Varieties of Greenstone. 



405 



it is of no importance in the present instance, whether it be added or 

 not. A mixture of hornblende and feldspar, the former in much the 

 largest proportion, and both of the minerals exhibiting but little of a 

 crystalline structure, constitutes the great mass of the trap rocks of 

 Massachusetts. Other varieties do, indeed, occur, composed of 

 different ingredients : but as these are found in such small quantity, 

 and are obviously accidental varieties, I have thought it most judi- 

 cious to describe them all under the term greenstone. Such a liberty 

 I have frequently taken in the case of the stratified rocks : and I 

 think it still less objectionable in the case of the unstratified ; because 

 there is much more diversity of opinion among geologists as to the 

 ingredients that compose the latter. These ingredients are often so 

 little crystalline, as to be exceedingly obscure in their characters ; 

 and it is, therefore, no wonder that such diversity exists even in the 

 statements of the ablest writers. Especially this is not surpris- 

 ing, when we recollect, that until recently, it was thought essen- 

 tial to a good description of these rocks, that the observer should be 

 able to prove that they belonged either to the primitive, transition, or 

 secondary class of the stratified rocks. It was bad enough to be ob- 

 liged to stretch the stratified rocks upon this Procrustean bed, al- 

 though here these artificial divisions had some appearance of natu- 

 ralness : but in the unstratified rocks, no facts could be found on 

 which to base such an arrangement ; and, therefore, imagination 

 must supply the necessary characters. The consequence was, that 

 minute and ever varying mineralogical characters in the trap rocks 

 were studied with scrupulous exactness ; while their geological posi- 

 tion and chemical and mechanical influence on other rocks, were 

 scarcely noticed. 



Mineralogical Characters. 



1. Hornblende and Feldspar : the mixture more or less granular. 

 Commonly the ingredients are so fine that they are with some diffi- 

 culty discerned by the naked eye. Hence it is not easy to determine 

 always whether the feldspar is compact or foliated. Frequently I 

 believe, however, that both varieties will be found, and that often in 

 the same specimen. The crystalline structure of the hornblende is 

 usually very indistinct. In the eastern part of the State, however, 

 where this rock is associated with sienite, the two ingredients are of- 

 ten very distinct, and the texture crystalline. A variety occurs on 

 Mount Holyoke and in West Springfield, in which the ingredients 



