174 BULLETIN : MUSEUM OF COMPAuAilVE ZOOLOGY. 



to islands with peaks ol over lour thousand feet, at the base of and 

 around which barrier and fringing reefs may be formed, or crescent and 

 horseshoe-shaped reefs might grow. 



The somewhat insignificant coral reefs of the Eastern Pacitic, as far as 

 known, occur upon volcanic islands. Those of the Sandwich Islands and 

 the unimportant coral patches of the Galapagos grow upon a basement of 

 volcanic origin, while the only atoll of the Eastern Pacific — Clipperton 

 Rock, off the Mexican coast — holds to its trachytic nucleus much the 

 same relation which some of the reefs of the Caribbean hold to the vol- 

 canic peak they surround.^ 



1 The absence of extensive coral reefs off the west coast of Tropical America 

 is probably due to several causes, the most important of which as regards the main- 

 land is the large amount of mud brouglit down the shore mountain slopes during 

 the rainy season. Similar conditions exist along the shores of tlie Galapagos, where 

 corals are formed merely in patches, as along the coast of the mainland, in addition 

 to whatever influences detrimental to their growth may be due to volcanic action. 

 (See Dana, Corals and Coral Islands; Pourtales, Am. Journ. Sci., 1875, Vol. X. 

 p. 282; and A. Agassiz, Bull. Mus. Comp. Zool., Vol. XXIII. No. 1, 1892.) 



It is interesting to add here some notes relating to Clipperton Rock, which prove 

 the existence of an atoll on the borders of the Panamic region, seven hundred miles 

 southwest of Acapulco. I am indebted to Professor George Davidson of the U. S. 

 Coast Survey for a sketch of Clipperton Island by Carl C. Jensen, made in 1893, as 

 well as for a specimen of the rocky nucleus. The latter was examined by Profes- 

 sor J. Eliot Wolff, who reports it to be " k volcanic rock with typical trachytic struc- 

 ture, of the same general class as the rock from the hospital quarry at Panama.'" 



The description drawn up from the notes and chart he kindly sent me agrees in 

 the main with the one given by Lieutenant Griswold.and published by W. H. Pease 

 in the Proceedings of the California Academy of Natural Sciences (1865, p. 199). 



Clipperton Rock was seen from the northeast by Sir Edward Belcher in 1839, 

 and described by him as a coral lagoon island with two entrances, both on the 

 weather side. It was, according to W. H. Pease, visited in 1861 by Lieutenant GriS- 

 wold, who landed upon the island, and says in his Journal : " It is girdled by a 

 broad barrier of coral about fifteen feet above the level of the ocean. There is no 

 entrance to the lag6on. The water inside is fresh and potable ; it is slightly brack- 

 ish. . . . The rock is a ragged pile of volcanic formation, on the south end of the 

 island. . . . The highest pinnacle of the rock is about 120 feet, and it covers about 

 two acres. It stands on the edge of the lagoon, or rather within it. . . . The lagoon 

 is a quiet fresh-water pond about two miles long and one broad, with a long spit 

 of mud running out into the middle of it, but elsewhere of a light green color which 

 seemed to indicate a considerable depth. [Light green does not in coral districts 

 indicate a considerable depth ; on the contrary, it may indicate a deptli of six to 

 seven fathoms at most. — A. Agassiz.] The shores are abrupt. ... I did not find 

 the least sign of vegetable life upon the island." 



Mr. Jensen says there are no trees on the island. It is surrounded by a reef of 

 coral over which the sea continually breaks. Outside of this unbroken reef the 

 bottom deepens rapidly, and vessels must anchor in not less than fifty fathoms of 



