33 



with the typical granite on the one hand, and with the 

 petrosilex on the other ; and it has been regarded by Prof. 

 Hyatt as transitional between them. Although this is undoubt- 

 edly its true position, yet it probably belongs on the petrosilex 

 side of the imaginary line separating these two rocks. A some- 

 what coarser, but otherwise identical, rock in the vicinity of 

 Woburn and Purchase Streets, in West Medford, appears to be 

 similarly related to the granite and petrosilex of that town. In 

 the immediate vicinity of the Clarendon Hills Station, on the 

 Boston and Providence Railroad, are outcrops of a rock which, 

 though marked on the map as petrosilex, and unquestionably 

 closely associated with the petrosilex of that region, probably 

 belongs, in a lithologic sense, with the fine-grained granite. 

 The coarse granite about Dedham Village passes on the north 

 and west into a finer-grained variety ; which becomes finer 

 the farther we trace it from the type, and appears to cover 

 all the northern and north-western parts of the large area 

 in Dedham, marked as granitic on the maps. This rock, 

 quarried in West Dedham, is the principal material used in 

 the construction of Trinity Church in Boston. Beyond the 

 Charles River, in JMeedham, it becomes so fine-grained and 

 cryptocrystalline as to appear at some points inseparable from 

 the adjoining granitoid petrosilex, or quartz porphyry. Here, 

 as elsewhere, a sharp line of demarcation between these rocks 

 cannot be avoided on the maps, but it probably does not exist 

 in nature. The rocks in Needham, however, are exten- 

 sively drift-covered ; and this inference as to the relations of 

 the granite and petrosilex of this town might not be sustained 

 by the facts, could they be fully known. The granite in the 

 southern part of Dover is similar to that just described ; and 

 there are abundant indications of a passage into the petrosilex 

 on the east, the evidence in this instance being clearer than in 

 the other. This variety of granite occurs in the western part 

 of Newbury, along the river Parker, and its characters are es- 

 pecially distinct south of this stream, on the road leading south 

 from By field Parish. The granite bordering either side of 



OCCAS. PAPERS B. S. N. H. — III. 3 



