BERKEY, GEOLOGICAL RECOXXOISSAXCE OF PORTO RICO 25 



Tujfs. — The most abimdant of all of the rock types is represented by a 

 great variety of volcanic tuffs. These accumulations seem to be the funda- 

 mental basis of the whole island. Whatever has been formed in the way 

 of shales, sandy beds or conglomerates or any other of the ordinary sedi- 

 mentary types seem to be directly derived from the same material that 

 makes up the tuffs and the rocks directly associated with them, related in 

 an interbedded succession. In most cases, the tuffs are essentially massive 

 in their structural habit and are made up of a complex accumulation of 

 large and small fragments of volcanic materials which prove to be chiefly 

 andesitic and closely related porphyritic types. There is great variety in 

 texture and minor structure and present condition, but on the whole there 

 is enough uniformity of actual composition to justify classing them all 

 as andesitic tuffs. 



In very many places there is obscure bedding structure indicating a 

 tendency to assort and work over this material at the time of its deposi- 

 tion. This is especially noticeable in the finer materials and some of 

 these beds are made up essentially of ash. These ash beds resemble the 

 type referred to as shales so closely, in some places, that it is impossible 

 to distinguish between them in their field appearance. I judge also that 

 there is practically a gradation from one rock to the other, the ash beds 

 showing transitions to shales, especially where they have developed under 

 conditions encourao'ino- much weatherino- and workiiio- over of the frao^- 

 mental materials and promoting the growth of organisms in sufficient 

 amount to make the accumulating beds somewhat calcareous in compo- 

 sition. 



Tuffs and ashes are well kno^Ti to be especially liable to attack by 

 alteration and to the ordinary changes that modify rocks. It so happens, 

 therefore, that many of these representatives are completely modified and 

 have become so dense that they exhibit none of their ashy or fragmental 

 structure without microscopic examination. In this condition they are 

 usually also exceedingly hard and as resistent to destruction as the hard- 

 est crystalline rock. The largest development of massive almost struc- 

 tureless tuffs which were seen occupy the Sierra de Cayey between Guay- 

 ama and Cayey and also the range along the military road toward 

 Aibonito; but there are extensive occurrences in many other sections. 

 Some of the most promijiently developed bedded tuft's and ash beds were 

 seen on the north flank of El Yunque along the Sabana Eiver and on the 

 north side of the divide l)elow Comerio, and also along the Ponce-Arecibo 

 Road both near the summit of the range and farther to the nortli midway 

 between Arecibo and Utuado. 



From what was seen of this type of rock, it was not possible to form a 



