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ANNALS NEW YORE ACADEMY OF SCIENCES 



■westward across the Descalabrado Elver to the Jacaguas Eeservoir. Sim- 

 ilar limestones are found at other points on the south side of the island 

 and are judged to belong to the same member of the older series. Be- 

 cause of the strong development in the vicinity of Coamo Springs, and 

 because of the fact that it represents a type so striking as to be recog- 

 nizable as a field unit, the limestone has been called by us in the field the 

 Coamo Tuff-Limestone. It is developed in the vicinity of the Coamo 

 Eeservoir to a thickness of several hundred feet and its most character- 

 istic appearance is the brownish mottled color effect produced by the 



Fig. 5. — Interbedded coamo limestone layers with massive tufts 

 This formation is seen at the military road crossing of the Descalabrado River. 



presence of fragments of tuff and accumulations of ash. In some beds 

 this material is so abundant as to make up almost the whole rock and it 

 becomes an interbedded tuff layer. Occasionally the limestone beds are 

 very pure and almost entirely free from volcanic materials, and there are 

 also numerous beds of real volcanic tuff, but typically there is an inter- 

 mixture of tuff materials with the lime in great enough abundance to 

 give a brownish spotted or mottled effect. An equally characteristic 

 feature of the rock is its concretionary or nodular appearance due to 

 algous growths to which the lime accumulation is chiefly due. The finest 

 development to be seen anywhere in the island is on the Descalabrado 



