45 



pointing to a division of the granite between two distinct 

 formations, the Huronian and something older ; but I have seen 

 no evidence of this character ; all the facts, so far as I have 

 observed, going to show that, as respects its origin at least, the 

 granite is a single rock. Although well satisfied that a large 

 proportion of the granite has been in a state of igneous plas- 

 ticity, yet its relations to the stratified petrosilex and the many 

 traces of bedding which it still retains forbid me to beheve that 

 the mass of this rock has been elevated from any vast depth ; 

 it seems rather like an extensive stratified formation which has 

 been softened in situ, and then to a greater or less extent 

 forced out of its normal position by the pressure of surrounding 

 and overlying terranes. 



PETEOSILEX. 



The determination of the relations of the group of rocks 

 here included under the general name of petrosilex is the most 

 difficult problem in the geology of Eastern Massachusetts. No 

 other rock in this region is subject to such great variations in 

 composition and structure, or is more puzzling in its petrologic 

 relations. Tliis diversity is not apparent at first view. On the 

 contrary, these rocks are likely to strike the casual observer as 

 comparatively uniform throughout their distribution and simple 

 in their relations ; and it is only by a careful study of the whole 

 field that the mind is finally divested of this idea, and the con- 

 clusion reached that these rocks are not one, but many. 



Porphyry is a much-abused word, which, like syenite (by 

 syenite is not meant the aggregate of orthoclase and hornblende 

 to which this term is now applied), has outlived its usefulness, 

 and should, in the opinion of the best geologists, be allowed to 

 become obsolete in its substantive use,- as a geological term. 

 Much of the rock in this region to which the term porphyry 

 is applied is not even porphyritic ; showing into what logical 

 errors we are led when we choose for the basis of a lithological 

 name a property common to many rocks, and which cannot be 



