1875.] 391 [Dodge. 



tained in the body of the rock, then becoming true porphyry. It fre- 

 quently holds translucent quartz grains. The compact felspathic vari- 

 ety is very different in appearance from (6.), the felspathic rock above 

 mentioned, which has also been called porphyry, being much less 

 coarsely crystalline; owing to its finer texture it changes less in ap- 

 pearance from its original condition under metamorphism. Pebbles of 

 it are abundant in the Brighton conglomerates. It probably belongs to 

 the crystalline group, but is puzzlirigly related to the slates in Maiden 

 and towns on the south shore. Perhaps some of the slates are so altered 

 in those two regions of great metamorphic effects, as to resemble the 

 real porphyry, thus leading to confusion of identity. Certainly the 

 slates do become so altered as to approach it in character by several 

 degrees of change which may be clearly traced. A third rock (c). 

 that has been called porphyry differs esentially from the other two. 

 It lies in the southern part of Needham, and may be examined at a 

 quarry about three-quarters of a mile north of Charles River village, 

 on Central Avenue. 1 It is associated with hornblendic rock which, 

 unlike that with the crystallines generally, is amygdaloidal. The 

 rock in question seems in places to be a greenish- white sandstone 

 with a high dip north-westward. So, also, at High Rock, so called, 

 east of the quarry; and along the track of the Woonsocket Division 

 of the New York and New England (formerly Boston, Hartford and 

 Erie) Railroad. A mile east, near a brook which flows south to the 

 river, it is reddish. Again, some specimens of the same granular 

 character contain crystals of hornblende. The crystallines in west- 

 ern Dedham are more markedly crystalline and redder. Farther 

 east, on the north side of the river, the rocks become more crystalline 

 or igneous in appearance, with many quartz veins. (77.) There is also 

 in Needham a sienitic rock, which is often porphyritic. The area of 

 felspathic rocks, of which two varieties these form a part, extends 

 into the southern part of Newton. 



Beside the clearly crystalline rocks, there are others apparently of 

 this group, which retain their stratification more or less. In Great 



1 To make the following local descriptions permanently useful, it will be neces- 

 sary to name a few current maps, on which the names of streets (which are espe- 

 cially liable to change), and other local designations adopted are laid down. The 

 map of Boston and vicinity, published by A. Williams (latest ed., 1874), is the 

 cheapest and most convenient for general reference. The Baker & Tilden Vicinity 

 Map, and "Walling's county and town maps, though now old, are very useful,, 

 being on a large scale. The Boston Directory Map already covers a large and 

 interesting portion of the ground. Atlases of Cambridge, Newton, and other 

 large cities, have lately appeared. 



