BERKEY, GEOLOGICAL BEC0XN0I8SANCE OF PORTO RICO 33 



miniferal matters representing organic growths accompanying the accu- 

 mulation of the deposit. 



The limestone members on the one side representing almost pure or- 

 ganic accumulations, and the conglomerates on the other representing the 

 simpler destructive volcanic f ragmental matters, give- the range between 

 which an exceedingly great variety of sediments are represeated. 



The shales of the yonnger series, represented by the Lares and the 

 Jnana Diaz shales, are more strictly detrital and of true ero^ional and 



Fig, 10. — Photomicrograph of the San Juan formation indurated dune sand, magnification 



28 diameters 

 The clearest grains are simple mineral fragments ; tlie dark ones and the grayish ones 

 with internal structure are fragments of calcareous organic growths. The grayish matrix 

 is a secondary binding material of calcific composition, in this case practically filling 

 the interstitial spaces. 



destructive origin, and in places they contain lignitic material which 

 suggests different physical conditions. This shows, however, in their 

 upper layers an increasing amount of organic content also, and finally 

 are succeeded by limestones of wholly organic makeup. The failure of 

 volcanic activity during and subsequent to that time gave no opportunity 

 for the amount of intermixture that is seen in the shales of the older 

 series, so that as a result the younger series of rocks is petrographically 



