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WAN AaAny, alana” | AAA ARAAR naan NNW Mnaan, va, AA AAA AA aM Ann, Ar ANAnAnrAn Acne aA AMAA anna annnnnnnnnn siposan eacacene Ann Ee nnn ar VAAAR AAA anne AANAA ama nnann Annan mannan aaa mAAAMOD VV a NY pa seenana . al ale AAR A ry pia) AnaaAanan AAAAAR Aaa rAnouAMAAhannanaesaga AAAAAAs a AAAs A “A ARAROP A: NEN BS ZR Fm vat aN AM ARAAR ee: AAAA MAAAARAAcAAAAARADE Ae a aye ax roe Co * YRenans NAAR : ASsaan ath AA AANA a, ay nall Ae a, WY ANAK | AAsa AAs SA MAAMAAMMANR AA nrwmannancrnnnntcne Anais ADARANAAA BAR ANAN AAAS An AMA b ay a aaa VA aan IANA ACAAAL AO AAO WANA ain On Annan Ap Uasasaaassapeaaaracascsd eee AANAAAAAAANAAA A é i AAA ac Annan, alae ~ AP AAA ASARAC AOA AA DANAAA AR AAA AAPA Wg nennnen in WinnAnaa Fete nana ano nnas AWW ANNA nanny nan ees uss She ieeea AAAMANAAAMARSA ee” DEPP D ON Anan aes aa , THE AMERICAN NATURALS VoL. xu. — FUNE, 1879. — No. 6. PROF. HARTT ON THE BRAZILIAN SANDSTONE REEFS. BY RICHARD RATHBUN. HE sea-coast of Brazil, from the mouth of the Amazonas to near Victoria, is generally low and of quite uniform character, being everywhere faced with broad, open beaches of sand that extend for miles and miles without interruption. A less inviting region than this to the explorer can scarcely be imagined, and it stands in marked contrast with the more tempting inland districts, especially those in the Amazonian valley, which have always caused Brazil to be considered a sort of naturalists’ patadise. It was little suspected, until a few years ago, that this unattractive ccast harbored very extensive and curious coral reefs, and dis- played, in connection with its beaches, interesting phenomena whose true character had been entirely misunderstood. Some of the earlier writers on Brazil endeavored to describe this region, relying either on their own imperfect observations, or on what others had seen, and adding, apparently, here and there, numerous details, the products of their own fertile brains, per- haps, to smooth over and complete the sketch. These very inaccurate accounts gave credence to the existence of a long bar or reef of stone, bordering the entire coast from north of Cape St. Roque to the southward of Bahia. Piso, in his work published in 1648, described this reef, and he has been copied over and over again by nearly every author who has written on this subject since his time, no one ever having taken the trouble to test the accuracy of his statements. Even Staff-Commander Penn, in the “ South American Coast Pilot,” VOL. X1II.—NO, VI. 25 348 Prof. Hartt on the Brazilian Sandstone Reefs. — [June, treats of “the vecife,a singular ridge of coral rock,” bordering the coast, at a distance from it of about half a mile to three miles or more, and extending from the north-east part of Brazil to south of Bahia. According to his accounts, the reef is about sixteen feet broad at the top, forming a natural breakwater, with smooth and shallow water inside, affording a channel for coasters. “It is broken occasionally, and forms, by the openings, entrances to the greater part of the ports, rivers and creeks on the coast.” Gardner describes the mythical reef in nearly the same terms, and there can be no doubting the fact that these two writers have reproduced Piso’s old description, with many additional details thrown in gratuitously. It is needless to state that these accounts of a continuous reef were without foundation, but they served, more or less, to mould the world’s ideas of the character of that coast up to the time when the late Prof. Ch. Fred. Hartt published the results of his careful and painstaking explorations of that region. Whether this observer was the first to rightly interpret the structure of the Central Brazilian coast or not, he, at least, has given the only extended and intelligible account of it. He has shown that although stone reefs occur at many localities, they are far from being universal, or even continuous over any great distance. Quite distinct from these are a series of coral reefs, often lying” near the shore, but much more developed farther out, rising up- ward from the surface of the submerged border of the conti- nent. Asa member of the Thayer Expedition in 1865, Prof. Hartt examined two quite perfect stone reefs, at Porto Seguro and Santa Cruz, in the province of Bahia, and at several places in the province of Espirito Santo he also found traces of the same structure, lying near the beach, however, and not forming true reefs. On his second trip, in 1867, he was able to study the larger and more perfect stone reef of Pernambuco, and to detect a similar formation on the beaches about Bahia. A full state- ment of these investigations, and of his theory of the origin of the reefs, is given in his “ Geology and Physical Geography of Brazil,” published in 1870. Darwin, who touched at Pernambuco on his memorable voyage, arrived at nearly the same conclusions regarding the mode of formation of the reef at that place, as did Prof. Hartt, but his description of it is very brief. 1879. ] Prof. Hartt on the Brazilian Sandstone Reefs. 349 Immediately upon the organization of the Geological Commis- sion of Brazil, in 1875, under the leadership of Prof. Hartt, active field operations were commenced in the province of Pernambuco, and another opportunity was thus afforded the late chief to exam- ine that most noted of all these natural breakwaters. With the larger force and more perfect appliances now at his command, he proceeded to carefully map out the reef and its surroundings, and to study in detail every feature connected with it. This extended investigation did not materially alter his previously-formed ideas as to the true character and mode of growth of the reef, but there were added many facts not before observed. Other stone reefs, at Cape Sto-Agostinho, Parahyba do Norte, etc., were explored by the Commission, and the conclusions arrived at, after comparing the results of all these studies, are of great interest and importance, demonstrating that on the coast of Brazil conditions obtain for the united working of a set of simple forces which, apparently, are not perfectly combined, at least with the same results, in any other part of the world. As we can best understand these phenomena after becoming acquainted with the structure and appearance of a single reef, we will first describe the one at Pernambuco, partly in the very words of Prof. Hartt, and then discuss the subject as a whole. Just to the south of the city of Pernambuco, a little river breaks through the land, and would open directly into the sea were it not that a narrow wall of stone, running in a general way parallel to the shore, carries its course some distance farther north, to beyond the limits of the city. Another small river, near its mouth flowing parallel with the sea, and separated from it by - only a narrow strip of land, runs through the city of Pernam- buco, and opens also behind the reef. There is thus formed a small and very shallow bay, which is con- tinued northward past the city, as a rather narrow channel, having a width of only a hundred rods or less, and a depth of but a few fathoms. The outlet to this channel is close to the northern end of the reef, and is so shallow as to admit only coasting steamers and vessels of ordinary draught. It is, however, the protecting reef and not the narrow harbor that interests us now. Standing upon some prominent point near the shore, in Per- nambuco, we can plainly trace this narrow strip of stone from its commencement, about opposite the ancient fortaleza do Brum, 350 Prof. Hartt on the Brazilian Sandstone Reefs. (June, situated to the north of the city, to the Ilha dos Pinhos, at the south. Its course is south, a few degrees west, and it runs in a nearly straight and unbroken line to near the latter place, where, however, it is cut through so as to afford a passage-way for small boats and jangadas. From this point to its southern end it is much broken up and generally irregular. But to examine it the more closely we must land upon it at low tide, and walk along its nearly level surface, stopping at times to investigate any unusual appearance that may attract our attention. The reef rock is a sandstone, of a light brownish tint when wet by the waves, and is formed of siliceous sand and rounded pebbles, mingled with which are many perfect and broken shells, all firmly cemented together by carbonate of lime. The most abundant shell in the sandstone is a species of Venus, still very common on the neighboring shores, and preserving in the rock its natural colors. At its northern end the reef is very narrow and has, apparently, been much undermined and worn by the waves, for it is lower here than to the south, and the sandstone lies in immense blocks, inclined toward the sea. To increase its height and better pro- tect the harbor at this place, an artificial wall, partly of brick and mortar, partly of reef stone clamped together, has been con-- structed. That the reef formerly extended much farther north- ward is evident from the existence of an isolated mass of reef rock just off the northern end, and of a stretch of submerged reef beyond the. channel. Upon the extreme point of the reef stands the Pernambuco light-house, and a short distance from it is a little old octagonal fort, called Picao, built of reef stone, and occupying the entire width of the reef. Following southward, the reef becomes broader and more level on top, but is still very irregular at the sides. In front of the city it has an average width of about two hundred and fifty feet, and a height equal to about that of high tide, though on account of the great commotion made by the waves at such times, it is impossible to exactly determine this fact. As to the thickness of the reef, it must reach down to below the level of low tide, because the rock is never entirely uncovered by the water. The highest part of the reef forms a broad belt of varying width, whose axis lies a little to the inside of that of the reef. 1870. | Prof. Hartt on the Brazilian Sandstone Reefs. 351 From this belt the surface slopes more or less strongly on both sides, but presents quite different characters, resulting from differences in exposure. The outer side of the reef has become very irregular from the constant beating of the surf, and is pierced with innumerable cavities of sea-urchins and thickly overgrown with sea-weeds and calcareous incrustations. The inner side, after a more or less rapid landward slope, breaks down abruptly and irregularly, and often presents an overhanging edge. The reef is cut up into large blocks by joints or cracks, which, though quite variable in their courses, may be reduced to two general series, one parallel to the axis of the reef, the other trans- verse to it; but many run obliquely or radiate from a common center, as though the reef at that point had settled down upon a hard underlying spot; sometimes they form a tangled maze. These joints are vertical or highly inclined, and the angular masses resulting from them are likely to be detached, on the outer side of the reef by the force of the waves, and on the inner side by the undermining action of the currents in the harbor. In this manner the margins of the reef have been made very jagged, the outer being the most irregular. On the upper surface of the reef, where there has been no dislocation, the joints tend to widen by the action of the surf and by chemical decomposition. There are thus formed open passages, a foot to a yard or more in width, and with a considerable depth of water. In these we always find a rich collection of marine animals, corals and other polyps pre- dominating. Now let us inspect more minutely the character of the surface, and the many foreign objects living or growing upon it, which tend either to protect it from wear, or to gradually and surely effect its destruction. As stated above, sea-urchins are burrowing into its outer edge. There is only a single species on the Brazilian coast that is able to excavate in the solid rock; it is the Achzno- metra subangularis, everywhere abundant, and possessed of stout, sharply-pointed spines. On abrupt slopes of the reef, this dili- gent worker forms rounded holes, having only a slight depth, but where the slope is gradual, the holes are much longer, running either directly inwards, or in a more or less winding way, being sometimes curved or bent upon themselves. They have often a length of four or five feet, and a width of three or four inches, the urchin apparently occupying the lower end of the elongate holes.