BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 63 



reaching from the sea to Lares so as to include a strip of the older com- 

 plex rock series beyond the inner margin of the Tertiary series of reef 

 limestones and shales which constitute the greater part of the bed rock 

 of the area. Beside the mapping and detail of structural relations, this 

 district presents the best opportunity to investigate the question of exact 

 age of the basal beds of the Tertiary series, the transition from lignitic 

 shales of perhaps fresh water alluvial origin to massive limestones of reef 

 type, a subdivision of the Tertiary series, the meaning of the thinning 

 out and disappearance of the Lares shales toward the east, and a paleon- 

 tologic study of the beds, all of which are fundamental in any additional 

 studv of other districts containing the Tertiary rocks. There are besides 

 good opportunities to study the meaning of the high terrace-like shelf 

 coming abruptly to the sea at Quebradillas river and the meaning of the 

 deep embayments now occupied by such playas as that at Arecibo. This 

 is also one of the best localities for a detailed study of the structural and 

 petrographic features of the San Juan formation as well as the behavior 

 of modern dune sands along the present coast. 



Another district of still different features, and giving foundation for 

 special studies of quite a different bearing, is that lying between Caguas 

 and the Caribbean sea and perhaps extending as far eastward as Naguabo. 

 This will include the largest massive igneous unit in the whole island 

 and promises information bearing upon magmatic differentiation, origin 

 of the magnetic iron ores, relation of the great intrusive masses to the 

 other igneous representatives, petrographic range of the igneous rocks, 

 and marginal metamorphic or other effects, — studies fundamental in a 

 final statement of the igneous history of the island. 



There are other districts which have special problems associated Avith 

 the regular areal geologic work, but these are sufficient to indicate the 

 range of such district studies and their variety. 



Certain special investigations are of a sort requiring comparison and 

 summan^ of many different localities, and for these it will not be wise 

 to handicap the investigator by limiting work to a single district. Some 

 of these are suggested below. 



REEF-BUILDING ORGANISMS 



The limestones of Porto Eico are remarkable for the great prominence 

 of alg?e and corals and other closely associated organisms lending them- 

 selves to the construction of reefs and accompanying deposits. These 

 forms belong to practically every limestone formation of both the older 

 and the younger series except those most closely related to the shales. It 

 is a study requiring the training of a specialist in such lines. 



