24 ANNALS NEW YORK ACADEMY OF SCIENCES 



evidence of marked unconformity between the conglomerate above and 

 the miderlying series of formations. But the fact that the conglomerate 

 beds, which follow to great thickness, are jjrevailingly of simpler struc- 

 tural habit, as compared with the calcareous shales, ash and tuff series 

 immediately below, suggests that there may be a break here of larger 

 consequence than is observed in other parts of the pre-Tertiary or older 

 series. 



The development of so extensive a series of conglomerates doubtless 

 does represent a considerable change in physical conditions, compared to 

 those controlling simpler deposits which preceded and followed them, and 

 it is possible that it may be found useful in separating the complex series 

 of older mixed bedded rocks and tuffs into an older and a younger divis- 

 ion by using this conglomerate as a dividing member. This is supported 

 to some extent bv the occurrence of a conglomerate of similar character 

 but of very much less extent on the north side of the island, several miles 

 south of Bayamon, and also one of apparently less prominence near La 

 Muda. If additional field work should show that the conglomerate belt 

 could be traced from one side of the island to the other, it seems to me 

 that it would be entirely practicable to make this division. 



The conglomerate is invaded by igneous intrusive material in much the 

 same manner as is observed in the other rock formations, but the massive 

 habit of the rock as a whole leads to a predominance of transverse dike- 

 like masses rather than the simpler looking sills. At one point in par- 

 ticular, however, near K-86 on the military road between Aibonito and 

 Coamo, the conglomerate has been invaded by a magma that must have 

 been fluid enough to penetrate the porous matrix surrounding the con- 

 glomerate pebbles where it now exhibits a crystalline habit. This in- 

 jected matrix is essentially a coarse diorite porphyry in composition, 

 through which the pebbles are distributed in tl»e manner that they seem 

 to have had in the original rock, so that there are still obscute traces of 

 bedding structure. There are additional petrographic peculiarities in 

 this rock that will be described under a different heading. This tendency 

 of the dioritic magma to penetrate and incorporate fragmental matters 

 was noted in several other places. It was most strikingly exhibited in 

 certain intrusive members cutting through tuffs and shales. In some of 

 these cases there is so great a quantity of fragmental matter as to wholly 

 obscure the true nature of the rock unless one can see the structural rela- 

 tions. In the case of the conglomerate, however, the crystalline habit of 

 the matrix is a striking feature and it is very evident that it is wholly 

 different from the regular conglomerate habit. 



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