248 



slate teaches that these rocks are not only cut off and faulted 

 by the amygdaloid transversely and along the strike, but that 

 they have also been forced asunder at intervals by intrusive beds 

 of this semi-igneous material. 



Immediately north of the ledge last described there is another 

 long narrow mass of the indurated red slate, with the usual 

 dip. It is of almost jaspery hardness, and is overlaid by 

 amygdaloid ; in fact, though much broken and contorted, and 

 of very irregular outline, it appears to be entirely isolated by 

 that rock which is part of a prominent east-west ridge of 

 amygdaloid extending from Cambridge Street nearly to the 

 Brookline boundary. 



The amygdaloidal texture is well developed in the most of 

 this great mass ; and it is an interesting observation that the 

 only two localities in all this region characterized to any extent 

 by a typical amygdaloid — Brighton and Nantasket — show 

 this rock occurring under precisely similar conditions as regards 

 its relation to the slate and conglomerate, and are at the same 

 time the points affording the most indubitable evidence of its 

 extravasation. Just west of Allston Street this ridge is flanked 

 on the north by a brownish slate dipping N. 40^—45°, and very 

 plainly cut off by the amygdaloid toward the east ; it is also 

 traversed by a north-south, vertical trap-dyke four feet wide. 



Though not traceable all the distance on the surface, this bed 

 of- slate, with a breadth of from one hundred to two hundred 

 feet, appears to extend to and across Cambridge Street, run- 

 ning between two amygdaloid ridges. Just south of Cam- 

 bridge Street the red slate can be seen passing beneath the 

 amygdaloid, or abutting against it with a northerly dip of 

 thirty-three degrees. On the north side of Cambridge Street, 

 near the Brighton Water-Works, the slate, considerably faulted 

 on a small scale, intersected by endogenous quartz, and dipping 

 N. 25°-30°, is very clearly overlaid by the amygdaloid; and 

 both rocks are cut by north-south vertical dykes of diabase, 

 one of which, about twelve feet wide, is well known as an 

 admirable example of the formation of boulder-like masses 171 



