BERKEY, GEOLOGICAL RECOXNOISSANCE OF PORTO RICO 37 



count of the great abimdance of the fragmental matter, it does not seem 

 possible that these intrusive masses could penetrate in that condition to 

 some of the positions where we now find them. It is more likel_y that a 

 rather fluid magma has penetrated some unusually porous fragmental 

 bed forming a matrix for it, perhaps also spreading it somewhat, and 

 then in breaking across to another bed, in some cases it was still mobile 

 enough to drag the mixture along into the larger transgressive structures. 

 This idea is somewhat supported by the finding of a conglomerate bed, 

 near K-86 on the militar}'- road west of Aibonito, impregnated with an 

 igneous matrix in essentially this same manner. In addition to these 

 forms, there are numerous larger intrusive masses, the largest of which 

 deserve a special name. I see no objection to calling them bosses. The 

 two largest occur, one between Caguas and the Caribbean sea toward the 

 south and southeast, and the other between Ja3niya and Utuado on the 

 north side of the divide.- 



Volcanic Vent Complexes 



A special igneous structure that has not been referred to except inci- 

 dentally is that composite of disturbed structures which represents the 

 location of old volcanic vents. They are essentially a complex of irreg- 

 ular intrusive units cutting and including masses and aggregates of 

 •various fragmental and sedimentary types in a mixed relation. In the 

 clearest cases, such a complex suddenly takes the place of a formation of 

 apparent promise of continuity such as a series of sedimentary beds, and 

 after an interval these beds are again found continuing as before. For 

 example, the Coamo limestone formation is abruptly cut off and its place 

 is taken for a mile or more by one of these igneous complexes, the lime- 

 stone continuing on the other side again. The Jacaguas reservoir, just 

 above Juana Diaz, lies in one of these old volcanic-vent complexes, occu- 

 pying, however, only a portion of the area. Another such case is repre- 

 sented by the complex cutting the great conglomerate beds on the 

 military road at about K-87— 88 west of Aibonito. Another is believed 

 to be represented by the very striking basin-like area crossed b}^ the road 

 between San German and Hormigueros. This one is now represented 

 by a very smooth plain five or six miles across surrounded on all sides by 

 more hilly country. The same conditions are imdoubtedly indicated by 

 the extremely complex structures seen on the Descalabrado river two 

 miles below, south of the military road. Some of these mark the sites of 

 ancient craters, cloorsred or choked with fragmental and intruding ma- 

 terials. 



