l8 NEW YORK STATE MUSEUM 



of apatite, zircon and zoisite. This rock is light pinkish gray and 

 with phenocrysts of microcHne. The analysis and quantitative 

 classification of a sample rock from this area is given below. 



A representative granite from two and one-half miles west- 

 southwest of Sacandaga Park is similar in composition to the rock 

 just described except that it is slightly richer in microperthite and 

 quartz and lower in orthoclase. This rock is gray weathering to 

 brown, thoroughly gneissoid and granulated, and with phenocrysts 

 of microperthite. The leaf-gneiss structure is beautifully developed 

 owing to the flattening out of quartz into large plates. 



Very similar in composition to these rocks is that from the small 

 area east of Batchellerville, except for a high biotite content and 

 the presence of i or 2 per cent of garnets scattered through the 

 mass. This granite is thoroughly gneissoid and granulated but ex- 

 hibits nothing of the leaf-gneiss effect. The reason for the garnets 

 here is not at all certain, but they may be due to a slight assimilation 

 of the surrounding Grenville by the molten granite. 



From these descriptions it is seen that the typical granite por- 

 phyry differs from the typical syenite chiefly in its prominent 

 porphyritic character, higher quartz and biotite content, presence 

 of microcline, and total absence of hornblende. 



Perhaps the best illustration of granite porphyry which presents 

 certain features very similar to the syenite is that from the area 

 just west of Sacandaga Park. A slide shows: 35 per cent ortho- 

 clase; 12 per cent microperthite; 18 per cent oligoclase to andesine; 

 25 per cent quartz ; 7 per cent biotite ; 2 per cent magnetite ; and i 

 per cent zoisite, zircon, apatite and garnet. Mineralogically this 

 rock is almost exactly like the more acid syenite except for the sub- 

 stitution of biotite for hornblende. The boundary line between the 

 granite and syenite here can not be drawn with accuracy because of 

 the apparent gradation of the one rock into the other and this fact, 

 together with the failure to find any evidence of one of these rocks 

 cutting the other, leads to the belief that the granite and syenite are 

 of practically the same age and that they are differentiation products 

 of the same magma. 



The following analysis of what is regarded as the most typical 

 granite (above described), from one and three-fourths miles north- 

 northwest of Northville, has been made for the writer by Professor 

 E. W. Morley: 



