GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 47 



nearly surrounding the mountain and across its strike. A very 

 similar example is furnished by the mountain south of Rock lake. 



The great igneous mass making up Blue mountain rises 2000 

 feet above the Grenville immediately to its southwest and south. 

 Also the presence of glacial boulders of Grenville limestone and the 

 nature of the topography very strongly suggest that Grenville now 

 is, or formerly was, present in the Tirrell pond valley. No rock 

 exposures occur around the shores of the pond or along its outlet 

 for some distance southward. At any rate, it is clear that much of 

 the great difference in altitude between Blue mountain and the 

 adjacent Grenville must have been caused by the intrusion of the 

 magma through the Grenville. 



Perhaps the finest example of marked irregularity of a bathy- 

 lithic surface is that of the great Panther-Snowy mountain mass 

 occupying the southern portion of the Blue Mountain quadrangle 

 and the northern portion of the Indian Lake quadrangle and already 

 described in detail under " foliation." This large body of syenite, 

 as above explained, apparently rose under and raised up (domelike) 

 the Grenville strata which have all been removed by erosion except 

 the peripheral belt now remaining on the west, north, and north- 

 east. As a result of this intrusion and subsequent removal of Gren- 

 ville, the higher portions of the syenite mass now rise fully 2000 

 feet above the Grenville. 



A striking illustration of another type of irregularity of bathy- 

 lithic surface is furnished by the basin (partly on the Blue Moun- 

 tain and partly on the Newcomb sheet) which contains the Chain 

 lakes (except the first and second lakes), Deer pond, Mud pond, 

 and Jackson pond. The bottom of this basin, which has a length of 

 4 miles and maximum width of i^ miles, is wholly occupied by 

 Grenville limestone and its associated hornblende gneiss at an 

 altitude of 1600 to 1700 feet. This valley is completely surrounded 

 by a body of syenite and granite which everywhere rises (usually 

 abruptly) from about 150 to over 800 feet above the valley floor 

 except in the vicinity of the outlet in the southwest. The distinct 

 basinlike character of the depression is clearly brought out on the 

 topographic maps. It is certain that, very largely at least, the 

 differences in altitude between the Grenville and surrounding 

 igneous rocks is due to original irregularities of the surface of the 

 igneous mass produced at the time of the intrusion. 



One further point needs emphasis, namely, that the figures above 

 given as representing the amounts of irregularity of bathylithic sur- 



