BERKEY, GEOLOGICAL RECONNOISSANCE OP PORTO RICO 27 



to the bedded rocks, shales, ash beds, etc., with which they are associated. 

 In some cases, these invading magmas have incorporated great quantities 

 of fragmental matter, giving the rock in its present condition a very 

 strikingly fragmental appearance. This habit associated with its per- 

 fectly apparent intrusive relation makes a very unusual combination in 

 the field. In many places there are included blocks of immediately adja- 

 cent rock such as one sees in the occurrence at the quarry at Fajardo 

 Playa, but, in extreme cases, the mass is chiefly fragmental in its make- 

 up and one could not readily interpret its history without complete field 



Fig. 6. — Dioiite porphyry sills 



These sills are intruded between layers of calcareous shales and ash beds on the road 

 near Comerio. The streaked or banded layers are shales, the massive portions, seen best 

 at the left side of the print, are sills. A transgressive relation can be seen between the 

 two layers at the extreme left. 



determination of its relations. Such occurrences may be seen in the 

 vicinity of Guayama on the road about a mile east of that place, and also 

 a short distance south of Eio Piedras. 



Few of these intrusive masses show anything but a rather uniform 

 medium grain texture and larger ones have the average appearance of a 

 rather fine or medium grain diorite. The granular appearance, however, 

 is probably deceptive, due to the way the rock disintegrates, for thin sec- 

 tions made from many typical intrusive representatives are nearly all 

 plainly porphyritic in texture. 



