32 



ANNALS NEW YORK ACADEMY OF SCIENCES 



the hasty examination given to them leads to the belief that clifEerentia- 

 tion effects can be traced. 



The closest relatives of the igneons rocks are the sediments, and be- 

 cause of the fact that the material constituting these sediments has been 

 furnished by the volcanic fragmental supply in large part, their character 

 and makeup is in many cases not strikingly different from the ashes and 

 finer tuffs. They do, lio-wever, represent an additional assorting, an ad- 

 ditional weathering and an additional opportunity for intermixture of 



Fig. 9. — PJiotomicro(/r(ii)h of a foraminifeial shale from the Bayamon-Comerio road, 



magniflciition 28 (liameters 



The dark areas are chiefly earthy aggi-egates or very fine texture : the whitish areas 

 are calcareous spots which in many cases still preserve the forms of foraminifera. It is 

 the removal of such materials from the shales that is believed to account for their 

 porosity as seen in weathered ontcioiis. 



materials from different sources and uf organic material developing at the 

 same time. These conditions give a great range of composition and 

 mineral makeup to the shales and sandstones- and they merge by imper- 

 ceptible gradations from simple tufaceous or arkosic sediments to cal- 

 careous rocks or even to fairly pure limestones. The common source of 

 the calcareous element in these rocks is from an intermixture of fora- 



