18 



ANNALS NEW YORK ACADEMY OF SCIENCES 



volcanic origin and are exceedingly abundant and extensive. They are 

 found at intervals in' all parts of the series, and it is impossible to say 

 that they are either more or less abundant in those portions which appear 

 to be older, rather than in those which appear to be younger or higher in 

 the series. The closest associate of these materials is the bedded tuif, 

 made up of volcanic fragments which have been somewhat assorted by 

 surface agencies so that they exhibit some sedimentary structural charac- 

 teristics. These are also exceedingly abundant and widely distributed and 

 they pass by insensible gradations of finer and finer materials into those 



Fig. 4. — Typical shale occurrence 



This is seen along the road between Ponce and Penuelas at K-10. The beds at this point 

 lie in a less distnrhed attitude than is usual in memhevs of the older series. 



that are recognized as true ash beds. Most of these have become so thor- 

 oughly cemented, or so much modified by secondary attack, that they now 

 present a perfectly sound and compact appearance. In thin section, 

 however, it is easy to see that the material is wholly volcanic and that 

 the bedding is the only secondary modification except that having to do 

 with the binding, induration or alteration of the rock. The ash beds are 

 probably close relatives of the so-called shales. 



Shales. — Eocks of tliis type are developed characteristically at Fa- 

 jardo, at Mayaguez near Baranquitas and at numerous other points, espe- 



