BERKEY, GEOLOGICAL REC0NN0IS8ANCE OF PORTO RICO 29 



examination of the water-washed pebbles in the stream beds that have 

 come do^vn the steeper momitain sides. An examination of such material 

 shows an extremely large varietal range, and, although by all means the 

 greater number are some variety, of diorite or andesite porphyry, there 

 are occasional more basaltic and more acid types represented. 



Summary. — From this description of the variety of rock types repre- 

 sented by the leading field units, it may be readily seen that a subdivision 

 into members of mapable constancy is no easy matter. 



It is the opinion of the writer that, for the present work and for in- 

 vestigations immediately to follow, local designations or names will be 

 of most direct usefulness, and that a correlation should be expected to be 

 the final outcome of a series of such studies. For the present, therefore, 

 it is Judged best to use the terms Fajardo shales, Mayaguez shales, Ja5^uya 

 road shales, Barranquitas shales, etc., as suitable names in these respective 

 districts for the shale member of the older series, without any intention 

 of suggesting their equivalence. In spite of the physical similarity in 

 these cases, it is not at all likely that they belong to the same horizon. 



Similar argument wdll hold for most of the other members, — the ash 

 beds, the tuffs, the limestones and the conglomerates, — and it is recom- 

 mended in these cases, also, that locality designations be used in the field 

 investigations. Those described in this report are not necessarily all that 

 deserve special designation, but the same rule may be applied to addi- 

 tional occurrences without in any way obscuring the ultimate solutions 

 of the problem of correlation. On account, therefore, of the complexity 

 of the structure and the limited amount of work yet done in connecting 

 the separated occurrences into continuous field imits, it is advisable to 

 retain such terms as Coqui limestone, Coamo limestone, Corozal lime- 

 stone. La Muda limestone, Trujillo Alto limestone, Cayey tuffs, etc., for 

 the earlier descriptions and special studies. 



EETriOGRAPIlIC EAXGE 



There is an exceedingly great variety of certain classes of rocks in the 

 Island of Porto Eico. Those most prominently developed and showing 

 the greatest variation in minor character, structure and relationship are 

 the volcanics, especially the volcanic fragmentals. All sorts of tuffs, cin- 

 der beds, ashes, mud flows and bombs are represented in great quantity, 

 in very wide distribution and in all stages of alteration and induration. 



Observations made on thousands of occurrences of this character of 

 materials leads to the conclusion that most of it is essentially of andesitic 

 composition. Although there is an occasional fragment of either more 

 basic or more acid composition, the predominant types are always of 



