Cushing — Geology of Clinton County. 



the height of 1,000 feet, and some <>f them are in such situations that they 

 can not possibly be regarded as river gravels and sands. This matter needs 

 further investigation. 



Mktamorpiiism of the Pre-Cambrian Rocks. 



After the intrusion of the gabbro, and prior to the commencement of 

 Potsdam deposition, the region was subjected to intense dynamic meta- 

 niorphism. The precise results produced would vary with the physical and 

 chemical properties of the rocks concerned, but, broadly speaking, they con- 

 sisted in the foliation of the rocks and the granulation of their contents, with or 

 without subsequent recrystalli/ation. The gneisses were granulated and sub- 

 jected to a stretching process, by means of which the foliation and some, 

 at least, of the banding was produced ; rocks like the gabbro dikes being 

 drawn out like the rest, and made to appear like an integral part of the series. 

 W hile some of the gneiss is characterized by a cataclastic structure, other 

 portions of it seem to have been completely recrystallized, and the different 

 minerals seem for the most part to have formed at the same time, few, if 

 any of them, showing idiomorphic boundaries against the rest. That this 

 reerystallization was not the final result of the metamorphism, is shown by 

 the 'frequent pronounced undulatory extinction shown by the component 

 minerals of such rocks. 



The anorthosites and gabbros were granulated and stretched in the same 

 manner as the gneisses, though apparently being more resistant to the latter 

 process. In the less feldspathic anorthosites, foliation was thus produced, 

 giving them their gneissoid aspect, and in them more or less reerystallization 

 took place with the formation of minerals foreign to the parent rock, by the 

 reactions of the various constituents on one another, much garnet, hornblende 

 and biotite thereby resulting. The stages of the process of granulation can 

 be studied to as great advantage in the Clinton county anorthosites as in 

 the Canadian examples so exhaustively described by Dr. F. I). Adams. 



The basic gabbros seem to have been characterized originally by an 

 ophitic structure, but as a rule they are more thoroughly and finely granulated 

 than the anorthosites. They are often of such tine grain, and their boundaries 

 against the enclosing rocks are so sharp that the resemblance to dikes is very 

 striking. Reaction rims are a prominent feature in these rocks. They almost 

 always contain hypersthene, which is not common in the anorthosites and less 

 basic gabbros. In many cases the rock seems t<» have undergone complete 

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