GEOLOGY OF THE NORTH CREEK QUADRANGLE 21 



very basic variety may come in and constitute the section for 50 

 or 100 feet or more. Yet, while the transition is sharp, there is 

 no evidence of separate intrusive masses nor is one justified in 

 inferring more than a differentiation of an eruptive mass into 

 layers or portions of contrasted composition. . . . That this 

 differentiation takes place in magmas is one of the growing con- 

 victions of students of igneous rocks.'' ^ Now, so far as the 

 writer's observations in the North Creek region are concerned, they 

 fully accord with Professor Kemp's interpretation of this puzzling 

 phenomenon. Of course, the rocks have been severely compressed 

 and possibly folded and the banded effect may thus have been ac- 

 centuated, but nevertheless there appears to be no getting away 

 from the apparent fact -of some sort of differentiation of the 

 granitic magma into layers of varying composition. 



GRANITE PORPHYRY 



This rock represents another phase of the great syenite-granite 

 intrusive mass and always shows a perfect gradation into either 

 the granite or syenite, so that sharp lines of separation between 

 these rock types can not be drawn. On the accompanying geologic 

 map this rock is seen to be rather widely distributed in small to 

 large irregular shaped areas making up altogether perhaps a little 

 less than one-ninth of the area of the quadrangle. A very similar 

 rock occurring in the vicinity of Northville has recently been de- 

 scribed by the writer - and that descri])tion applies almost perfectly 

 to the North Creek granite porphyry. Exactly the same evidences 

 which were presented to prove that the Northville granite porphyry 

 is really only a facies or differentiation product of the great syenite 

 magma, may also be applied to this granite porphyry. Still more 

 recently such rock has been found by the writer in the northern 

 portion of the Lake Pleasant quadrangle. Thus it is quite certain 

 that granite porphyry is a rather widespread rock in the southeast- 

 ern Adirondacks. 



The typical rock is gray to pinkish gray, thoroughly gneissoid,- 

 .- nd with beautifully developed porphyritic texture. The i)hcno- 

 crysts of feldspar are usually from one-half to one inch long and 

 more or less flattened parallel to the foliation. Carlsbad twins are 

 often easily recognizable. Feldspar, ([uartz, and biotite or horn- 

 blende are always plainly visible to the naked e\c. Often large 



1 N. Y. State Mus. P.ul. 138, p. 48 and 128. 

 - N. V. Stale Mus. lUil. 153, p. 17-Jo. 



