436 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



Page 



Riebeckite-segirite granite 463 



Quantitative studies 465 



Inclusions 467 



Riebeckite-bearing granite porphyry 467 



Diamond Hill quartz deposits 471 



Sheldonville quartz vein 473 



Post-Permian diabase dikes 474 



Summary 475 



Introduction 



The area which it is the purpose of this paper to describe is of interest 

 to petrologists because of the occurrence within its borders of the rare 

 ultrabasic rock cumberlandite, of a considerable mass of riebeckite gran- 

 ite and associated porphyries, and of the remarkable quartz deposit known 

 locally as Diamond Hill. As will appear beyond, all three of these for- 

 mations, as well as the general geology of the region, have been made the 

 subject of more or less study by workers in this field. A desire to ascer- 

 tain more fully the relations and characteristics of the riebeckite rocks 

 was the particular motive that led the present writers to take up the 

 study of the region, and while this phase of the investigation was the 

 main object, it has been thought well to extend somewhat the scope ol" 

 the paper, so as to include a description of the surrounding formations 

 for a few miles on all sides. 



Besides giving the relations of the riebeckite granite, it is hoped tliat 

 the present paper will contribute something to the knowledge of the bio- 

 tite granites, whose identity or non-identity are the subject of so much 

 dispute. The riebeckite rocks near Cumberland Hill mark, so far as is 

 known, the southernmost extension of rocks characterized by the essen- 

 tial minerals, microcline-microperthite and alkali hornblendes or pyrox- 

 enes, or both, which are so prominent a feature of the petrology of tho 

 eastern part of Massachusetts and which reach their most important 

 development in the Quincy-Blue Hills area and from Salem and Marble - 

 head northward to the extremity of the Cape Ann Peninsula. 



The accompanying geologic map, based on a large amount of detailed 

 work, has been prepared to give as accurate an idea as possible of the 

 distribution of the various rock types. The boundaries are in part hypo- 

 thetical, particularly for that part of the map which lies in Massachusetts 

 , and in the vicinity of the Blackstone River, where outcrops near contacts 

 are rare. In places where bedrock is covered by drift so efficiently as to 

 render the mapping of the former impossible, a hlank space has been left 



. . . ... .V .r.^.; . . .... 



