36 AXXAL^ XEW YORK ACADEHY OF SCIEXCES 



definite conclusion concerning the age represented excej^t by their rela- 

 tion to certain interbedded shales and limestones. It appears that the 

 nnderhing older j^ortions of the series of tnffs and ash beds have com- 

 paratively little of such interbedded calcareous material and haye cTery- 

 where^been modified or altered or metamorphosed to a greater degree than 

 beds that lie higher in the series. But beyond this there is little to Judge 

 of the actual age. As one goes higher in the series, however, there arfe 

 occasional prominent limestone members with which tuffs are intimately 

 associated or interbedded, and it may be possible, by reason of these rela- 

 tions, to form a more accurate estimate of the geologic age of this later 

 portion of the series. 



Volcanic Flows. — In addition to the sedimentary beds of various sorts 

 and the related tuffs, there are at occasional places evidences of volcanic 

 lava flows. These were seen at several places on the road between Baya- 

 mon and Barranquitas. They are amygdaloidal in present habit and 

 represent vesicular basalt and andesites. On the whole, evidences of lava 

 flows on a large scale are wanting. This kind of product seems to have 

 been very much more rare than the fragmental t^^^e. A more prominent 

 thing as a structural feature is the occurrence of very numerous intrusive 

 bodies. 



Intrusives. — The intrusive masses in Porto Eico occur in all parts of 

 the island and in all of the formations except the Arecibo and the over- 

 lying alluvial deposits. Xo such evidence was seen in any part of the 

 Tertiary of yomiger series: but the comj^lex series of rocks representing 

 the pre-Tertiary, here referred to as the older series, are cut in all sorts 

 of wavs by both laro^e and small intrusive masses. The smaller intrusives 

 are chiefly andesite porphyry in composition and have everywhere pene- 

 trated the shales and ash beds. The commonest occurrence is in the form 

 of small sills or sheets conformable to the bedding structure and varying 

 in thickness from only a few inches to many feet. These sills are so per- 

 fect in form, have so little disturbed or modified the adjacent beds, and 

 are so similar in general composition and appearance, after weathering, 

 to the associated sedimentary beds, that it is quite impossible to deter- 

 mine in all cases how much intrusive and how much original sedimentary 

 rock is involved. The only thing noticed as a rule is the uniformity' of 

 petrographic structure that seems to be characteristic of the intrusive as 

 compared with the associated beds. The simplest occurrences of sills of 

 this kind, which at the same time show their igneous intrusive character, 

 were seen near Fajardo, near Eio Piedras and in the vicinity of Comerio. 

 But occurrences of tlie same kind are exceedingly numerous in nearly 

 every district and in total amount form a very great additional thickness 



