38 



AXXALS XEW YOJRK ACADEMY OF SCIEXCES 



Folding 



Most of the rock formations representing original bedded types have 

 been more or less tilted or otherwise do not now have their original atti- 

 tude. Those belonging to the younger series, the Tertiary limestones 

 and shales on the north side of the island, are comparatively little dis- 

 turbed, and in some cases do not have a very different dip in spite of th-j 

 fact that they have changed very much in position with respect to sea 

 level. On the south side of the island, however, even these later beds are 

 in many places tilted at a higher angle than they had in the beginning 



Fig. 12. — Overturned fold and crush zone in finely tedded sliales on the Jayuya rood near 



the summit of the range 



and occasionally show high angles and even gentle folds. This condition 

 may be seen on the Jacaguas Eiver. near J nana Diaz, better than at most 

 places, but similar conditions are indicated by the relations at certain 

 points farther west. This condition on the south side of the island indi- 

 cates more extensive and more violent djmamic disturbance on this side, 

 which is further supported by the presence of faults cutting and affecting 

 the Tertiary series on its present inner margin. 



The older series, the pre-Tertiary, is still more profoundly affected 

 and, in almost all districts, shale and ash beds may be found standrag at 

 high angles, in many cases practically vertical, and in occasional instances 



