* ft 



MUSEUM OF COMPAKATIVE ZOOLOGY. 



171 



granite and felsite near together, are to be found upon Break-heart Hill in 

 Saiigus. It in to this locality that Mr. Crosby refers when ho says : * 

 " On the high hills to the west of the private road running from Forest 

 Street and Central 13rook to Water Street we have, perhaps, the faiest 

 example of the extravasation of the granite yet observed in this region. 

 The exposures of the rock here are remarkably good ; and the granite 

 is coarse and sharply defined where it penetrates the adjoining petrosilex 

 and liorubleude slate in irregular dykes, or envelops isolated masses of 

 these rocks that have been wrested from the parent beds." 



Concerning the fact that the granite envelops detached fragments of 

 the stratified group there can bo no doubt • but when we come to exam- 

 ine the relations of the felsite and the granite, the evidence is not so 

 easily deciplicrcd. The top of the hill is composed of a mixture of 

 felsite and volcanic ashes, which extends over the southeastern slope, 

 meeting the rocks of the stratified group ; to the northward, a short 

 distance, this mixed mass is liautcd by the coarsely crystalline granite 

 containing the stratified fragments ; and to the westward it connects 

 with a large area of similar rocks forming the high hills near by in that 

 direction. Within the small area of this complex rock there are two 

 patches of granite. One of these, with a diameter of about twenty-five 

 feet, upon the western brow of the hill, has its side penetrated by a 

 tongue, at least six feet in length, of very distinctly banded felsite. 

 The banding is also very distinctly marked at several places along the 

 periphery of the granite, and in every case it is parallel to the line of 

 contact between the two rocks. Tlie smaller patch of granite upon the 

 southern slope of the hill is completely enveloped by felsite. Although 

 the felsite in this case is not banded, it apparently sends tongues into 

 the granite, and, near the junction of the two rocks, it envelops small 

 fragments of the larger mass which it surrounds. Although there are 

 other exposed contacts of the felsite and granite in the neighborhood, 

 none were observed to furnish important evidence bearing upon the 

 relations of the two rocks. The banding of the felsite is undoubtedly 

 of igneous origin, and its occurrence about the granite, together with 

 the tongues of felsite penetrating the granite, and fragments of the 

 latter rock enveloped by the former, are phenomena which, as it seems 

 to us, can be explained only by supposing that the felsite flowed through 

 and around the granite. 



The well-marked dikes of felsite cutting the granite between Fishing 

 Point and Phillips Beach on the Swampscott coast, as first pointed out 



*' Contributions to the Geology of Eastern Massachusetts, p. 78. 



