458 Haivkins — Notes on the Geology of Rhode Island. 



numerous basic inclusions in the granite of the Westerly 

 region, as suggested to the writer, may indicate that the 

 roof of the batholith was lower toward the south; and 

 perhaps also that the present coast line may be imagined 

 to have resulted from lack of resistance to erosion on 

 account of predominant amounts of the weaker basic 

 schists in the granite still farther southward. A similar 

 lowering of the batholithic roof to the eastward may 

 explain the present distribution of exposures of schist in 

 the Blackstone Valley and gabbros farther west; or 

 downward tilting toward the east (indicated by the atti- 

 tude of the sediments in the Woonsocket basin) may have 

 caused erosion to expose deeper seated rocks to the 

 westward. 



The arrangement of parallel belts already mentioned 

 suggests a synclinal or anticlinal structure. Minor 

 structures so obscure the larger ones, and exposures are 

 so scanty, however, that its exact nature has not been 

 discovered. The fact that two rock types on opposite 

 limbs of the fold (those at West Greenville and Moosup 

 Valley; see analyses) are closely related in chemical com- 

 position might be construed as favoring this hypothesis. 



The chemical analyses of the gabbros taken collectively 

 (Table II) show them to be uniformly of very basic types, 

 but varying systematically in content of femic and salic 

 constituents. The most basic example in the State is 

 the peridotite or cumberlandite of Cumberland Hill. This 

 may represent the ancient center of igneous activity in 

 intrusion. Westward and southwestward from it the 

 gabbros steadily decrease in femic content, rapidly at 

 first, then more and more slowly, to Moosup Valley, and 

 finally to Preston, Conn. When plotted graphically, the 

 line connecting the points representing femic content of 

 the various types form a continuous curve (fig. 2). This 

 would seem to indicate something of the nature of the 

 areal distribution of igneous intrusive activity in west- 

 ern Rhode Island before the great granite batholith 

 invaded the region. The writer also ventures to suggest 

 that the study of related rock types from Cumberland 

 Hill northward into Massachusetts might add further 

 interesting facts with regard to this basic intrusive 

 series, and ultimately establish proof whereby the geo- 

 logic age of the whole peridotite-gabbro-green schist 

 series might be established. 



