PREFACE. 



Eaely in the summer of 1875 I began a systematic study 

 of the rocks in this vicinity, with the view of collecting such 

 data as, added to my previous knowledge, would enable me to 

 prepare a thesis for graduation in the department of Natural 

 History of the Massachusetts Institute of Technology, on the 

 <' Geology of the Environs of Boston." In the beginning of 

 the following winter I was charged by Prof. A. Hyatt, under 

 the direction of the Massachusetts Commission to the Centen- 

 nial Exhibition, with the task of preparing a geological map 

 of the State, for exhibition at Philadelphia. Opportunity was 

 afforded me by the Commission to visit all parts of the State. 

 The Boston Society of Natural History generously permitted 

 the use of my time as assistant in their Museum for this 

 purpose, and, as that winter was unusually favorable for ex- 

 ploration, I was in the field almost constantly. The Centen- 

 nial map and the report on the same were completed in the 

 following spring. The wider range of observation which this 

 work for the State had afforded seemed to justify me in ex- 



