GEOLOGY OF THE NORTHERN ADIRONDACK REGION 273 



this change from their original character was effected. But the 

 character of the change brought about by compression varies with 

 the depth of the roclvS beneath the surface because of hightened 

 temperature and pressure, and rocks may become so greatly 

 modified as to lose all trace of their original character. 



Precambric history 



Old sediments. The oldest rocks which have with certainty been 

 recognized in the Adirondack region consist of certain well 

 banded gneisses and schists, with bands of varying thickness 

 of coarsely crystalline limestone. They are believed to be old 

 water-deposited rocks, ancient sheets of sand, mud and calcareous 

 mud, deposited on the floor of some large body of shallow water, 

 in all probability the sea. They are now so greatly changed from 

 their original condition that the structures and textures char- 

 acteristic of rocks so formed have been almost wholly destroyed, 

 being replaced by others which are not characteristic, since they 

 may be produced in igneous rocks as well. The inference as to 

 their original condition is based partly on their composition, and 

 partly on the fact that they show frequent and rather abrupt 

 alternations in character, as if they had originally consisted of 

 beds and layers of varying composition, as water-deposited rocks 

 do. There is apparently a great thickness of these beds, but 

 their base has never been made out with certainty nor is their 

 summit known, so that our ideas concerning their thickness are 

 of the vaguest. They must have been laid down on a surface 

 of older rocks; but we are at present wholly in the dark as to 

 whether or not these older rocks are anywhere exposed in the 

 district at the present time. Rocks which may not improbably 

 represent them, are present and will be shortly described, but 

 no exposures which will enable a decision in the matter have been 

 discovered, nor are they likely to be in the immediate district, 

 though perhaps such may be found to the west or south. 



Though unknown, the thickness of these deposits is great, with 

 repeated changes in character, so that it is beyond question that 

 the submergence endured for a long, a very long time, during 



