Topography of Greenstone. 



415 



are mere varieties of the same family, would such a demarcation be 

 of any great use? although I could wish to see it done ; since in that 

 way many facts might be brought to light, important to geology. 



Rarely does the greenstone under consideration form ridges or ele- 

 vations of any considerable height. In Weston, Waltham, Lincoln, 

 Lexington, and West Cambridge, this formation attains its greatest 

 elevation ; which is never as much as 500 feet above the ocean. 



The greater part of the greenstone under consideration is exceed- 

 ingly hard and compact, and the ingredients are with difficulty dis- 

 tinguished. When passing to sienite, however, they become coarse 

 and highly crystalline. Very frequently the rock has a greenish as- 

 pect, from a quantity of epidote which is disseminated in it, or forms 

 narrow veins, or a coating upon the surface. It is not common, ex- 

 cept where it is associated with the graywacke, to see it exhibit that 

 brown dirty aspect so common in the trap rocks of posterior date. 



Occasionally we find examples of a slaty structure in this green- 

 stone. And it must be regarded as really a slaty structure, not the 

 result of a concretionary deposition.* For the slate generally ap- 

 pears to be genuine hornblende slate, sometimes rather less crystal- 

 line, however, than that rock generally is. I recollect at this mo- 

 ment but three places where this slaty greenstone was observed : viz. 

 in Lincoln, on the turnpike between Andover and Boston in Stone- 

 ham, and near the line between Reading and Wilmington. In a the- 

 oretical point of view this fact seems to me important ; and I shall 

 recur to it in- the sequel. 



Variety No. 4, that has been described above, is found in connec- 

 tion with sienite, a mile or two north of Byfield Academy. Near the 

 academy we find red compact feldspar : but I do not know that this is 

 at all connected with the greenstone. On the north side of Merrimack 

 river, in Salisbury, opposite Newburyport, this same variety of rock 

 occurs in juxtaposition with sienite. Its aspect not a little resembles 

 the varioloid wacke in Saugus ; and I am not without strong suspi- 

 cions that it may be the same rock highly indurated. And it strength- 

 ens this suspicion to find that sometimes in Newbury it exhibits a va- 

 rioloid structure. 



The map exhibits the most northerly of the greenstone ranges in 

 the Connecticut valley with which I am acquainted ; though in Ver- 



* Dr. Macculloch/jhowever, regards the slaty structure as a variety of the con- 

 cretionary. 



