234 MINERALOGY AND LITHOLOGY. 



gion is just as distinctly characterized by iine-grained argillitic rocks and 

 green schists, and I am going on to describe other schists and slates that 

 are widely spread over this region, which have only attained a half crys- 

 talline condition. Now, though quite coarse-grained crystalline rocks 

 occur in this valley, still, when a rock which has the same ultimate com- 

 position as a granite is found, it is usually an argillitic schist or clay 

 slate. The most interesting exceptions are the coarse-grained and mas- 

 sive diorites, which, on account of their condition and composition, were 

 long held, and are still held by some, to be eruptive rocks. Though their 

 relationships to the surrounding stratified deposits have been clearly 

 pointed out by the geologists already quoted, I would point to the fol- 

 lowing confirmatory circumstances, which are drawn from the litholog- 

 ical study of them, and which also explain in some degree the reason for 

 their massive condition. 



In the first volume of the report of this survey there is a series of maps 

 prepared by Prof. Hitchcock, which indicates that the strata that form 

 the interior of the state were first accumulated and elevated, and the 

 valley of the Connecticut was left as a long open sea, in which the sedi- 

 ments which now form these schists and greenstones were deposited. 

 Therefore they were not subjected to the powerful influences, the opera- 

 tion of which in the interior is shown by the highly crystalline rocks 

 which are much folded, elevated into mountains, and very frequently cut 

 by eruptive masses. In the valley region eruptive rocks are rare, and in 

 marked contrast we see what can be accomplished by gentle metamor- 

 phic action on stratified sediments ; and the study of lithological spec- 

 imens indicates that the following results have been produced by it. 



In the case of pure clays, but little more was accomplished than the 

 consolidation and production of very minute and formless crystals. The 

 same influences seem, however, to have been quite sufficient to develop 

 hornblende crystals of considerable size, though the form of these crys- 

 tals is imperfect. The amphibolites can therefore always be recognized 

 macroscopically. Again : when triclinic feldspars appear in these rocks, 

 they, too, commonly form crystals of considerable size, which are often 

 well developed in an otherwise very fine mixture of minerals. If, then, 

 the composition of the sediments were such as might be entirely recrys- 

 tallized into triclinic feldspar and hornblende, we might expect a massive 



