34 AXXALS NEW YORK ACADEMY OF SCIENCES 



more simple and less modified. The organic content is of greater variety, 

 however, and because of the reef -building tendency the primary structure 

 is more varied than is seen in the limestones and shales of the pre-Ter- 

 tiary representatives. 



The most striking petrographic type is, probably, the solidified dune 

 sand making- up the San Juan formation. Its uniformity of grain, 

 strong cross-bedded character, porous habit, together with its rather sur- 

 prising stability, make it an object of some considerable interest. Sev- 

 eral of these classes of rocks, therefore, represent petrographic series of 

 unusual range and variet}^, and because of their perfection of develop- 

 ment would seem to warrant detailed study. 



There are no foliated metamorphic rocks so far as yet seen in Porto 

 Eico. One specimen of such rock, a mica schist, was shown to the writer 

 as having come from the Portuguese river not far from Ponce, but a hur- 

 ried reconnoissance in the vicinitv failed to uncover anvthinsr even re- 

 sembling it. The specimen probably does not belong to the rocks of 

 Porto Eico. jN'o evidence whatever has been seen of conditions that 

 would be expected where such rocks occur. 



The most profoundly modified rock observed is a massive serpentine. 

 Such material was seen at two places by this party, one near Yauco and 

 another on the road to Comerio, and the same type is reported by Pro- 

 fessor Crampton on a much larger scale near Maricao. But they are all 

 simple petrographic cases after all, being ordinary intrusive units of 

 heavier ferro-magnesian content than the average which have been heav- 

 ily altered, especially by hydration processes, to the present condition. 

 Genetically and historically, the serpentines are not materially different 

 from the other intrusive bodies. 



Depth of Decay 



Alteration has affected the rocks at most points to considerable depth, 

 but in spite of this there are plenty of outcrops, and along the roads there 

 are many cuts exposing fresh rock. The stream beds are strewn with 

 fresh pebbles and boulders. Although decay obscures the character of 

 the rock in most of the outcrops, the structure is usually fairly well pre- 

 served, and in most cases enough can be seen to enable one to determine 

 the format ional habit. 



The most striking thing about many of these badly decayed outcrops 

 and cuts is the remarkable way they stand against destruction or removal 

 by ordinary weathering and erosion agents. At many points, road cuts 

 are made, with side walls absolutely vertical, through wholly decayed 

 rock material, that stand for years without crumbling down. Embank- 



