LITHOLOGY. 235 



and comparatively coarse-grained rock as the result of the metamor- 

 johism; and as less basic strata are little modified by the same meta- 

 morphism, we might expect that such massive crystalline diorites would 

 appear between layers of less crystalline slates and schists. We might 

 expect, also, if the composition were such as to convert the larger part 

 into hornblende and triclinic feldspar, these predominant minerals would 

 determine the structure, and the excess of quartz might be included 

 between the crystals of a massive quartz diorite. It is well known that 

 in still waters fine sediments are deposited, and that these may possess 

 a very basic or a very siliceous character, as has been shown by the study 

 of the composition of the materials on the bottoms of bays and harbors 

 and on the ocean's bed. Such a variety of sediments, when subjected 

 to the influences that have operated on the materials of the White Moun- 

 tains, would all be changed into highly crystalline rocks; but under these 

 more gentle influences, only those rocks would reach a highly crystalline 

 state which are composed of minerals most easily developed. The gentle 

 nature of the metamorphism in these regions is indicated by the strati- 

 fied, almost fragmental character of rocks of the same nature as granite, 

 and the very common association of basic and acidic elements, which 

 under the influence of greater heat or pressure would react on one 

 another. 



Hence, as sediments of almost all varieties, from pure carbonate of 

 lime on the one hand to pure sand on the other, are found at the bottom 

 of still waters, it is natural that among them there will be some deposits 

 of the composition of diorites ; and as diorites are easily made, then they 

 may be expected at times among stratified deposits. The assumption, 

 that diorites are all eruptive because the larger part of them are so, is a 

 position to which exception can well be taken. 



Again : by some, rocks of this class, when plainly stratified, have been 

 claimed to be the consolidated tuff or debris of basic eruptive rocks. 

 Of course this may be true in some cases, yet I think that no such 

 direct origin is to be attributed to these rocks which are so widely dis- 

 tributed in locahties all over this part of America. The question of chief 

 interest is. What is the difference in microscopic structure, or other char- 

 acters, between metamorphic and eruptive diorites ? This question, so 

 far as New Hampshire rocks are concerned, I have attempted to answer. 



