GEOLOGY OF THE SCHROON LAKE QUADRANGLE 



73 



the strata to be notably bent upward along those sides. It should 

 be noted, in accordance with this view, that the strikes of the Gren- 

 ville strata and the adjacent igneous rocks are essentially parallel. 

 In the northern portion of the Olmstedville area five good obser- 

 vations show the Grenville to have persistent dips of from 20 to 

 70 degrees northward, which is precisely the opposite of the Gren- 

 ville of the northern portion of the nearby Minerva area, this 

 latter being on the same strike. Such a sharp change would 

 scarcely be expected as a result of ordinary erogenic folding, and 

 it is more likely the result of the magmatic intrusion. The granite 

 magma north of Olmstedville apparently broke through and flowed 



Fig. 7 An early ncrth-scuth section through Olmstedville illustrating t^e 

 geologic structure and the probable mode of origin of the valley in the 

 vicinity of the village and the escarpment on the north. Several stages 

 of erosion are shown from the late Meso^.oic or early Cenozoic penepia n 

 level (dotted upper line) to the present surface (heavy lower line). Length 

 of section, 4 miles. Vertical scale, 2^2 times the horizontal. 



intrusively upon the Grenville strata, thus accounting for the north- 

 ward dip of both granite and Grenville with the latter dipping 

 under the former. Because of the much greater resistance of the 

 granite to weathering and erosion, it stands out as a local scarp 

 (see map), while the much weaker Grenville has been notably worn 

 down to form the valley around Olmstedville. Figure 7 illustrates 

 the principles here involved. 



The very irregular dips and strikes in the small area of Gren- 

 ville just east of North pond are no doubt due to deformation of 

 this block of strata by one or more of the various intrusive masses 

 which rose adjacent to, or possibly engulfed it. A similar con- 

 dition is true of the Grenville south of Adirondack village. 



The other mapped areas of Grenville are relatively small, and 

 they are merely inclusions in the syenite-granite series. They 



