BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 11, pp. 115-126 March 22, 1900 



VOLCANICS OF NEPONSET VALLEY, MASSACHUSETTS 



BY F. BASCOM 



(Presented before the Society December SO, 1899) 

 CONTENTS 



Page 



Introduction 115 



Structure of the region 115 



Age of the rocks 116 



Acid volcanics 116 



Macroscopic characters and occurrences 116 



Petrographic characters and mineral constituents 117 



Fragmental acid volcanics . . . 118 



Chemical composition. 121 



Basic volcanics 122 



Occurrence and petrographic characters 122 



Basic fragmentals 123 



Chemical constitution and name , 124 



Porphyritic volcanic 125 



Conclusions 126 



Introduction 



While the igneous rocks of the Boston basin have been the subject of 

 considerable investigation and discussion, the volcanics of the limited 

 portion of" the basin drained by the Neponset river have received little 

 attention. No petrographic study has been made of these volcanics. 



Recently Professor W. O. Crosby afforded the writer an opportunity 

 to visit this district and to collect specimens, and further encouraged a 

 more detailed study of the material collected. 



The general geology of the region has been investigated by Professor 

 Crosby and is quite aside from the purpose of this paper. A brief sketch 

 of the results of the investigation,* so far as they have appeared, may 

 not, however, be out of place. 



Structure of the Region 



The prevailing rock is a conglomerate and the prevailing structure is 

 anticlinal. Along the northern margin of the belt the conglomerate 



♦Physical History of the Boston Basin, 1899. The Neponset Valley, p. 6. 

 XVI— Bull. Geol. Soc. Am., Vol. 11, 1899 (115) 



