GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 603 



in the direction of elongation of the Jamaican bank, Sud (Haiti), 

 and Porto Rico. 



The relative truncation of the west end of Porto Rico, except tho 

 protuberant which forms Cabo de San Francisco, is striking and 

 suggests faulting. The declivities both to the north and south of 

 the island are great, over 4,000 fathoms in depth being reached 

 within 40 miles of the north coast, while 2,000 fathoms are attained 

 within a shorter distance from the south coast. 



A submarine bank extending from the east end of Porto Rico to 

 Ancgada Passage is known as Virgin Bank. The depth of water 

 between the islands rising above this bank is less than 20 fathoms, 

 which is a minimum for the amount of submergence they have recently 

 (geologically speaking) undergone. These islands are detached out- 

 liers of Porto Rico. 



86. Saint Croix. — -Although St. Croix is separated from the Vir- 

 gin Islands by a depth as great as 2,400 fathoms and is joined to the 

 St. Christopher chain by a ridge less than 1,000 fathoms deep, it 

 possesses great similarity to members of the Virgin group. The west 

 end is truncate and the submarine slope precipitous; the submarine 

 slope to the north is also steep. There is clear evidence of faulting 

 on the west and north sides. A ridge, largely of igneous rock, stands 

 against the north shore from the west end of the island for some dis- 

 tance to the east. South of the ridge is a sloping, rolling, calcareous 

 plain. The east end has a submarine continuation in a bank less 

 than 50 fathoms deep. The tectonic axis is east and west, the rocks 

 resemble those of the Virgins, and the zoogeography indicates former 

 connection with them. For these reasons it seems probable that this 

 island was formerly a part of the Porto Rican- Virgin Island land-mass 

 and has been sundered irom it by diastrophic processes. However, 

 Saint Croix might be accorded separate status as a province, or 

 referred to the St. Christopher axis; but it appears to me preferable 

 to classify it with the Virgin Islands. 



9. Costa Rica-Panama. — Between the Nicaragua-Costa Rican bound- 

 ary and the mouth of Rio Atrato is an S-shaped land area which 

 does not exhibit striking major tectonic lines, although some de- 

 formation axes are obvious in Panama. The region is largely one 

 of vulcanism, present or past, which although occurring within 

 definable limits does not follow continuous straight axes but occurs 

 in a curving belt. The topography appears disordered, with volcanic 

 protuberants here and there without perceptible system. The vol- 

 canic heaps range from a few hundred to nearly 10,000 feet in 

 altitude. 



10. Andes. — The south-north trending ranges of the Andes reach 

 the shores of the Caribbean Sea between the Gulf of Darien and 

 Venezuela, and send a spur, Cordillera de Merida, northeastward to 



