102 



seemingly gradual transition between this granite and the diorite 

 is undoubtedly real and perfect. All observers of these two 

 rocks will agree with me that they admit of neither a lithologic 

 nor a geographic separation. As a rule they are both evidently 

 eruptive, and over large areas they have been extravasated 

 through each other so extensively, and the action has been so 

 mutual, that the confusion is complete, and I have long been 

 accustomed to speak of these as ' ' mixed rocks ; " and I know 

 now of no term that will better express their relations lithologi- 

 cally or petrologically. Those desiring to study the complication 

 of these rocks, in the field, will find the best exposures for that 

 purpose in Marblehead and Salem, especially along the shores 

 in the former town. 



Most observers have failed to discriminate between the 

 various granitic and dioritic rocks of Eastern Massachusetts, 

 classing them all together under the general term " syenite." 

 At some points there is at best but little difference, but the 

 same is true of any series of rocks ; the adjacent members of 

 the series must approximate. This is a fundamental principle 

 of the science, growing out of the mode in which sediments 

 laid down by water must necessarily be arranged. The law of 

 continuity certainly holds in lithology. Yet, though we cannot 

 separate the fine-grained, dioritic granite from the diorite, we 

 can and ought to distinguish between the latter and the coarse- 

 grained, little hornblendic granite. I have been able to recog- 

 nize two varieties of the hornblendic granite associated with the 

 diorite. In one the hornblende is very abundant, the feldspar 

 is gray, and the texture of the rock granular or crystalline. 

 This variety is far more abundant than the other, in which the 

 hornblende is little conspicuous, and the feldspar usually red, 

 sometimes gray, and always more or less compact, resembling 

 petrosilex. The second variety is more quartzose than the 

 first, and the quartz is in the form of rounded grains. 



The term diorite, as here used, probably covers a whole 

 family of basic rocks ; these are supposed to consist essentially, 

 however, in each case, of a plagioclase feldspar and some 



