146 J. C. Branner — Geology of Fernando de Noronha. 



ings show a depth of more than 6,000 feet, while to the south- 

 east, at the same distance, the depth is 3150 feet, and at twelve 

 miles, 4920 feet. This group of islands, therefore, rises 

 abruptly from the ocean's floor. The currents and surf that 

 strike it from the east are unchecked by shallows on that side, 

 so that it receives the full force of the waves and is conse- 

 quently being cut away at a rapid rate. 



The island is of volcanic origin, there being no sedimentary 

 rocks upon it. The volcano which once existed here ceased 

 action long ago, and the powerful surf which constantly beats 

 upon the island has since cut away the cone, and is now fast 

 diminishing the remnants of the original island. Moreover, 

 the usual chemical processes of disintegration, hastened and 

 deepened here by a very great precipitation falling upon rocks 

 highly heated by exposure to a tropical sun, has covered the 

 interior of the island with a deep soil mingled with rock frag- 

 ment which, together, obscure the geological details over the 

 main body of the island. Its original lofty central portion has 

 gradually yielded to these disintegrating influences till only the 

 great Peak and its smaller companions remain to suggest the 

 former elevation of the group. A large portion of the island 

 is now under cultivation, and the loose blocks, which might 

 otherwise have been of some service in suggesting, at least, the 

 distribution of the rocks, have been gathered from the fields 

 and built up in stacks or stone walls, or used to make roads or 

 houses. The lands that formerly sloped at a low angle into the 

 sea have been encroached upon, cut down and swept away by 

 the ocean currents until the island is now walled in for the 

 most part by high, precipitous cliffs ; the ancient sandy beaches 

 which at one time bordered its southeastern shores, and which 

 were probably fringed by coral reefs, have been almost com- 

 pletely obliterated. The more rapid destruction along the 

 shores and the slower weathering of the interior brings into 

 close proximity two topographic types, and almost any view 

 which includes both shore and inland topography shows the 

 more graceful lines of the old topography in strong contrast 

 with the newer, bolder, and more angular cliffs and escarp- 

 ments made by the steady encroachments of the sea upon the 

 land. 



The best, and almost the only, good rock exposures are about 

 the shores ; but many of these are difficult or impossible of 

 access on account of the lack of boating facilities at the island, 

 and because of the violent surf which so generally prevails. In 

 addition, the rocks are so broken, faulted, and thrown into con- 

 fusion, that it must be confessed my study of the place was far 

 from satisfactory. I endeavored especially to construct an 

 accurate map of the island, and to collect typical rock speci- 



