BERKEY, GEOLOGICAL EECOXXOISSAXCE OF PORTO RICO 23 



"Mountain Limestone." — In some localities, such as that near Barran- 

 quitas, and probably at other points along the same divide, the shaly beds, 

 become very calcareous, as has been pointed out by Mr. Hill. These 

 were referred to l)y liini as limestones and are probably included in his 

 "■'mountain limestone" and estimated that the thickness of such beds 

 amounted to fully a thousand feet. The prominence of the shaly struc- 

 ture in these l)eds and their apparently close genetic relation to the type 

 described in this discussion as shales, leads us to regard this occurrence 

 as essentially the calcareous extreme of the shale series. iVs already 

 noted, the shales are characteristicallv calcareous and some of them are 

 predominantly so. 



Corozal Limestone. — At Corozal, a fragmental limestone was seen just 

 south of the village. Its relations were not worked out and its meaning 

 is therefore not well understood, but its structural peculiarities lead to 

 the suspicion that it may be associated with volcanic fragmental accumu- 

 lation. It was found that in at least one place in another part of the 

 island there had been volcanic outbreaks through heavy limestone beds, 

 and it is evident that fragmental material from such activity miijht 

 therefore include a good deal of simple limestone fragments. It is hardly 

 conceivable that they would in most places accumulate in enough abun- 

 dance to make a limestone bed a second time, but such a thing is doubt- 

 less possible. 



Conglomerates. — There is a very extensive development of conglomer- 

 ate occurring in a belt whose general trend seems to be from southeast 

 toward the northwest, crossing the military road between Aibonito and 

 Coamo. There must be a total thickness of strata, including shales and 

 interl)edded tiiffs with occasional small limestone, of perhaps several 

 thousand feet. In all parts of the formation where conglomerate is de- 

 veloped, the pebbles represent the same kinds of rocks as were encoun- 

 tered in the tuffs and intrusive masses. Actual representatives of previ- 

 ously solidified bedded material or indurated ash or shales are very rare, 

 but in one case at least a pebble was observed that was judged to repre- 

 sent a fragment from an older silicified tuff. As a matter of fact, the 

 materials are practically all of simple igneous character and the matrix 

 in most parts of the formation is very abundant, or even predominant, 

 the particles of which are of the same igneous material. The distribu- 

 tion of material and the range of composition leads one to believe that 

 this conglomerate represents a special state or condition whereby ma- 

 terials of essentially tufaceous origin were, immediately after their vol- 

 canic eruption, worn, rounded, somewhat assorted and bedded and mixed 

 with related material. At the point examined, there was no satisfactory 



