BERKEY, GEOLOGICAL RECOXNOI8SAXCE OF PORTO RICO 31 



would probalily ineliule all of the habits known to this family. In addi- 

 tion to the porphyries of this family composition, there are less common 

 occurrences of felsite, quartz porphyry-, granite porphyry and basalt 

 porphyry. In surface flows there is, besides the andesites, an occasional 

 amygdaloidal basalt, but so far as observed there was no rhyolite or other 

 very acid surface type. Among the massive larger intrusives, the com- 

 monest and most abundant type is a granite porphyry or granite varying 

 in some parts to the composition of syenite. A massive rock of the na- 



FiG. 8. — Photomicrograph of a typical iceathered shale from Fajardo Playa, magnification 



28 diameters 



The dark field is made up of an extremely flue aggregate of earthy materials. The 

 white circular and irregular spots are entirely empty and constitute the porosity of the 

 rock. The circular forms of these voids suggest that they represent former calcareous 

 content in the form of foraminifera, now completely removed by weathering. 



ture of a diorite is also represented, as is a very coarse rock of the nature 

 of a giant granite. How these are related, how many intermediate 

 varieties there may be and whether this variation represents magmatic 

 differentiation within a single mass or instead different units of intru- 

 sion, has not yet been determined in enough detail to make a positive 

 statement. But in at least two cases where these large masses were seen. 



