302 NEW YORK STATE MUSEUM 



is a range in composition from granites through syenites and 

 diorites to gabbros, all intermediate gradations appearing. If 

 there be any rocks exposed in the region which are older than the 

 Grenville rocks, they are found here. Unmixed with Grenville 

 rocks, they extend along the Potsdam boundary on the north side 

 of the Adirondacks for a distance of 70 miles. Nowhere else in 

 the region is a belt of such length known, though there may prove 

 to be one of even greater dimensions in the little studied south- 

 western area. Smyth has shown that a great, unbroken extent 

 of gneisses occurs there, but these may prove in large part to 

 belong with the later intrusions. Grenville gneisses may be also 

 found more abundantly than yet appears, when the region is 

 covered in more detail. Such gneisses are abundant north from 

 Little Falls, though no limestones occur. 



These gneisses present just such an igneous complex as is found 

 in all parts of the earth's surface where these very old rocks are 

 exposed, which are thought by many to represent the original 

 cooled crust ^of the earth, or rather its downward extension. 

 There is much to be said in favor of this view, though it can by 

 no means be held to be fully established. The main difficulty of 

 its adoption so far as these special rocks are concerned is that 

 very similar, or identical, gneisses are found either interbanded 

 with the Grenville rocks or else cutting them intrusively, as has 

 already been noted. If the two are not identical, it should be 

 possible to demonstrate differences between them, and the future 

 may show this possibility. If they are identical, the only pos- 

 sible way in which they could represent the floor on which the 

 Grenville rocks were laid down would be to hold that in many 

 places, after the deposition of the Grenville rocks, these under- 

 lying rocks had been rendered plastic by heat and compression 

 and had thus comported themselves as igneous rocks. The 

 difficulties against this view are great, and the whole question is 

 a most perplexing one. 



To the northward in Canada there are great stretches of country 

 occupied by similar rocks, and the name " Ottawa gneiss " is there 

 given to the formation. Uncertainty as to the equivalency of the 



