THE PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, 



MASSACHUSETTS. 



By F. Bascom. 



Introduction 131 



General Geology of the Valley 132 



Petrography : 



Plutonic Igneous Rocks: 



Granite 134 



Microgranite 137 



Rhyolite 138 



Intrusive and Effusive Igneous Rocks: 



Granite Porphyry and Rhyolite Porphyry Dikes 140 



Aporhyolite Lava 141 



Aporhyolite Pyroclastics 145 



Aporhyolite Dikes 147 



Trachyte Lava 149 



Andesite Lava 151 



Andesite Pyroclastics 154 



Andesite Dikes 154 



Acid Andesite Dike 155 



Diabase Dikes 156 



General Discussion and Conclusions 159 



INTRODUCTION. 



While the igneous rocks of the Boston Basin have, from an early date, been 

 the subject of considerable investigation and discussion, only recently have they 

 * been subjected to detailed petrographic and chemical examination. Such a study 

 has been made of the Essex County Petrographic Province by Dr. Henry S. 

 Washington 1 and of the Blue Hills Complex by the late Dr. Theodore G. White. 2 

 Adjoining the Blue Hills' province on the north and including the whole of Hyde 

 Park, and portions of Canton, Dedham, Quincy, Milton, Mattapan, West Rox- 

 bury, and Dorchester townships, is a district of considerable topographic diversity , 

 drained by the Neponset River. Within this district, which is some three miles 

 wide and ten miles long, are exposed a seemingly lawless assemblage of intrusive 

 and effusive igneous rocks. 



* Henry S. Washington, Journal of Geology, vol. VI, pp. 787-808 

 105-121, 284-294, 463-482, 1899. 



2 Theodore G. White. Proceedings of th* Boston Soniptv of Natural 3 



64 



PP. 117-156, 1897. 



XXVIII 



131 



