No. 2. — The Felsites and their Associated Eochs North of Boston. 



By J, S. DiLLEK. 



The extremely interesting and complicated petrology of Eastern 

 Masyacliusetts has been the subject of a great deal of discussion, and 

 at no time in the past have opinions concerning the origin and relations 

 of the rocks been more at variance than at present. It is important, 

 therefore, that the facts to be found in nature be carefully observed and 

 described in order that the various differences of opinion may, as far as 

 possible, be removed and the truth demonstrated. There are, however, 

 certain localities whose facts appear, under the eyes of some observers 

 to be very different from those seen by other observers in the same place ; 

 and it seems necessary in such cases, where the facts are questioned, to 

 describe them with more than usual detail. 



The field we have explored includes Melrose, Maiden, and the southern 

 portions of Medford, Stoneham,* Wakefield, Saugus, and Lynn. Within 

 this area felsite is the chief rock, and with it are associated, besides a 

 complex group of tufaceous rocks, commonly called breccias, a group 

 of stratified rocks, granites, dioritos, slates, and diabases. The recent 

 "Contributions to the Geology of Eastern Massachusetts," by W. 0. 

 Crosby, contains a map upon which the general distribution of the 

 rocks, so far as we know, is quite correctly given; but, as Mr. Crosby 

 is doubtless well aware, there are many places, and some of them im- 

 portant, too, where the map is in error. We cannot expect to have for 

 many years to come a very accm-ately detailed geological map of this 

 neighborhood, for even if we had, what we have not, a correct topo- 

 graphical map to use as a basis of geological work, the extreme com- 

 plexity of the rocks, together with their highly altered condition and 

 the existence of quite extensive covered areas must necessarily require 

 a longtime for correct delineation. In determining the relative age of 

 eruptive rocks we have been guided by the generally accepted criterion, 

 viz. that of two rocks, the one which penetrates the other in dike-like 

 masses or contains fragments of the other is the younger. In consider- 

 ing the rocks associated with the felsites, we shall notice them in the 

 order of their ages, beginning with the oldest, and endeavor to describe 

 the facts apart from theoretical considerations. 



VOL. VIL — NO. 2. 11 



