58 ANNALS NEW YORK ACADEMY OF SCIENCES 



Gases in solution per litre of vmter at 0° of temperature and 760 mm. of 



pressure 



Nitrogen 13 ec. 740 



Oxygen 1 761 



Sxilphj'dric acid 1 967 



Total 15 2468 



Historical Statement 



A complete or even a reasonably full account of the geological history 

 of Porto Rico cannot be written at this stage of the investigations. Such 

 a statement is necessarily the end product or climax of the whole series 

 of studies that are proposed, but it may not be out of place to outline 

 some of the leading and most clearly marked steps as a rough sketch or a 

 preliminary attempt. 



At the outset, it is well to appreciate that the Island of Porto Rico is 

 geologically young. There are no traces, so far as known, of any of the 

 so-called ancient rocks. It is quite true, of course, that the older series 

 of formations is largely a volcanic complex whose exact age may never be 

 accurately determined, but there is no occurrence of profoundly meta- 

 morphosed members or other evidences of great geologic age. Besides, 

 the series, complex as it is and difficult to group into suitable divisions as 

 it may be, undoubtedly forms a very closely related succession of minor 

 formational miits whose uppermost members are determinable as to age 

 with reasonable accuracy. It would appear also from the nature of the 

 deposits and their structural relations that the accumulation must have 

 been, for the most part, a rapid i)rocess. 



There is no good reason, so far as any of these facts are concerned, why 

 the whole of the "Older series" could not have been accumulated in a 

 single geologic period. The fossil content of the upper members of this 

 series indicates that this period was the Cretaceous, as used in the broader 

 sense in geology. Whether or not the older members date back to an im- 

 mediately preceding time cannot yet be definitely stated, but whatever 

 there is, is clearly so closely associated with the Cretaceous beds that they 

 can all be treated as a single historical unit. 



This earlier period is characterized by volcanic and other igneous activ- 

 ity on a very large scale. Beds were accumulated both above and below 

 sea level. There seem to have been oscillations of level accompanied by 

 recurrences of similar beds, and apparently much shifting of the supply 

 of material accompanied by great variation of character laterally. There 

 is good evidence that succeeding volcanic outbursts broke through these 

 beds nt many places. 



