrystalline structure. In the other case the matrix 
is largely isotropic, and is principally of secondary silica in 
the form of opal. These latter areas have usually a small 
irregular central cavity somewhat rounded or oblong in 
shape. In some cases these centers have been filled with 
secondary quartz. These darker areas of secondary silica 
are usually separated from the lighter ones by an irregular 
and generally narrow band of a yellowish green, showing 
high polarization colors. Under crossed nicols and with 
higher powers, this is seen to be composed of finely polar- 
izing aggregates, doubtless of some secondary ferromagne- 
sian mineral. Similar minute, radial aggregates occur 
scattered through the lighter areas of the slide. 

The slide from the central portion of the dike does not 
show the division into light and dark areas, though it con- 
tains a small amount of opaline silica and quartz. There is 
a larger amount of the greenish yellow ferromagnesian 
mineral, which is more evenly distributed through the rock, 
a part in the form of radial aggregates and a part as minute 
flakes. The matrix is cryptocrystalline, showing a feeble 
polarization. The ground-mass is filled with opaque, dust- 
like, microscopic particles. 


Occurrence. The rhyolite occurs in a single area, to 
the west of the main area of the andesite, in the Little 
Harbor region. It caps the summit of the ridge at this 
point, and extends as a light covering to the basement rocks 
for several hundred feet down the southern and western 
slopes, the underlying formations appearing here and there. 
At a point about midway down the western slope the rock 
has a roughly bedded appearance, dipping toward the west 
at a rather high angle. The relation of these rocks to the 
other igneous rocks of the island was not learned. 

Macroscopic Characters. The rhyolite varies from com- 
pact to very vesicular, and is of a light color, nearly white 


or with a tinge of pink. It appears to be considerably 
altered. It contains scattered phenocrysts of quartz^ (up to 
3 mm. in diameter) with smaller and more numerous crystals 
of biotite. The vesicles indicate flow by their pronounced 
compression in one plane. Some of the cavities have a 
smooth, lustrous surface, and appear to be regular in shape, 
as if due to the leaching out of phenocrysts once contained 
in them. The form of two or three of these cavities strongly 
suggested a simply twinned feldspar. 

Microscopic Characters. In thin section the open-textured 
facies of the rock is seen to be composed largely of a dirty 
brown, very vesicular ground-mass, in which occur scattered 
phenocrysts of quartz and biotite, besides more or less mag- 
netite in small grains or crystals. No phenocrysts of feld- 
spar were seen, but one cavity was found which clearly 
had the form of a Carlsbad twin of feldspar. This cavity 
had a very narrow border of some secondary product strongly 
stained with limonite. 

The quartz is in general quite clear, and occurs in idio- 
morphic forms which are usually more or less corroded. 
Frequent cracks traverse the sections. Besides brownish 
patches of included glass, the quartz contains occasional 
small spherulites, and sections of biotite partly or wholly 
included. No liquid inclusions were seen. 

Biotite occurs in scattered, idiomorphic sections, generally 
with clear boundaries. A few of the rectangular sections 
are somewhat frayed at the ends. The crystals range in 
length from .15 mm. to .7 mm. The mineral exhibits the 
usual strong pleochroism. 

The ground-mass consists in large part of feebly polarizing 
feldspar microlites in a dark isotropic matrix. A few of the 
vesicular cavities of the slides are nearly round, the rest 
being elliptical in form, and occasionally drawn out at the 
ends. Some contain a small amount of a clear, secondary 
mineral, and others, nearly spherical, are completely filled 
with almost opaque secondary products, dirty brown to black 
in color. 



Occurrence. There is one main area of the andesitic 
rocks with several smaller occurrences. These rocks are 
all, both macroscopically and microscopically, identical, and 
undoubtedly indicate an originally continuous area, cover- 
ing the larger part of the eastern division of the island. 

Ascending the ridge next the ocean, to the south of Mid- 
dle Ranch Canon, this rock is first met with at an altitude 
of about 300 feet, where it forms a small patch extending 
from this point to an elevation of about 500 feet. The next 
area occurs at an altitude of about 1,100 feet, where the 
rock not only forms the summit of this part of the ridge but 
caps, as well, a minor ridge which extends into the adjacent 
canon. The third occurrence is near the head of this canon. 
Besides these more definite areas, the soil at a number of 
places in this region has a purplish tinge, and the general 
appearance points to a more extended areal distribution of 
the andesite. Erosion has, however, entirely removed the 
rock in some places, while in others it has left only the thin- 
nest coating on the rocks beneath, or the former covering 
remains simply as a coloring to the soil, in places occupied 
by other rocks which normally weather to a yellow. 

There is a small area of andesite at the extreme south- 
eastern end of the island. Here much of the andesite con- 
tains inclusions in varying amounts, the rock in some places 
being well filled with this fragmental material, which is de- 
rived in large part from earlier andesitic flows. 

The main area of the andesite has a general easterly and 
westerly trend, and extends from the shore on the north 
side to the lower slopes south of Little Harbor. It reaches 
an extreme altitude of 2,109 ^ eet - ^ consists of a series of 
volcanic flows which present a distinct banding on the face 
of the cliffs northwest and southeast of Swain's Landing. 
These bands have a width of four or five feet and upwards. 
To the east of Swain's Landing they have a dip of io-i2 
toward the Landing, while on the other side they dip in the 
opposite direction and at an angle of about 3. Midway 


between this point and Whitley's Cove the bands are more 
or less irregularly flexed, though preserving a general par- 
allelism to the shore-line. 

Another small area of these rocks is found along the coast 
to the east of Empire Landing. 

The area of andesite near the isthmus is also formed by a 
series of flows which show a distinct banding along the cliff 
on the ocean side. This banding preserves a course rough- 
ly parallel to the water-line till near Isthmus Cove, where it 
changes its direction, dipping at an angle of about 25 to- 
ward the point at the entrance to the cove, as shown in the 
section on the map. This formation reaches its greatest 
altitude near the southeastern end, where it is about 900 
feet above sea-level. Along the northern shore-line of this 
area adjoining Isthmus Cove numerous faults are seen (not 
shown on the map), ranging in throw from a few inches 
to a hundred feet or more. 

The most marked feature of this area is a distinct white 
band following the upper line of the cliff for some distance, 
and overlying the volcanic rocks. This is the bed of tuff 
and diatomaceous earth already mentioned as occurring with 
the volcanics here. That the andesite lies above as well as 
below it is plainly seen at a number of points. An espe- 
cially good section showing the upper contact of the tuff is 
obtained in the little bay to the east of Isthmus Cove. Here 
the tuff is overlain by porous andesitic rocks. 

The rocks along this part of the shore have been hollowed 
out in places by the force of the waves, forming caves, pil- 
lars, and blow-holes. 

The coarse banding of the andesites along the cliff sec- 
tions shows a variety of colors, the rocks weathering in dark 
grayish or purplish with occasional reddish tints. The soil 
formed by this series of rocks is always purplish in color, 
and is easily distinguished, even at a distance, from the soils 
which the other rocks of the island form, the latter being 
either reddish or yellowish. 

Macroscopic Characters. The freshest specimens of the 
andesite are black or nearly so. Though the rocks are in gen- 


eral dull, some of them have an almost greasy luster. A few 
of the specimens, purplish in color, appear to the eye to be 
fresh and compact, but with a lens it may generally be seen 
that they are more or less altered. Though usually compact, 
the rocks are vesicular in places, the irregular vesicles more 
or less compressed in the plane of flowage. This was noticed 
particularly about Isthmus Cove; also in places back of 
Whitley's Cove. Rarely the rock is amygdaloidal, as in 
the neighborhood of Whitley's Cove. The rock, though 
usually fracturing irregularly, at times breaks into plate-like 
pieces a centimeter or more in thickness. These pieces 
may sometimes be broken into thinner plates, owing to a 
laminated condition of the rock. These latter sheets vary 
in thickness from two millimeters to several centimeters, 
and their surfaces are generally yellowish from decomposi- 
tion products. In other cases the rocks break into irregular 
masses, while showing a phenomenon similar to the fore- 
going in a series of fine parallel lines on those fractured 
surfaces at right angles to the bedding. One and only one 
glassy specimen of the rock was obtained, from near the 
small bay to the east of Isthmus Cove. 

Microscopic Characters. Microscopically the rocks, with 
the exception of two of the specimens examined, are pyrox- 
ene-andesites. They are usually porphyritic with a hyalopil- 
itic ground-mass. The phenocrysts consist essentially of 
labradorite, augite, and hypersthene. Secondary silica is 
usually present in greater or less amounts. This is largely 
opal with occasional chalcedony. The first mineral to sep- 
arate from the magma was magnetite, followed by the pyrox- 
enes, and finally by the feldspars. 

The magnetite occurs either as small octahedrons or in 
irregular patches, and appears to be in two generations. 
The largest grains are about .2 mm. in diameter. These 
are not very numerous. The smaller grains are more abun- 
dant, and are distributed more or less evenly through the 
ground-mass. These have an average diameter of about .04 
mm. This scattered magnetite forms one of the most marked 


features of several slides which have very few phenocrysts. 
It amounts in one to perhaps one-third as much as the feld- 
spars of that slide, and equals or very slightly exceeds the 
amount of pyroxene. The magnetite occurs as inclusions 
in all the other phenocrysts. 

The augite is usually idiomorphic, though where not well 
developed as phenocrysts it occurs as minute flakes. It is 
very pale green, almost colorless, and without noticeable 
pleochroism. Its habit is prismatic and the resulting forms 
are generally octagonal. The crystal outlines are usually 
very sharp and clear, though rounded and resorbed sections 
are not uncommon. The cleavage, in general, is not visible 
except with higher powers. Cracks are common, traversing 
the crystal in every direction. The augites generally are 
remarkably clear and free from alteration products. A few 
of the larger sections, however, are much dulled and 
cracked. Twinning parallel to the orthopinacoid is com- 
mon. Liquid inclusions are numerous, occasionally reaching 
a diameter of .02 mm. Some inclusions of magnetite also 

The hypersthene differs but little from the augite in gen- 
eral appearance, in form, habit, inclusions, or its relation to 
the essential minerals, and cannot always be readily distin- 
guished from it. The former, however, is very slightly 
pleochroic, and all sections give parallel extinction. It has 
a somewhat weaker double refraction than the augite, and 
the interference colors of the sections are therefore in general 
lower, showing yellow of the first order in the majority of 
cases. Further, favorable sections give characteristic inter- 
ference figures. Though its habit is like that of the augite, 
the prismatic faces occasionally are little developed, or 
rarely are entirely wanting. The two minerals are occa- 
sionally intergrown. 

Whenever the two pyroxenes are developed as definite 
phenocrysts the hypersthene is always in excess of the 
augite. The number of the pyroxene phenocrysts in the 
rocks is always much smaller than that of the feldspars, the 
ratio being about one to four or five. 

( 3 ) January 9, 1897. 


The feldspar phenocrysts occur in idiomorphic sections 
and are usually lath-shaped, presenting a tabular develop- 
ment parallel to the brachypinacoid. There is a slight tend- 
ency to the formation of ruin-like terminals. The crystal 
boundaries are in many cases clear and sharp, though most 
of them show a varying amount of resorption both on the 
sides and terminals of the sections, oftener the latter. Some 
of the feldspars clearly show a second period of growth 
after having been in part resorbed into the magma. Zoning 
is rather common. Twinning is in accordance with both 
albite and Carlsbad laws. Occasional cracks penetrate the 
sections and some of these are brought out more clearly by 
a staining of limonite. In sections cut approximately per- 
pendicular to the albite lamellation the extinction angles 
show the species to be labradorite. As a rule the sections 
of the feldspar are very fresh and clear, being free from 
decomposition products. Inclusions are not uncommon, 
consisting chiefly of brownish glass with a few small and 
irregular pyroxenes and occasional small grains of magnet- 
ite. The glass is usually in irregular patches and is either 
centrally or zonally arranged. In some cases it occurs in 
roughly rectangular forms zonally arranged, and with their 
longer axis parallel to the longer axis of the crystal. The 
length of the sections of labradorite varies from perhaps 
.06 mm. to about 1.5 mm. The sections are usually quite 
numerous, though their number is very variable, and some 
of the specimens show few phenocrysts of any sort. 

The phenocrysts of the various minerals generally occur 
as scattered crystals, but they occasionally form small ag- 
gregates. In this case, the boundaries of the feldspars yield 
to those of the ferromagnesian minerals with which they 
are in contact. This, with the occasional complete inclu- 
sion of these minerals already mentioned, shows the later 
development of the feldspars. 

The ground-mass consists largely of feldspar microlites 
with a varying amount of interstitial glass. Pyroxene oc- 
curs occasionally in flakes in the ground-mass, when but lit- 
tle developed porphyritically. The feldspars have a low 


extinction angle, most of them extinguishing parallel to their 
length, thus placing them in the oligoclase-andesine series. 
Many of them appear to be simply twinned. In length they 
range up to about .04 mm. In nearly all cases they show a 
general parallelism due to flow. The, amount of glass in 
the ground-mass is very variable, some of the slides being 
almost holo crystalline. In other cases the glass and second- 
ary silica compose nearly one-half the ground-mass. This 
glass is readily distinguished from the opal by its color. 
The latter is yellowish brown and is quite clear, while the 
glass is dark and usually filled with small, irresolvable dots 
which may be magnetite. 

Glassy Fades. The glassy facies of the andesite, found 
as a very small occurrence near the isthmus, differs both 
macroscopically and microscopically from that just described. 
The rock is dark, almost black, and glassy, with yellowish 
patches scattered over the surface, the largest seen being 
13 mm. in length. Occasional kaolinized feldspars occur, 
either within these yellowish patches or alone, up to a length 
of about 2 mm. With a lens very small hexagonal crystals 
of biotite are seen here and there. The dark ground-mass 
constitutes the bulk of the rock. 

Under the microscope scattered and aggregated crystals 
of magnetite, hypersthene, augite, biotite, and feldspar are 
seen here and there in a glassy matrix. Magnetite, in grains 
and small octahedra, occurs as inclusions in the other min- 
erals, and scattered through the ground-mass. There is 
only a moderate amount of biotite in the slides. It occurs 
as isolated idiomorphic sections, and was seen nowhere in 
contact with the other minerals. No basal sections were 
seen. The mineral everywhere exhibits the usual strong 

Augite occurs either alone or with hypersthene, and while 
generally more or less rounded, it occasionally shows good 
crystal boundaries. It is pale green in color and non-pleo- 
chroic. One section of the augite showing rough crystal 
boundaries is wholly surrounded by a somewhat rectangular 


growth of hypersthene, the augite occupying a nearly cen- 
tral position. The vertical axes of the two minerals are 
similarly oriented. The augite is simply twinned, the two 
halves of the section showing a very slight difference of 
extinction. The hypersthene is much fractured and pre- 
sents the appearance of a number of rods placed in parallel 
position, side by side and end to end. The interstices 
between the rod-like parts are filled with a brownish yellow 
isotropic substance closely resembling the opaline silica in 
the slides already described. This mineral also forms an 
intricate network in most of the feldspars, and constitutes a 
considerable proportion of many of the yellowish patches 
which are so numerous in the rock. These yellowish areas 
are largely aggregates of hypersthene and feldspar. 

The hypersthene is prismatic in habit and occurs, in gen- 
eral, either alone or in aggregations together with occa- 
sional feldspars. The feldspar and hypersthene are the 
most abundant minerals of the slide, the former being some- 
what in excess of the latter. The hypersthene has a pro- 
nounced pleochroism, c being light green in color, 6 reddish 
brown, and a very pale reddish. The absorption formula 
is c>B>a. 

In one slide two small sections of free quartz were 
seen occurring quite close together and showing rounded 
and somewhat corroded boundaries. The sections are both 
crossed by numerous cracks. 

The feldspars are all much resorbed, and seldom show 
crystal boundaries. The brownish network which most of 
them contain is frequently central, leaving a narrow border 
free from inclusions. 

The ground-mass, with a high power, is seen to consist of 
glass filled with crystallites and microlites, with here and 
there perlitic cracks. With a low power the crystallites 
and microlites appear only as dusty particles. The perlitic 
cracks are distributed very irregularly, some portions of the 
slides being entirely free from them. They are usually 
quite numerous near the larger aggregates of the pheno- 
crysts, and frequently those bordering these areas are stained 


yellow. Such stained cracks show, under crossed nicols, a 
faint polarization, as if from some radially arranged sec- 
ondary product. The microlites are capillary in form, 
averaging .01 mm. in length, and exhibit a slight polariza- 
tion. They also show a pronounced flow structure. In 
smaller amount are the margarites and trichites, the latter 
in the form of tufts and wisps, and sometimes curled at the 
ends. The margarites, also, are occasionally gathered into 
loose tufts, radiating in all directions. 

Basaltic Fades. It was stated (page 32) that the speci- 
mens examined were all pyroxene andesites, with but two 
exceptions. One of these was found on the minor ridge 
bounding the valley of Whitley's Cove on the north, not far 
from the contact between the basement rocks and the vol- 
canics. The other is from the slopes to the east of Isthmus 
Cove. The rocks differ from those already described, in 
containing iddingsite. 1 The specimen from near the isthmus 
is dark gray in color and very much altered, but the iddings- 
ite occurs here in good crystal forms, while in the other 
specimen, which is fairly fresh, the crystal boundaries show 
more or less resorption. As the structure of the latter rock 
differs considerably from that usually found in the andesites, 
it will be described in some detail. 

The rock is purplish and compact, but passes into a black 
vesicular facies, apparently differing microscopically from 
the compact form only in the size of the component minerals 
and in the amount of magnetite contained. The minerals 
are much smaller in the vesicular portion of the specimen, 
and it is almost black with magnetite, in the form of grains, 
long, roughly bordered rods, and irregular areas. The 
glassy ground-mass of this portion of the rock is filled with 
minute black dots, doubtless magnetite. The compact por- 
tion of the specimen has very little glass, though it contains 
a large amount of secondary silica, chiefly in the form of 
opal. The rock is nearly holocrystalline, and contains 

1 See "Geology of Carmelo Bay," by Andrew C. I^awson. Bull. Dept. Geol., Univ. 
Cal., Vol. i, pp. 31-36. 


very fresh, lath-shaped feldspars in two generations, besides 
amber-colored iddingsite and pale green augite. The 
specific gravity is 2.770. 

The opal has the usual appearance of that mineral macro- 
scopically. On one face of the specimen there is a con- 
siderable crust of hyalite showing a distinctly botryoidal 
surface when viewed with a lens. It is colorless and nearly 
transparent, with a vitreous luster and a hardness of about 
5.5. It is infusible, dissolving in soda, with effervescence, 
to a clear glass. It is in large part soluble in caustic pot- 
ash, and in the closed tube gives water. The areas of 
opal in the slides are all isotropic. The sections readily 
take a stain after heating with concentrated hydrochloric 
acid, which, however, scarcely attacks the powdered mineral. 
The color of the opal in thin section is light brown. 
Cavities occur in it, occasionally lined with chalcedony. 

Both generations of feldspars appear to be labradorite, 
and the larger ones form the most prominent feature of the 
slide, being rather numerous and of considerable size, 
ranging in length from about 2 mm. to 3.5 mm. They are 
allotriomorphic, and contain, as inclusions, considerable 
iddingsite, besides a little glass, an occasional smaller 
feldspar in the largest sections, and rarely augite. The 
small included feldspars always show more or less resorp- 

The augite occurs in small grains with very irregular 
boundaries. These contain many cracks which give them 
a granular appearance. The mineral shows no alteration 
and contains as inclusions occasional grains of magnetite, 
besides partially included small feldspars. Several sections 
were seen with a few small feldspars wholly enclosed. 

The most characteristic mineral of the rock is the iddings- 
ite. In amount it slightly exceeds the augite, and equals 
about one-third of the feldspar. It varies in size from .06 
mm. to nearly .4 mm. It was the second mineral to separate 
from the magma, preceded by the magnetite. It occurs as 
usually elongated grains with very irregular boundaries, 
frequently marked by bays due to magmatic corrosion. 


Where the original form is indicated it is very similar to that 
of olivine. The form is best shown in the rock from near 
the isthmus. That rock, however, is very soft and much 
altered, and no satisfactory microscopic sections of the 
mineral were obtained, except for the determination of the 
outline. Several small but good crystals were made out in 
the rock, with a lens. As seen thus, the general form tallies 
with that described by Prof. Lawson. 1 The mineral here 
is deep brownish red in color, with a pronounced cleavage, 
the cleavage surfaces presenting a somewhat metallic luster. 
The central portion of the crystals is usually dark green. 
The forms which the slides present are of two types, both 
hexagonal, one with a pronounced cleavage, the other with- 
out a cleavage but with a distinct fibration at right angles to 
the direction of elongation. The cleavage subtends an 
angle, two measurements of which gave 131 and 133, 
respectively. Referring the mineral to the same system of 
axes to which Prof. Lawson has referred it, a is perpendic- 
ular to the cleavage, b in the cleavage and parallel to the 
fibration, c in the plane of cleavage and perpendicular to b. 

The cleavage is well shown by a series of parallel and 
narrow, open seams, to which the extinction is in all cases 
parallel. In sections in which the cleavage is wanting the 
extinction is always parallel to the longer direction of the 
section and to the fibration. The latter sections show a 
fair biaxial interference figure. The mineral is therefore 
orthorhombic. The emergence of the acute bisectrix is 
perpendicular to the plane of cleavage and b . The 
optical character of the mineral, as determined by means of 
the quartz wedge, is negative, a is therefore the acute bisec- 
trix, and =; c=. 

The color of the mineral in thin section is very variable, 
especially in different parts of one and the same section, 
ranging from a golden brown to a clear though not bright 
yellow, with occasional dull greenish areas in or near the 
center. The deeper colors are usually marginal or along 


the frequent cracks, and appear to be due to limonite formed 
by oxidation of the contained iron. In all the attempts made, 
however, this color could not be leached out by acids. The 
deep color of the sections and strong absorption of light 
prevented an entirely satisfactory determination of some of 
the optical properties. 

The pleochroism is marked in sections transverse to the 
cleavage, but is not so strong in sections showing no cleav- 
age. The absorption formula is c > 6 > a. The mineral 
possesses a rather low mean index of refraction. In the 
thinner sections the iddingsite may be seen to possess a 
strong double refraction, though the polarization colors are 
usually masked by the deep color of the mineral. In sec- 
tions parallel to the plane of cleavage, though the transverse 
fibration parallel to b is distinct, the color of the mineral 
conceals the fibration at right angles to this. 1 

No satisfactory material could be obtained for investi- 
gating the mineral chemically, nor was any attempt made 
to analyze the rock as a whole, on account of the secondary 
silica contained in it. One of the slides was uncovered and 
an attempt was made to stain the mineral. This was suc- 
cessful only after several trials had been made, both with 
concentrated and dilute acids. Dilute boiling sulphuric 
acid finally caused the mineral to take the stain, the results 
thus agreeing with those obtained by Dr. Ransome. 2 

No definite information was obtained from the Santa 
Catalina specimens as to the origin of this mineral. Its oc- 
currence in a rock of this type, and possessing the form 
characteristic of olivine, would certainly, in the absence of 
any evidence to the contrary, point to the strong probability 
of its being a pseudomorph after that mineral. If the min- 
eral described by Iddings 3 is the same as that under discus- 
sion as it appears to be it leaves little doubt on the ques- 

iSee "The Eruptive Rocks of Point Bonita," by F. Leslie Ransome. Bull. Dept. 
Geol., Univ. Cal., Vol. i. No. 3, p. 91. 

*Loc. cit., p. 92. 

8 "Geology of the Eureka District, Nevada." Monongraph XX, U. S. G. S., Appen- 
dix B, pp. 388-390. 


Analysis. The following analysis was made from a 
particularly fresh specimen of the andesite, obtained from 
the ridge forming the eastern boundary of the valley back 
of Swain's Landing: 

Si O 2 61.05 

TiO 2 09 

A1 2 O 8 * 18.30 

Fe 2 3 3-49 

Fe O i. ii 

Mn O trace 

CaO 7.75 

Mg O 2.59 

Na 2 4.06 

K 2 1.36 

H 2 71 

P 2 O 5 trace 

100.51 2.668 


The porphyrite was nowhere found in a fresh condition, 
while the andesite at many points is very fresh. The pro- 
nounced difference in the amount of alteration which the 
two rocks have undergone would suggest that the andesite 
is the younger. When the feldspars of the two are com- 
pared this difference amounts to more than a mere sugges- 
tion. These feldspars are closely related chemically, and, 
other things being equal, those of the older rock should 
show a greater amount of weathering. In the fresher an- 
desite the feldspars are remarkably free from decompo- 
sition products, while those of the porphyrite always show 
a greater or less degree of alteration. In none of the spec- 
imens do the feldspars compare with those of the andesite 
in freshness. 

More positive evidence as to the relative age of the two 
rocks was obtained on the western slopes of the andesites, 
in the Little Harbor region. Here, at an elevation of about 
1,200 feet, numerous inclusions of the porphyrite are found 
in the andesite. They are of very irregular shape, averag- 
ing two or three inches in diameter. Differential weathering 


frequently causes them to stand out on the surface of the 
rock. These inclusions show beyond a doubt that the por- 
phyrite is the older of the two rocks. 


This material has already been mentioned as occurring 
intercalated with the andesites of the isthmus region near 
their upper limits, and forming a single composite bed of 
considerable thickness. So far as known there is but this 
one occurrence. Though there are excellent exposures to 
the east of Isthmus Cove no complete section of the bed 
was seen, but from the several parts it is estimated to have 
a thickness of from one hundred to two hundred feet. 

Besides this bed on the island, there is doubtless a consid- 
erable deposit of similar material just outside of Isthmus 
Cove, as indicated by the sounding contours. The large 
scale Coast Survey map of the isthmus emphasizes this, 
and shows, by mapping in the contours, a more or less con- 
tinuous submarine ridge, extending out some distance. 
This ridge is marked in its course by a shoaling of the 
water at one point, and by two small islands. These islands 
are within the 200 ft. and 300 ft. contours, and are 29 feet 
and 66 feet in height, respectively. The nearer of the two 
is composed of tufaceous material, and the outer one is 
probably of the same, though it was not seen near at hand. 

The bed of the island is not homogeneous but is composed 
of numerous minor beds of varying thickness and color, 
now of the white shale and now of reddish, yellowish, or 
dark grayish tufaceous material. The thickness of a given 
bed is seldom the same for any distance, and it frequently 
happens that a bed will wedge out and disappear within a 
few rods. One of the beds noted changes in thickness 
from about six feet to one foot in a little more than fifty feet. 
The various beds of the formation therefore occur in no 
fixed order, though the bulk of the deposits is always of the 
diatomaceous earth. 


2. TUFF. 

The tufaceous beds are not wholly of volcanic material, 
but contain more or less of the shale fragments, besides 
having occasionally a matrix of the shale (diatomaceous 
earth). They also contain fragments of the metamorphic 
material similar to that composing the underlying basement 
rocks. The rock as a whole is rather soft, and usually 
somewhat compact. One of the highest beds is made up 
of moderately fine angular material and is somewhat porous, 
as it has very little cementing material. Usually fragmental 
shaly material appears to form most of the finer portion of 
the rock. Some of the beds are more or less even-grained, 
composed wholly of moderately fine material, and contain- 
ing nothing larger than half an inch in diameter. Others 
contain, besides this, large angular blocks, the largest attain- 
ing a length of about two and a half feet, though most of 
them are less than a foot and a half in length. By the 
weathering of the face of the cliffs many of these blocks 
project from the surface. In this way both the larger and 
smaller fragments gradually work out and fall to the base of 
the cliff. These blocks are composed almost wholly of 
very vesicular and usually much reddened, andesitic lava. 
A few large fragments of shale were seen in these coarser 
beds and a number of angular blocks of the metamorphic 
material, some of them a foot and a half in length. These 
beds, are in places fossiliferous, and it is said that large 
pectens have been found in them. No fossils were found 
by the writer, however. 

3. SHALE. 

The shale wherever found is white or light gray in color, 
but it varies considerably in texture and composition. As a 
rule the rock is very soft and earthy, and can be easily 
scratched with the nail. In this condition it has a low 
specific gravity. The more earthy and less compact the 
rock, the more easily it splits into thin sheets. The lightest 


separates into paper-like fragments, almost thin enough for 
microscopic sections. The rock is found in two other con- 
ditions, one opaline and the other calcareous, between 
which and this type there are all gradations. Both these 
less common types are hard and compact, and are quite 
brittle, breaking with a conchoidal fracture. The opaline 
variety has a hardness of about 5, and in places has a luster 
like that of opal. This is No. 89, described by Dr. Hinde 
on page 48. The calcareous rock is somewhat darker in 
color than the others. It effervesces quite freely with strong 
acids, while with dilute acids it behaves like dolomite. The 
gradation of this rock into the light, earthy shale is seen by 
testing the different specimens with acid. The different 
grades of the rock show different degrees of effervescence, 
while the most earthy specimens give apparently none. The 
opaline variety does not effervesce with acid. The effer- 
vescence is due in many of the specimens to minute cal- 
careous remains, but in the darker rock it results from the 
calcium-magnesium carbonate which makes up the mass of 
the rock. All the specimens give water in the closed tube. 
Heated they turn black, then white, giving off bituminous 
odors. The specific gravity of the earthy specimens could 
not be determined on account of their porous character. 
That of the limestone is 2.69. 

No fossils of any considerable size were found in these 
rocks. Some of the more siliceous specimens contain 
minute, empty molds, arranged along the bedding planes. 
In one of the more compact specimens two fragmentary 
shell casts were seen, besides a small cast of what is prob- 
ably Tellina congesta, Conrad. Fish-scales are quite com- 
mon, with their delicate markings well preserved. 

Microscopic Characters. Under the microscope the shale 
is seen to consist largely of isotropic material, in which are 
scattered angular crystal fragments. The isotropic portions 
of the slides appear to be, not of glass, but wholly (or nearly 
so) of organic remains. The crystal fragments vary in amount 
in different specimens, or even in different parts of a single 
slide, but on the whole they form but a small percentage of 


the entire rock. They are largely microscopic in size, with 
here and there a larger fragment reaching an extreme length 
of about .1 mm. This fragmental material is largely of 
feldspar some of which shows twinning, besides an occa- 
sional dull greenish patch of chloritic material. A few 
quartz fragments occur, but the source of these may be the 
quartzite of the basement series, as several fragments were 
seen in one of the slides, composed of very small inter- 
locking grains of quartz. Most of these fragmental crystals 
show a feeble polarization, particularly the smaller ones. 
In one of the slides were seen several larger fragments of 
andesite, somewhat altered, but still fresh enough to show 
the twinning of the porphyritic feldspars. 

A section of the calcareous rock shows that it is appar- 
ently free from the angular fragments of the shaly speci- 
mens, while, as before, the mass of the rock appears to 
consist of organic remains. With crossed nicols the larger 
molds are seen to be filled with calcite. The ground-mass 
of the rock is not isotropic, but gives the delicate polariza- 
tion tints of calcite. With a high power the entire rock is 
seen to have a microcrystalline structure, being made up of 
irregular grains of calcite. This structure bears no rela- 
tion to the distribution of the organic material in the rock, 
except in the case of the Foraminifera. 

Character of the Organic Remains. A number of small 
fragments of the shale were forwarded by Prof. Lawson to 
Dr. George J. Hinde for examination. He has kindly 
placed his conclusions at Prof. Lawson's disposal, in the 
following note. 

"From small samples of these rocks sent over to me by 
Prof. A. C. Lawson I have prepared thin microscopic sec- 
tions where the material was sufficiently coherent to allow 
of such being made, and in the case of the very soft rocks 
the fine powder has been mounted just as it occurs, without 
washing away the finer debris. I have only aimed in the 
following notes at giving a general idea of the nature of the 
organisms of which the rocks are composed, for the task of 
recognizing even the genera present would prove too long 


and difficult to be undertaken, and moreover, the material 
at hand, though sufficient to show the general character of 
the organisms, would not be enough for determination of 
particular forms. 

11 No. 90. This soft, white, earthy rock is essentially 
diatomic in character. Both in section and in powder it is 
seen to consist of a mass of heterogeneously mingled frag- 
ments of diatom frustules, with a small proportion of com- 
plete forms. By far the larger mass of the rock is formed 
by the broken up and disintegrated particles of the diatoms, 
and the smallest and finest portions recognizable under the 
microscope are clearly organic debris. Coscinodiscus appears 
to be the predominant genus. Detached sponge spicules are 
fairly numerous. They are principally pin-shaped and 
styliform; also a few simple fusiform rods occur belonging 
to the Monactinellid division of siliceous sponges. The 
Tetractinellid sponges. are represented by a few fragmentary 
trifid spicules and globate forms. Only one or two some- 
what doubtful fragments of Radiolaria were noticed, and 
these organisms must have been very sparsely present, for 
their structures are stouter and more capable of preservation 
than the diatoms. The silica of these organisms diatoms 
and sponges appears to be unaltered in the fossilization 
it retains the same glassy aspect as in recent examples. In 
addition to the siliceous organisms, Foraminifera are like- 
wise present, and they yet retain the calcareous structure of 
their walls, though hardly so well preserved as in the case 
of the siliceous fossils. A rather large form of Textularia 
is the most common of the Foraminifera. It is to these 
organisms that the calcareous portions of the rock are due. 
The rock is very finely laminated, showing a series of well 
marked undisturbed layers of organic remains in which are 
scattered some minute angular chips of minerals here and 

"No. 118. A whitish, comparatively soft, earthy rock. 
Examined both in section and in powder. Very similar to 
the preceding in consisting nearly wholly of diatoms and 
diatomic debris. Coscinodiscus is very numerous; some 


forms relatively large. Sponge spicules are also present, 
but I could not certainly distinguish any Radiolaria. No 
Foraminifera to be seen in this specimen, and there was no 
reaction of the rock in acid. It is not unlikely that the 
calcareous organisms have been leached away, for minute 
empty pores can be seen in transverse sections of the beds. 
It has a lesser proportion of angular rock chips than the 
preceding (No. 90). 

" No. 147. Very soft, earthy white rock, readily break- 
ing up into fine flaky laminae. No reaction in acid. Like 
the preceding this is also nearly entirely a diatomic rock, 
but the diatoms are here of different forms, Melosira? and 
Grammatophora being most conspicuous. There are fair 
numbers of sponge spicules, usually broken; they are chiefly 
pin-shaped and styliform. Neither Foraminifera nor Radio- 
laria were recognizable in the material examined. The 
angular rock chips were fewer in this rock than in the pre- 
vious specimens (90 and 118). 

" No. I52. 1 A pale gray, hard rock just scratches with 
knife compact, flinty fracture, readily effervesces in acid. 
Examined in section only. It consists, like the soft rock 
above referred to (No. 90), mainly of diatoms and diatomic 
debris; the ground-mass of the rock is, as far as can be seen 
under the microscope, wholly of the broken up diatom 
frustules. Both the minute fragments and the entire forms 
are as unchanged as in the soft rocks. Coscinodiscus is 
abundant, also Navicula, Grammatophora and other forms. 
Some of the spaces between the diatom frustules have been 
infilled with calcite. There are a few Radiolaria present, 
spheroidal and discoidal forms, but their numbers are insig- 
nificant in comparison with the diatoms. Sponge spicules 
are apparently absent. Foraminifera are fairly common and 
well preserved, showing their wall structures; the most 
abundant is a large species of Textularia, probably the same 
form as that in No. 90. The interiors of the Foraminifera 
have been infilled with calcite. Angular chips hardly to be 
seen in the sections of this rock. 

1 No. 152 is the limestone, the analysis of which is given later, p. 50. 


" No. 89. Pale gray or cream-tinted hard rock just 
scratches with knife no action in acid. The section 
examined showed numerous minute pores, but whether these 
indicated spaces where organisms had been is doubtful. No 
organisms could be recognized in this rock, which, never- 
theless, appears to be of opalized silica. A few angular 
chips could be distinguished in polarized light. 

" With the exception of this last specimen, the siliceous 
and silico-calcareous rocks of the island of Santa Catalina 
are remarkable for the very slight amount of alteration 
which the structures of the siliceous and calcareous organ- 
isms have undergone in the fossilization. Both the most 
delicate diatoms and the Foraminifera occur in these beds 
together, in nearly as well preserved condition as in deposits 
now forming. The beds may well be compared with recent 
diatomic oozes, and, as in these latter, there is a small per- 
centage of sponge spicules, Radiolaria and Foraminifera 
mingled with the prevailing diatoms. The paucity of Radi- 
olaria in the beds is a peculiar feature. Sections of these 
rocks show very distinctly that the entire material, down to 
the smallest particles, is of organic remains mostly now 
broken up, for the proportion of perfect forms is small 
compared with the large quantity of fragmental debris. 
The amount of the foreign angular mineral particles is insig- 

Chemical Characters. This note would seem to leave no 
room for question as to the organic origin of the shale, but 
that the point might be considered from all sides, a chemical 
determination was made of the amount of soluble silica in the 
most earthy and least calcareous of the specimens. About a 
a gram of the roughly powdered material was used, in a ten 
per cent, solution of potassium hydrate. For the piirpose of 
comparison specimens of pumice and nearly pure volcanic 
ash were taken and subjected to the same treatment as the 
shale. All the material was well dried at 100 (C) before 
weighing and adding to the solution. The solutions were 
brought to boiling twice, being allowed to stand some hours 
in the interval, and for about a day after the second heating. 


The residues were then filtered off, and the silica was precip- 
itated in the filtrate by acidifying with hydrochloric acid, and 
evaporating to dryness. The weight of the silica obtained 
by this process was compared with the weight of the resi- 
dues, and except for the shale they all tallied very closely. 
All lost some silica, and on the addition of ammonia after 
the precipitation of the silica a slight amount of alumina 
was precipitated in all the solutions, showing that the alumina 
in the rock was acted on to some extent. The same test 
applied to the potassium hydrate (which occasionally con- 
tains alumina) gave no precipitate. The results showed 
that the pumice had lost 3.2 per cent, of silica, the volcanic 
ash 4.2 per cent., while the shale had lost 70.3 per cent. 
The powdered residue from this shale was subjected to- 
microscopic examination, and with the higher powers was 
found to contain a large percentage of minute crystalline 
fragments. Nothing could be made of the isotropic material 
of the residue. A considerable amount of the residue 
thrown into dilute acid produces momentary effervescence, 
showing that a part of it is calcareous, doubtless organic 

Origin of the Shale. These results show that the shale is 
largely composed of opaline silica, and, together with the 
statement of Dr. Hinde, are sufficient to disprove, for this 
region at least, the hypothesis tentatively advanced by Prof. 
Lawson 1 that the Miocene shale of the coast of California is 
largely of volcanic origin. That this shale is a part of the 
same Miocene shale which is found so extensively developed 
along the coast, there can be little doubt, although the proof 
obtained is not positive. It has a similar appearance, presents 
the same variations, contains abundant micro-organisms (a 
characteristic feature of the Miocene shales), while the 
occurrence of fish scales adds another link to the chain of 
evidence, as this is another marked characteristic of the 
coastal shales. Further, the Miocene shale occurs at San 

1 " The Geology of Carmelo Bay," by Andrew C. lyawson. Bull. Dept. Geol., Univ. 
Cal., Vol. i, pp. 24-26. 

(4) January n, 1897. 


Clemente Island, some twenty-five miles further south. 
If, then, these deposits are a part of the extensive Miocene 
shales, we cannot consider their development as in any way 
local, or as influenced by the deposits with which they are 
associated, for the conditions favorable for the development 
of the micro-organisms of the shale at that time must have 
been far reaching. Except for the few microscopic frag- 
ments, the shale as it occurs here is in general peculiarly 
free from the tufaceous material with which it is associated, 
indicating a considerable interval of quiet deposit, inter- 
rupted at times by violent local volcanic outbursts. Santa 
Catalina at this time was an island, as now, though doubtless 
separated from the mainland by a considerably greater 
expanse of water. The true relative attitude of island and 
mainland and the intermediate channel can be revealed 
only from the neighboring coastal formations. In addition 
to other evidence, the porous character of some of the 
lavas associated with the tuff and diatomaceous earth, and 
the remains of large pectens found in the tuff, indicate that 
these deposits could not have been formed in abyssal depths. 
It would appear from other reasons that they were laid down 
in not more than 600 or 1,000 feet of water. 

Analysis of Limestone. For the purpose of ascertaining 
the character of the limestone, a partial analysis was made 
by dissolving fragments in acid, and determining the amount 
of the constituents in the solution thus obtained. The 
results were as follows : 

Insoluble residue 8.234 

Fe 2 2 3 3 (alittle)} 2 ' 862 

CaO 27.944 

MgO 13.012 

Ignitionandj 43 6l5 

No determination of the alkalies was made. A qualita- 
tive examination was made for phosphoric acid, but no 
quantitative determination, though considerable was found 
to be present. The residue in this case also was examined 


microscopically. While the slide of the rock showed no 
fragmental material, this powder showed a very fewiscattered 
mineral fragments here and there, but even in this con- 
centrated form they are not so numerous as in the slides of 
the shaly specimens. Aside from these the residue is wholly 
isotropic and consists largely of the remains of diatoms, 
with occasional Radiolaria. It is interesting to note, in com- 
paring the chemical and microscopical characters of the 
rock, what a small percentage of organic remains is neces- 
sary to give a slide the appearance of being well filled with 

The results thus show that the rock is a magnesian lime- 
stone. Without doubt the carbonates are original and not 
secondary, their source probably being the sea-water in 
which the organic forms were laid down. 


Besides the beds at the isthmus, two minor deposits of 
sedimentary material were found, both within the main area 
of the andesite. One occurs in a saddle on the principal 
ridge to the west of Orizaba, at a distance of not more than 
a mile; the other, shown by a dotted surface on the map, is 
on the lower slopes of the andesite in the Little Harbor 

The first mentioned is only a small deposit in the lowest 
part of the saddle, extending not more than five or six feet 
up the slope on either side. The rock which forms the 
deposit proper is more or less open and is composed almost 
wholly of coarse shell fragments roughly cemented with 
secondary calcite. These fragments, though too small for 
a specific determination, appear to be the remains of large 
pectens. In addition to this, there were seen a number of 
rolled pebbles of andesite and porphyrite, and several large 
blocks of a white earthy material, all of which appear to 
have been deposited here. It is possible, however, that they 
were brought here by the Indians who formerly occupied 
the island, since there is unmistakable evidence that this 


saddle was one of their camping grounds. The specimens 
from the earthy blocks show a free effervescence with dilute 
acid, and contain in places rough, free, calcite crystals, 
several millimeters in diameter. 

The lower slopes of the andesite in the Little Harbor 
region, up to an altitude of six or seven hundred feet, are 
everywhere strewn with rolled pebbles of andesite, por- 
phyrite, and quartzite. Near the northern border of these 
lower levels there are two small deposits of white, earthy 
material. Along the southern border, on the ridge adjoin- 
ing Middle Ranch Canon, there is a considerable deposit of 
sandstone and conglomerate, and a little above this on the 
same slope another deposit of the earthy material. All the 
specimens of the latter rock wherever found effervesce 
freely with dilute acid. A very few rough shell casts were 
found in one of the areas of the earthy rock. The rock 
powder under the microscope showed no organic remains. 
The bulk of the powder gives the high polarization colors 
of calcite. Some of the specimens contain occasional small 
pebbles. In the coarser deposits there are all gradations, 
from conglomerate, with pebbles averaging one-half an 
inch in diameter, to a fine-grained, yellowish, micaceous 
sandstone. None of these effervesce with acids. A search 
for fossils revealed a few indeterminable shell-casts. These 
deposits of sandstone and conglomerate are in general 
thin, though at one point they reach a thickness of about 
fifteen feet. 


Beginning near the extreme southeastern point of the 
island, and extending along the coast to the northward, is a 
small area of quartzite breccia. As seen in the gulches it is, 
in part at least, bedded, the beds varying in the coarseness 
or fineness of their material. The coarser beds contain 
occasional large blocks two feet or more in diameter, and, 
rarely, reaching a length of several yards. The material 
composing this breccia, so far as can be made out with a 
lens, is wholly quartzite, except for occasional blocks and 


fragments of andesite seen near the upper part of the series. 
The bedding of the breccia is seen on the upper p^art of the 
cliffs at the northern end of the area as a rather coarse 
banding, approximately parallel to the shore-line. Toward 
the south the banding becomes somewhat irregular and is 
lost to view some time before the extreme point is reached. 
Here the breccia is seen at the base of the cliffs, and so far 
as could be determined it is, in part at least, included in the 
porphyrite (see Plate III, fig. i) which occurs here on the 
cliffs just above. 

This porphyrite is a white, much weathered rock, and it 
is possible that it occurs here as a dike of considerable size, 
or as an intrusive sheet, and does not belong to the main 
occurrence of this rock. At any rate it is in some respects 
unlike the porphyrite as it usually occurs. Within the area 
just described the porphyrite outcrops along the shore at one 
other point, at least, where the rock is to all appearances 
like that of the main area. The breccia at the point of the 
island is cut by a dike of greenish porphyrite about two 
feet wide, which also cuts the white porphyrite mentioned 

About midway between Pebbly Beach and the extremity 
of the island there is a small beach at the base of the cliffs, 
which is partly made up of boulders and smaller masses of 
a conglomerate resembling the breccia in the material of 
which it is composed. This has apparently fallen from the 
cliffs above, although no rounded material was anywhere 
seen in place by the writer. So far as observed, these 
boulders are composed of water-worn metamorphic pebbles, 
imbedded in a large amount of compact, greenish cement. 
This cement shows a marked effervescence with dilute acid, 
and under the microscope it is seen to be composed in large 
part of angular fragments of quartz and quartzite, in a thin 
cement which is largely calcite. Several small sections 
were seen, closely resembling the porphyrite in appearance, 
and containing porphyritic feldspars, but much altered by 
decomposition products. 

The observations made were too limited to prove conclu- 


sively the relations of the breccia to the main body of the 
porphyrite and to the andesite, but some of the evidence 
points to the probability of its being older than either. 


In surface area the rocks of the basement series cover a 
little more than half the island. They consist mainly of 
quartzites and mica -schists, with several smaller areas of 
talc- and amphibole-schists and serpentine. 

The occurrences of the other basement rocks within the 
quartzite area were not exactly mapped, and for that reason 
they appear on the map without definite boundaries. The 
actinolite areas, in particular, are more extensive than is 
here indicated. The main occurrences of these rocks are as 
indicated, but smaller areas of all of them occur elsewhere 
within the quartzite area. The actinolite-schists, besides 
their main occurrence, are found in the area of the quartzite 
about Middle Ranch Canon. The areas of the basement 
rocks found within the porphyrite area northwest of Avalon 
are in part of actinolite- and talc-schists, with some ser- 
pentine. Talc-schist is also found near the center of the 
west end, toward the northern coast. Besides the ser- 
pentine areas mapped, a small patch was found on the ridge 
to the south of Middle Ranch Canon, and another not far 
from the extreme northwestern point of the island. A 
patch of garnet amphibolite is found just to the west of the 
border of the andesite in the Little Harbor region. 


The quartzite occurs distinctly bedded, and wherever it 
was possible observations for dip and strike were made, 
though these were insufficient to warrant a statement with 
regard to the beds as a whole. In many places these read- 
ings give no real indication of the general dip and strike of 
the series, owing to local folding with minor plications, and 
to occasional faulting. By a comparison of the various 
readings made, however, the western division of the island 


appears to have a synclinal structure. Whether this is true 
for the division to the southeast of the isthmus_ cannot be 

An excellent cliff section showing the general stratifica- 
tion is seen on the southern coast of the west end, where 
the bedding is distinctly visible for three or four miles along 
the shore. For the greater part of the distance the dip of 
the beds is quite uniform, though the minor beds and sheets 
observed show intricate folding and crumpling. The dip 
ranges from S. 15 E. to S. 45 E., at an angle varying 
from 15 to about 30. 

On the northern coast of this part of the island the dip, 
so far as observed, is northerly, and varies considerably in 
amount, the average lying between 25 and 50. A char- 
acteristic section of the bedding is shown in Plate III, fig. i, 
a view of the shore at the north end of the beach of Cherry 
Valley, the second small bay to the north of Isthmus Cove. 

The quartzites are nearly everywhere intersected by nu- 
merous veins of secondary quartz, usually of small size and 
running in various directions. In places, however, these 
veins attain a width of a foot or more. At a number of 
points some of the veins contain a small percentage of min- 
eral ores. 

The quartzite is usually bedded in thin and more or less 
irregular sheets. They range from a fraction of an inch to 
two or three inches in thickness, averaging perhaps half an 
inch. These sheets are usually separated by partings of a 
dark earthy character, varying in thickness from the thin- 
nest film to about a quarter of an inch. In the more thinly 
bedded quartzite these partings are frequently thicker than 
the quartzite sheets. 

Macroscopic Characters. The quartzite is occasionally 
milky white; usually, however, as seen with a lens, it ap- 
pears colorless and glassy. Rarely it is found black, while 
here and there it occurs with a tinge of pink, or even con- 
siderably reddened, owing to the presence of minute gar- 
nets, either scattered through the sheet or arranged in bands 


parallel to the bedding. Many of these quartzite sheets 
appear to be wholly free from mica, the surface of the sheet 
glistening from the minute quartz crystals composing the 
rock. Other specimens, in cross-section, appear to be of 
clear quartz, but when viewed in the plane of the bedding 
numerous minute scales of muscovite are seen scattered 
over the surface. In all fractured surfaces these flakes are 
seen to be arranged with their planes parallel to the plane 
of the bedding. There are all gradations between this and 
specimens in which the mica is the most prominent mineral. 

The layers which form the partings of the quartzite beds 
are quite dark, varying from a dark gray to a yellowish or 
reddish color due to iron stain. They are finely schistose 
and readily flake off; are quite soft and have usually a 
smooth, silvery surface. Even where this silvery luster is 
not at first apparent it may easily be made out with a lens. 
The layers appear to be composed of mica or its decompo- 
sition products. Tested chemically the mineral shows the 
presence of a large amount of alumina, a little iron, no lime 
and a little magnesia, besides giving a decided flame reac- 
tion for potassium. The optical characters of the flakes 
could not be determined, owing to their want of transpar- 

A considerable proportion of the rocks of the west end 
have much the appearance of gray sandstone to the unaided 
eye, though with a lens they are seen to be composed 
largely of this micaceous material, with minute lenses or 
grains of the quartzite scattered through it. These mica- 
schists occur indiscriminately with the rocks which are 
more properly quartzites, and occasionally lens -shaped 
masses of the quartzite are found in such areas. There 
are all gradations between these rocks in which the mica is 
predominant and those in which the quartz predominates. 

Besides the micaceous partings of the quartzites there 
were found at a number of points partings of blue amphi- 
bole, having frequently a silky luster. This amphibole also 
occurs in larger masses in a schistose condition. The oc- 
currences of this rock were not mapped, but they are found 


particularly in the Little Harbor region, apparently confined 
to the neighborhood of the areas of the amphibole- and talc- 
schists and serpentine. It is probable that here, as else- 
where in California, these blue amphibole-schists are due to 
local contact metamorphism occasioned by the intrusion of 
basic irruptives. 1 

Microscopic Characters. Only one slide was made of 
the quartzites, which, however, is doubtless typical of the 
purer quartzites in general. It consists almost entirely 
of a mosaic of clear quartz grains of irregular shape and 
size. Many of them are flattened in a direction parallel to 
the plane of schistosity, thus giving frequently very much 
elongated sections. Their boundaries interlock in an ex- 
tremely intricate manner. Occasional pale pink garnets 
occur as inclusions in the quartz, averaging a little less than 
.1 mm. in diameter. They are for the most part rounded, 
though two or three present crystal boundaries. Long nar- 
row sections of what is probably sillimanite are compara- 
tively numerous, nearly all arranged with their longer axes 
parallel to the plane of schistosity or to the direction of the 
flattening of the quartz grains. The terminals taper more 
or less gradually to a point. No cross -sections were seen. 
The mineral is colorless and has a moderately high index 
of refraction, somewhat higher than that of quartz, and it 
may therefore be readily distinguished from the latter in 
ordinary light. The double refraction is considerable, giv- 
ing brilliant, though somewhat mottled, polarization colors. 
The extinction is in all cases parallel and perpendicular to 
the longer axis of the mineral. In all the sections observed 
the longer axis is the axis of less elasticity as shown by the 
quartz wedge. No optical figure was obtained. 

1 " A Contribution to the Geology of the Coast Ranges," by Andrew C. Lawson. Am. 
Geol., Vol. XV (June, 1895), p. 352. 

" The Geology of Angel Island," by F. Leslie Raiisome. Bull. Dept. Geol., Univ. Cal., 
Vol. i, No 7. 



The actinolite-schist occurs bedded, showing greater 
variation than the quartzite in the thickness of the beds. It 
also frequently exhibits plications such as occur in the 
quartzite. Some of these schists occur in rather thin 
beds, with a finely schistose structure, the slender needles 
of actinolite parallel to the plane of schistosity. In other 
cases, especially the coarser forms of the rock, it is 
found showing no marked schistose arrangement. The 
crystals in these coarser schists frequently have a length of 
three or four centimeters. The rocks are more or less 
compact, and in general are composed of columnar or aci- 
cular actinolite crystals, but always associated with a greater 
or less amount of other minerals. The most common 
mineral accompanying the actinolite is talc. This is usually 
in small amounts, but rarely it becomes the dominant min- 
eral, forming a matrix in which the needles of actinolite are 
embedded. Chlorite occasionally occurs with the actinolite, 
and like the talc, this sometimes, though rarely, becomes 
dominant. The chlorite varies in occurrence from minute 
flakes to plates several centimeters in diameter. A small 
amount of quartz is frequently found in these schists, and 
occasionally both quartz and feldspar, in varying amounts, 
occur associated with the actinolite and hornblende. 

Almost the entire area of actinolite- and hornblende-schist 
is composed of the former. The latter is confined to the 
area which contains the serpentine, occurring here with the 
actinolite-schist. The rocks are coarse-grained, compact, 
greenish black in color, and are composed of coarsely pris- 
matic crystalline hornblende. A small amount of mica is 
occasionally associated with it. 


The serpentine of this same area is found on the summits 
of these hills of amphibole-schist. The hills are in the 
neighborhood of 1,000 feet in height, and the serpentine 
which outcrops here is two or three hundred feet in thick- 


ness. It occurs stratiform, with an average dip of from 20 
to 30 in a northerly direction. The rocks are very hard and 
compact, and in weathering present an extremely rough 
surface, with projecting fragments, many of which have 
sharp, jagged points. It is doubtless owing to this bold, 
irregular surface that one of these hills has received the 
name of Granite Peak. The surface of this rock is also 
irregularly pitted. The occurences are almost wholly of 
this facies, and little evidence was seen of internal move- 
ment, causing a slickensided appearance. The general 
appearance of the rock in the field is in most respects quite 
unlike that of the serpentine of the Potrero, San Francisco, 
described by Dr. Palache, 1 which is typical of much of the 
serpentine of the Coast Range. There are a few small 
patches of magnesite within the serpentine area. The hand- 
specimens of the serpentine vary in color from a dirty greenish 
white to a dark bluish green, more or less mottled with 
limonite. The compact specimens show an indistinctly 
banded structure, and have a rather uneven fracture. This 
surface is entirely different from the smooth and somewhat 
polished surface of the pale green, slickensided specimens. 
Traversing the surface in lines approximately parallel to the 
banding are occasional fine veins and threads of chrysotile, 
with their fibres at right angles to the enclosing walls, and 
stained here and there with iron. More numerous and finer 
threads cross the surface at right angles to the larger veins, 
and nearly all are stained with limonite. Threads of mag- 
netite run through the rock, in no fixed direction. In some 
places the rocks contain many minute veins of secondary 
silica, running at right angles to the banding. Cross-sections 
seen on the surface show that they are filled with the silica 
arranged in concentric rings. No remnants of the minerals 
from which the serpentine was derived were seen in any of 
the specimens, but it doubtless consisted in large part of 
olivine, for the mesh-structure characteristic of the serpen- 

^'The Lherzolite-Serpentine and Associated Rocks of the Potrero, San Francisco." 
Bull. Dept. Geol., Univ. Cal., Vol. i, No. 5, pp. 161-179. 


tines so derived is seen throughout the greater part of the 

Areas of a somewhat different fades of the serpentine 
occur within the talc and garnet-amphibolite area. This 
rock is hard, compact, occurring massive, and not strati- 
form. As in the serpentines just described, minute veins 
of silky chrysotile traverse it here and there. The rock is 
very dark green, and scattered through it are aggregates of 
a magnesian mineral, with pearly luster, whose optical prop- 
erties were not investigated. Besides this mineral, there 
occurs in various amounts, associated with the compact ser- 
pentine, a pale green, lamellar mineral with the optical 
properties of bastite. 


The area in which this serpentine is found is largely of 
talc-schist, usually found as a soft, foliated rock, stained 
yellow with limonite. It has a silvery luster, and when 
looked at closely the talc is seen to be of a pale green color. 
It is quite smooth, with a greasy feel, and is easily scratched 
with the nail. The rock splits readily along the schistose 
surfaces. Near the western end of the area, back of 
Empire Landing, there is a soapstone quarry where is found 
a facies of the schist, which but little resembles the foliated 
form just described. This is massive, not schistose, and 
has a dark gray color with a tinge of green. The rock 
may be scratched with the nail only in places, showing that 
it is not wholly talc. The chief difference between this 
rock and the foliated schist is the presence everywhere 
through it of a mineral with a pronounced lamellar struc- 
ture, occurring in moderately small, bladed forms, which 
are interlaced in all directions. This mineral appears to be 
the chief constituent of the rock, and at least equal to the talc 
in amount. It is pale green in color, with a metalloidal luster, 
and a hardness of about 4. Before the blowpipe it gives 
the characteristic reactions of serpentine. With a lens the 
silvery flakes of talc may be seen here and there, besides 
scattered grains of pyrite. 


Under the microscope the rock is seen to be composed in 
part of an allotriomorphic aggregate of bastite, and partly 
of irregular areas of talc, with several small patches of 
magnesite. Small amounts of pyrite are scattered through 
the slide. 

The bastite occurs in plates or somewhat lath-shaped forms, 
and is colorless or with the faintest tinge of green. The 
mineral is non-pleochroic, even in moderately thick cleavage 
flakes. It has a pronounced fibration parallel to the vertical 
axis. Its extinction is characteristic of a rhombic mineral, 
being in all cases parallel to this fibration. It has a low 
index of refraction, and gives low interference colors, much 
like those of feldspar. The cleavage flakes show the fibra- 
tion which is observed in thin section. Rarely a needle of 
pyrite is seen in the fibration. Cleavage flakes give a good 
biaxial interference figure, and show that the plane of the 
optical axes is at right angles to the plane of cleavage and 
parallel to the fibration. The optical character of the min- 
eral is negative, as determined both by the mica plate and 
quartz wedge. The bastite is everywhere altering to talc, 
and all stages of the process may be seen. Alteration begins 
along the margin and along the cleavage planes, and works 
inward. Occasionally the talc occurs as a pseudomorph 
after the bastite, giving a parallel extinction, owing to a par- 
allel arrangement of the fibres of the talc. Usually, how- 
ever, the talc occurs in patches of irregular shape, and with- 
out a definite extinction throughout an entire revolution of 
the stage, owing to the compensatory effect of the irregu- 
larly oriented talc fibres. 


No detailed petrographical study was made of the serpen- 
tine rocks of the island, but such as was made proves them 
to be variable in their microscopic structure, and therefore 
different in their origin. At no point was there seen any of 
the unaltered rock from which the serpentine was derived, 
so that the conclusions must be drawn from the microscopic 


structure of the serpentine itself. Judging from this, the 
serpentines may be roughly divided into three groups accord- 
ing to their probable origin: (i) those derived from pyrox- 
enites, (2) those from rocks composed largely of olivine, 
and (3) those from a rock in which both rhombic pyroxene 
and olivine were among the essential constituents. The 
first are now characterized by the bastite structure, the 
second by the mesh-structure. It is probable that the whole 
of the talc-schist is derived from the first form of serpen- 


Along the ridge near the upper limit of the talc-schists, 
and within that area, are found here and there small, pro- 
jecting bosses, with occasional larger areas, of garnet-am- 
phibolite. This rock usually presents a somewhat roughened 
surface, more or less reddened with iron oxide. It is not 
compact, and readily crumbles under the hammer. The 
fresher material is dark or almost black in color, and appears 
to be composed wholly of a brownish or greenish hornblende, 
with roughly rounded red garnets in varying size and 
amount. In some places these garnets attain a diameter of 
about 3 mm. and form the principal feature of the rock, 
while in other cases the rock is composed almost entirely of 
a somewhat fine-grained hornblende, and an occasional 
minute garnet may be made out only with the aid of a lens. 
At a few points the rock occurs as a black, rather coarsely 
granular aggregate, composed entirely of hornblende, so far 
as can be determined with a lens. 

A slide was made of the facies of the rock containing 
the largest garnets. There are nine of these garnets in the 
slide, ranging from 2 to 3 mm. in diameter. Microscopically 
the rock is composed of scattered, pale pink garnets in a 
matrix of hornblende. Here and there are small grains of 
rutile. The hornblende is brownish with a tinge of green, 
and occurs in allotriomorphic plates, with seldom a hint of 
crystallographic form. The boundaries are usually well 
marked by a limonite stain. The sections themselves are 


quite fresh and free from products of decomposition. The 
mineral has a pronounced prismatic cleavage. _It is quite 
strongly pleochroic, c being dark, greenish brown, 6 deep, 
yellowish brown, and a very pale, brownish green. The 
absorption scheme is c^l6>a. Inclusions are common, and 
in many of the sections abundant. They are largely minute 
flakes of a mineral with low polarization colors, and a 
refractive index somewhat higher than that of the horn- 
blende. The same mineral occurs in scattered flakes in the 
garnets also, and they are there seen to be colorless or 
nearly so. In the hornblende these flakes are in small, 
open areas, usually collected near the center of the includ- 
ing crystal. Besides these inclusions, occasional small 
grains of rutile are found. 

The rutile, in general, through the slide, occurs as rounded 
and usually oblong grains, in color deep yellowish to red- 
dish brown, varying with the tints of amber. These grains 
are usually found along the lines which mark the bounda- 
ries between the hornblendes, and generally several together 
occur along the same line. They have an extremely high 
index of refraction, and on account of the consequent dif- 
fusion of light the extinctions are not sharp and clear. The 
direction of extinction in the grains which are distinctly 
elongated is parallel and at right angles to the axis of elon- 
gation. The mineral shows a pronounced though not 
strong pleochroism, and has a strong absorption. As the 
grains could not be distinguished from the dark hornblende 
in the crushed rock, the hornblende was dissolved out by 
means of hydrofluoric acid, when the minute, dark grains 
of the rutile could be readily distinguished from the pale 
red fragments of garnet, neither of these being attacked by 
the acid. Some of these grains were then separated and 
tested for titanium, with favorable results. 

The garnets have quite irregular boundaries, and along 
the margin are frequently intergrown with the hornblendes 
which surround them. In a number of cases minute frag- 
ments of the garnets are completely enclosed by the border- 
ing hornblendes, while occasionally a fragment of horn- 


blende is wholly surrounded by the garnet near its margin. 
Frequent cracks, many of them iron -stained, intersect the 
garnets, without definite direction. Macroscopically the 
garnets appear to have a zonal structure, with a narrow and 
somewhat clouded outer zone, a broad middle zone, seem- 
ingly of the clear pink garnet, and a slightly darker inner 
zone. Under the microscope this structure is seen to be 
due to inclusions in the garnet. With crossed nicols the 
sections are, as a whole, not perfectly isotropic, but trans- 
mit a faint light in all positions. This is due to a multitude 
of microscopic, dust-like inclusions, which fill the central 
portions of the garnets. With higher powers these inclu- 
sions cannot be resolved, but are seen to be of some rather 
brightly polarizing mineral. They do not always occur in 
solid areas, for portions of the space are free from them, 
these isotropic portions running like veins through the mass. 
These areas fill the greater part of the sections, but there is 
always a narrow, irregular band along the margin which is 
free from these minute inclusions, and is isotropic under 
crossed nicols. The darker, central areas appear to be due 
to a clouding, the nature of which could not be determined. 
Besides these minute inclusions there are others scattered 
through the slide, which have been mentioned in connection 
with the hornblende. Some of the garnets contain here and 
there, particularly along the isotropic borders, minute needles 
of a yellowish to brownish mineral, with parallel extinction 
and high refractive index, and giving high polarization col- 
ors. This mineral is probably rutile. A few of these nee- 
dles were seen in some of the hornblendes bordering the 
garnets in which the inclusions are found, and some of the 
needles were seen extending from the one mineral into the 
other. There are rarely inclusions of large grains of rutile 
and small hornblendes. 




The submarine contours surrounding the island have been 
represented for depths of 200, 300, 400 and 600 feet. The 
discussion of the results arrived at by a study of this feature 
of the topography has been left till this point, as these re- 
sults are so closely connected with the geological history of 
the island. By mapping in the deeper contours, it is seen 
that the general form of the island is preserved to a depth 
of at least 1,800 feet, and doubtless somewhat beyond this, 
though the indications are that the pronounced trench out- 
side Little Harbor gradually loses its character, so that at 
some greater depth the outline of the entire mass may be 
much simpler. 

In looking at the map it will be noticed that the average 
distance from the shore to the 200 feet contour is much less 
than the average distance from the 200 feet to the 400 feet 
contour. This is particularly marked in those parts of the 
island where the cliff cutting is the most rapid. By map- 
ping in the contours on the large Coast Survey map of the 
isthmus these features are strikingly brought out. Here, 
since there is more detail, it is readily seen that the more 
rapid deepening of the water near the shore extends to about 
250 feet, and to this level the details of the present outline 
are fairly well preserved. Beyond the 250 feet contour 
there is a broad platform with a very gentle outward slope 
(of about i) to some point beyond the 300 feet level. 

Beyond the 400 feet contour the water deepens rapidly 
on the southern side of the island, while on the north the 
widely separated contours indicate a gradual slope. The 
pronounced difference between the two sides is well shown 
in the accompanying sections (figs. 5 and 6) , which were 
chosen as most fairly representing the average character of 
the two sides respectively. The first is the section along 
a line at right angles to the outermost point north of Whit- 
ley's Cove; the other, along a line at right angles to the 

(5) January 12, 1897. 



shore on the opposite side of the island, at a point to the 
southwest of the most southern occurrence of the andesite 
on the map. These sections suggest the possible origin of 
the island as a tilted orographic block, the rapid descent on 
the southern side contrasting strongly with the moderate 
slope on the other. The contrast is similar to that of sec- 
tions taken on opposite sides of San Clemente Island which 
is almost certainly such a block. 1 The platform mentioned 

FIGURE 5 Submarine profile, north side of Santa Catalina. 

FIGURE 6 Submarine profile, south side of Santa Catalina. 

as occurring above the 350 feet contour is well brought out in 
the section from the southern side, though the slight increase 
in slope above the 200 feet contour is not marked. The 
features make it clear that before the present sinking of 
the island began it stood some 350 feet higher than now. 
That this platform is later than the andesite is seen from the 
fact that it has been cut in the tufaceous deposits at the 
isthmus. The island stood for some time at or near that 
level, while rapid cutting was going on both along the cliffs 
on the most exposed sides, and in the softer tufaceous 
deposits near the isthmus, where the erosion of the harder 
rocks is comparatively slow. When the island had been 

1 "The Post-Pliocene Diastrophism of the Coast of Southern California," by Andrew 
C. Lawson. Bull. Dept. Geol., Univ. Cal., Vol. i, No. 4, p. 129. 


reduced to the form approximately shown by the 200 feet 
contour the present sinking began. This is_ shown not 
only by the increase of the submarine slope, but also in the 
isthmus chart, by the preservation of the main features of 
the present drainage system, showing that the recent stream- 
valley flooding took place at that level. The information 
on this point obtained from this latter source, however, is 
reliable only within certain limits, as a rapid, partial or com- 
plete filling up of these sunken channels in their lower 
levels might easily cause an error in the interpretation of 
the facts. But, taken in connection with the other evi- 
dence, they may be considered as trustworthy to a certain 
extent at least. 


The alteration and deformation of the basement rocks of 
Santa Catalina probably took place before the individualiza- 
tion of the mass now forming the island. In the opinion of 
the writer, the history of Santa Catalina began with the 
tilting of an orographic block formed of the already altered 
basement rocks. This view is based on the character of 
the submarine contours and the slight recent tilting shown 
in the slope of the summits (see figs. 2 and 3, page 7). 
The tilting was no doubt gradual, and has continued inter- 
mittently to comparatively recent times. This is shown in 
connection with the figures just referred to, as it was pointed 
out that the angle of the slope of the crest is about i from 
the horizontal, and in the direction of tilting toward the 
north. No further evidence of the original crust-block is 
seen on the land, owing to the extensive erosion to which 
the mass has since been subjected. 

The time of the original tilting is not known, but the 
crust-block must have been at that time a part of the main- 
land. By long continued erosion the crest of the mass was 
carried northward so that it occupied a position now approx- 
imately represented by the main ridge from Whitley's Cove 
to the west end. Following this came the irruption of the 


porphyrite laccolite, possibly preceded by a further tilting 
of the block. This irruption led to the formation of a struc- 
tural valley in the Little Harbor region, between the por- 
phyrite area and the ridge just mentioned. This valley was 
subsequently enlarged and deepened by an extensive ero- 
sion which followed. At this time Catalina probably stood 
some two or three thousand feet higher than now. The 
mass then had the general form of two long ridges, the one 
already referred to, and another having the general trend of 
the porphyrite area as seen on the map, and being possibly 
connected with the former ridge not far from its eastern 
end. The drainage of the large valley just mentioned was 
to the west. Its remnants still exist on the island, forming 
the amphitheater of the Little Harbor region. 

This period of erosion was followed by the eruption at 
intervals of andesite, which completely filled a portion of 
this valley and covered the adjacent ridges. The source of 
these outpours appears to have been local. They were 
accompanied by a slow settling of the land area to which 
this mass then belonged, and Santa Catalina became an island, 
probably for the first time in its history. The evidence 
shows that it has remained an island ever since. That it 
was sinking at this time is shown by the deposits of tuff 
intercalated with the lavas. 

This submergence continued after the andesite flows had 
ceased, for the higher portions of the andesite were some- 
what eroded before the island had reached its lowest level, 
as is shown by the fact that the shelly deposit near Orizaba 
(see page 51) lies in a saddle several hundred feet below 
the peaks bordering it on either side. The amount of this 
depression was between 1,400 and 1,600 feet below the level 
at which the island now stands. That it was at least as 
great as this is shown by this same shelly deposit, which 
occurs at an elevation of about 1,360 feet; and that it was 
not greater is shown by the base-levelled summits of the 
island at an elevation of from 1,400 to 1,600 feet. This 
took place during Miocene times, as the deposits of shale 
near the isthmus bear witness. This submergence may 


have been sufficient to form two islands of the mass, the 
channel between them extending from a little to the west of 
the isthmus, three or four miles to the east. This is based 
on the fact that between these points the main ridge falls 
considerably below the 1,400 ft. level. It may be, however, 
that this decrease in altitude is a part of the local depression 
hereafter suggested in connection with the isthmus. 

The submergence was followed by a long period of ero- 
sion, during which the then existing island (or islands) was 
reduced to the peneplain condition. The main body was a 
low and nearly level area, above which, near the center, 
projected the higher andesitic peaks. This area contained 
a bay of considerable size, occupying the Little Harbor 
region. The reduction of the island to a peneplain was 
followed by an elevation, the amount of which is approxi- 
mately indicated by the 350 feet submarine contour, thus 
making the altitude of the peneplain, roughly, 1,850 
feet. This movement was gradual, and was interrupted 
by at least one pause, at an elevation of 600 or 700 feet 
above the present sea-level. This is shown by the levelled 
slopes in the lower portion of the Little Harbor region, and 
by the sedimentary deposits found on these slopes. The 
island remained at its highest level long enough to carve the 
broad submarine bench on the most exposed side. A very 
slow subsidence may have taken place at this time. It was 
followed by the present period of comparatively rapid sink- 

This most recent period has been a short one, as is shown 
by the small amount of cliff cutting, which has taken place 
since it began, on those parts of the island most exposed to 
wave action. It was during the period of rapid submergence 
that the stream valleys of the present drainage system were 
flooded in their lower portions (see Plate III, fig. i, and 
fig. i, page 4). For while the broad submarine platform 
was being carved about the island, whatever subsidence 
there may have been was not too rapid for the cliff cutting 
easily to keep pace with it. Thus no valley drowning could 
take place, and no trace of buried channels or sunken 


valleys (belonging to the present drainage system) is found, 
in general, below a depth of 250 feet. 

The recent tilting of the island, which has been mentioned, 
appears to have occurred largely if not wholly during the 
island's emergence after its reduction to a peneplain. For 
the constancy of the depth of the more recent submarine 
features clearly shows that their relative attitude cannot have 
been appreciably altered since the time of their formation, 
and therefore that the tilting must have preceded this in 
greater part, at least. To this recent differential elevation 
is due, in part at least, the long, narrow channels of the 
southern side of the island, as contrasted with the open valleys 
on the north; though these are doubtless due in part, also, 
to the more rapid cliff cutting on the southern coast. 
. The present drainage system of the island was begun at 
the time of the last rise, after the formation of the pene- 
plain. This peneplain has since been deeply dissected and 
eroded, till only the roughly levelled summits of the ridges 
remain to mark its former existence. Sufficient time has 
elapsed since the streams began their work for the gorge of 
Silver Canon to be cut down through more than 1,400 feet 
of rock, while in the same time the broader valley back of 
Avalon has been excavated and its slopes minutely carved. 
The topography, then, is by no means young, but it has not 
passed its prime. The submergence and rapid cliff reces- 
sion tend to preserve the youthful appearance of the island, 
by shortening the stream channels, thus increasing their 
grade and causing the streams to continue their sharp, 
incisive cutting. To such a cause is due the dissection of 
the alluvial fan back of Avalon. 

The isthmus is a particularly interesting feature of the 
island, for the mass is nearly separated at this point. A 
very slight further subsidence would be sufficient to form 
two islands. That the isthmus once formed a watershed, 
which separated the two stream valleys to the north and 
south, there can be no doubt. These drowned valleys now 
form the harbors on either side, and constitute the most 
pronounced example of valley drowning on the island. The 


drainage into them was principally from the tributary canons. 
The valleys were shallow, with only a gentle grade from the 
divide to their mouths, so that a comparatively slight sub- 
sidence has almost completely drowned them. The divide 
of the isthmus was at one time somewhat lower than at 
present, the pass having been filled in to a certain extent by 
alluvial deposits from the neighboring slopes. 

Although the harbors at the isthmus conform to the types 
of the present stream topography, we cannot suppose that 
the isthmus itself has been formed wholly by steam erosion 
during the present topographic cycle. The break in the 
continuity of the mass, which is found at this point, is too 
sudden and complete to be considered as due to the forces 
of erosion alone, in view of the fact that no such effect has 
been produced in any other portion of the island. The 
origin of the isthmus must be otherwise explained. The 
most reasonable hypothesis is that of a local sag at this 
point. This is borne out by the sudden change in the dip 
of the bed of tuff and diatomaceous earth, as it approaches 
Isthmus Cove (shown in Section A on the map). If this is 
due to a local depression, that depression must have occurred 
before the island had reached the highest point in its last 
rise, and after the deposition of the tuff and shale. The 
submarine platform at this point shows no apparent depres- 
sion, so that any sag which there may have been must have 
taken place before the platform was carved. 

In conclusion it must be said that the writer's work upon 
the island was, owing to limited time, necessarily incom- 
plete, and many details remain for future investigation. 

The writer wishes to express here his gratitude to Prof. 
Lawson for his kindness in giving advice and assistance 
throughout the work. Acknowledgements are also due to 
Dr. J. C. Merriam. 

Geological Laboratory, 

University of California, Oct. ist, 1896. 










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