BERKEY, GEOLOGICAL REGONNOISSANCE OF PORTO RICO 59 



An occasional more prominent change of conditions, more or less 

 clearly marked in the structural relations and character of material, may 

 possibly be used as a basis for epochal subdivision. It is quite clear, how- 

 ever, that there was no profound change of geologic control throughout 

 the whole of this earlier time, — it was strictly a volcanic period. The 

 succession of disturbances by which it was affected is represented in part 

 by dynamic modifications of the nature of folding, crushing and faulting, 

 but this is probably an accompaniment of the more profound igneous 

 activities also, and need not be regarded as evidence of any strikingly 

 different causal process. If there were contributory causes of a regional 

 sort controlling the folding, they are essentially simply superimposed 

 upon or introduced into the larger, more profound and longer continued 

 igneous activities which prevailed both before and after that time. 



This long period, characterized by great complexity of formational 

 development, including tuffs, agglomerates, conglomerates, shales, lime- 

 stones and immense numbers of intrusives of great variation in size, form 

 and composition, finally came to an end by the dying out of the volcanic 

 energy, and greater stability of the whole with respect to elevation and 

 subsidence was established. Erosion cut down the exposed formations, 

 the sea attacked the margins and in time most of the projecting moun- 

 tain mass was reduced near to base level, the sea encroached far onto the 

 former land areas and a new historical chapter was begun. 



It is not possible to say, with the data in hand, that the entire island 

 was reduced to a peneplain, or perhaps a conoplain, but there is good evi- 

 dence, from the traces still left of former planation and from the dispo- 

 sition of the remnants of overlying formations still preserved, that the 

 greater part of the present area was worn down to base level and sub- 

 merged. The process of base leveling was going on before the close of 

 igneous activity and it was continued long enough to bevel across rocks 

 of all sorts with marked success, but there is no necessity for regarding 

 it as a very long geological time. 



As erosion proceeded, sediments were deposited unconformably around 

 the margins of the island of that time and perhaps also in some of the 

 marsfinal vallevs above sea level. These constitute the earliest shale beds 

 of the "Younger series'' and are believed to be of Eocene age. They are 

 at least early Tertiary. Where more simple marine conditions came into 

 control, as would happen when submergence or planation had masked or 

 destroyed the more elevated sources of supply, the deposits became almost 

 wholly reef limestones and shell limestones, with only minor amounts of 

 strictly detrital material irregularly distributed. This gave a succession 

 of somewhat irregular beds which are abundantly supplied with organic 



