26 



success. More might have been accomplished with a longer 

 time for exploration ; yet much must ever remain undone, on 

 account of the great extent to which the rocks are, in some dis- 

 tricts, concealed by superficial deposits. A special color has 

 been used to represent the general Huronian formation wherever 

 the particular lithological representative is not known ; but the 

 probabilities are great that the rock, whatever it is, belongs to 

 this age. 1 



The Huronian system in this region, like the Naugus Head 

 series, though in a somewhat less degree, exhibits great disturb- 

 ance. Distinctly bedded rocks are the exception ; and, although 

 many apparently structureless rocks are probably really strati- 

 fied, it is undoubtedly true that a large part, perhaps the greater 

 part, of the formation has been more or less fluent, and ex- 

 travasation may be set down as a characteristic structural 

 feature. The extent to which some of the rocks of this series, 

 in Eastern Massachusetts, are characterized by a condition ap- 

 proaching chaos, can be fully appreciated only by those who 

 have studied them somewhat in detail in the field. The strati- 

 fied portions of this series have usually a N. EL — S. W., vary- 

 ing to E.— W., strike ; and the areas of unstratified and extrava- 

 sated rocks generally exhibit in their trends a tendency to 

 parallelism with the strike of the stratified rocks. The latter 

 usually dip steeply to the north-west. 



The Huronian series of Eastern Massachusetts is principally 

 composed of the following rocks, or, rather, groups of rocks : — 



1. Granite (hornblendic and binary). 



2. Petrosilex (passing into felsite and quartzite). 



3. Diorite (unstratified and largely exotic). 



4. Hornblendic Gneiss, Stratified Diorite, etc. 



5. Limestone. 



i In adopting the plan, wherever practicable, throughout this work, of mapping the 

 various formations lithologically rather than geologically, I have done much to keep 

 my facts and theories separate. 



