486 



Scientific Geology. 



(20) Mica Slate in Granite ; Chester. 



No. 21 differs but little from the preceeding cases. Two granite 

 masses appear to be connected by two veins ; the widest of which is 

 two feet, and the other only six inches thick. Thus a piece of mica 

 slate is insulated, and both this and the granite are cut by a more re- 

 cent vein of granite, fifteen inches wide. This case occurs three 

 miles northeast of the meeting house in Williamsburgh. But I have 

 not personally examined it, and am indebted to Mr. Alanson Nash 

 for the sketch. 



(21) Granite and Mica Slate ; Williarnsburg-h. 



Veins of granite traversing granite are more frequent in Massachu- 

 setts than in any other rock. Generally the veins are composed of 

 much coarser materials than the rock that contains them ; and by 

 this mark alone can they be distinguished, except that sometimes the 

 color of the materials of the vein and that of the containing rock, 

 are different. The following case occurs in the west part of Whate- 

 ]y, and exhibits a mass of granite of fine texture, about fifteen feet long 

 and ten feet wide, with mica slate on one side. The dark part of the 

 drawing represents this granite, and the white strips crossing it are 

 veins of coarse granite. Must we not suppose such veins produced 



