20 



NEW YORK STATE MUSEUM 



The potash-soda ratio is such that the rock comes close to sub- 

 rang 2, Dellenose. 



Mode calculated from measured sections 



Microperthite 



Orthoclase 



Microcline. . . 

 Plagioclase. . . 



Quartz 



Biotite 



Magnetite 



Zoisite 



Zircon 



Apatite 



Total Relative 



diameters | volumes 



1788 



19 



II55 



12 



I8I0 



19 



IOI4 



II . 



2717 



29. 



500 



5- 



90 





60 





21 





13 





9168 



100. 



Sp. gr. 



2.6 

 2.6 

 2.6 



2.63 



2.65 



3.00 



5-25 



3.26 



4-5 

 3-2 



Units 

 by weight 



4648 

 3003 

 4706 

 2667 

 7200 

 1500 



473 

 196 



94 

 42 



24529 



Percentage 

 weights 



i«.95 

 12.24 

 19.18 

 10.87 



29-35 

 6.12 



1-93 

 .80 

 •38 

 •17 



99-99 



According to the old classification this rock is a biotite-granite- 

 porphyry while under the new classification it is a biotite-grano- 

 phyro-toscanose. 



From the above it is seen that the Northville syenite and granite, 

 though in different classes, fall in exactly corresponding orders, 

 rangs, and subrangs and this, together with the fact that the syenite 

 is close to the Persalane border, shows that these two rocks are 

 closely related in the quantitative chemical classification. Thus the 

 field relations, examination of thin sections and chemical composi- 

 tion of these two rocks, which present such a marked difference in 

 appearance, afford practically conclusive proof that they are of the 

 same age and represent differentiation products from the same 

 magma, the granite representing merely somewhat more salic 

 (richer in quartz and feldspar) portions of the cooling magma. 



In the following table the Northville syenite and granite are 

 compared with certain other carefully studied Adirondack rocks : 



