GEOLOGY OF THE NORTH CREEK QUADRANGLE 6l 



The sharpness of the contact between the igneous rocks and the 

 Grenville and the altitude of the igneous masses above the Gren- 

 ville have often been accentuated by the faulting, but in spite of 

 this some idea of the irregular surface of the great intrusive mass 

 may be gained. Thus, the Grenville between Kelm and County 

 House mountains occupies a depression fully 300 or 400 feet deep 

 in the granite porphyry; while between Mill and Oven mountains 

 the Grenville occupies a depression some 700 or 800 feet deep in 

 the granite. Even if we grant the possibility of some faulting of 

 the Pine-Gage mountain mass, it seems clear that this igneous rock 

 rose by intrusion some hundreds of feet through the Grenville. 

 In spite of the accentuated heights of the igneous masses of Hack- 

 ensack, Moon, Heath, Potter, Huckleberry, and Crane mountains, 

 it seems necessary to regard a considerable amount of the eleva- 

 tion of the igneous rocks above the Grenville as due to the intru- 

 sion itself. The only other alternative is the untenable view that 

 faults completely surround these igneous bodies. 



In general, then, we see that the great intrusive body often 

 shows irregularities on its surface which vary in altitude by hun- 

 dreds of feet within from i to 3 or 4 miles. 



TOPOGRAPHY 



RELATION OF TOPOGRAPHY TO ROCK CHARACTER 



The surface configuration of the North Creek sheet is almost 

 perfectly adjusted to rock character. A glance at the geologic 

 map will show that the Grenville rocks, with few exceptions, occupy 

 the lowlands; this is because of the relative weakness of those 

 rocks as compared with the intrusives. The limestone areas or 

 belts, being weakest of all, are invariably found in the valleys, 

 and stream courses have commonly developed along such belts. 

 ( )ccasionally the more resistant Grenville gneisses, even where un- 

 affected by faulting, have stood out fairly well against erosion 

 as, for example, south of Valentine pond and south-southwest of 

 Unirman. The Grenville quartzite being quite pure and very 

 resistant usually stands out fairly conspicuously in the Cirenville 

 areas. This is well shown in the area south of Warner j^ond, 

 though the height is there accentuated by faulting. The steep 

 slope of the (iuartzite ridge east of Tripp ])ond appears to be due 

 to more rapid wearing away of the much weaker underlying Gren- 

 ville. In the mixed gneiss areas, particularly where the Grenville 

 is abundant, the topographic development has been very similar 

 to that of the Grenville areas. 



