438 Pirsson and Washington — Geology of Red Hill, JV. H. 



Chemical Composition. — From the mode just given, if we 

 assume, as mentioned above, that the alkalic feldspar has the 

 average composition Or 3 Ab„ and the oligoclase, as shown by 

 its optical properties, is Ab 4 An a we can reckon the mass com- 

 position of the rock to be as follows : 



Si0 2 A1 2 3 Pe 2 O a FeO MgO CaO Na 2 K 2 H 2 Total 

 77-9 11-6 0-2 0-4 0-3 0-2 2-9 5'9 0-5 = 99-9 



The small amount of biotite is assumed to be like that of an 

 average granite rock. The composition thus reckoned must 

 be rather close to the actual one, the only important uncer- 

 tainty being in the relations of soda to potash. 



Classification . — In the quantitative system the rock has 

 practically a normative mode, the biotite being negligible. It 

 is persalic, quarfelic and peralkalic, and is therefore alaskase. 



If we accept the relations of soda to potash as given in the 

 analysis, the molecular ratios are K 2 : Na 2 :: 0"063 : O047 ; it is 

 sodipotassic and considering the texture is grano-alaskose. In 

 Rosenbusch's classification it is a typical granite aplite; in 

 ordinary usage a fine-grained biotite granite. 



Paisanal Liparose (Paisanite). 



This rock occurs in a narrow dike cutting the grano-mias- 

 kose or nephelite-syenite of the Home quarry. The quarry is 

 a large shallow pit from which the crumbling gravelly debris 

 has been carried away for road-making or other purposes, 

 leaving bowlders and exposures of fresh massive rock, in place. 

 It is in these that fragments of the dike occur, and it is also 

 seen in place on the edges of the excavation cutting the altered 

 rock, but it cannot be traced in the grass-covered pasture field 

 beyond. 



Megascopic characters. — Holocrystalline ; fine-grained ; light- 

 gray ; sparsely flecked with minute black dots ; full of con- 

 cealed joint planes or contraction cracks along which it readily 

 splits, making good specimens difficult to obtain. Tough; hard 

 and very fresh ; weathers brown in color. 



Microscopic characters. — The minerals seen in thin section 

 are riebeckite, epidote, biotite, alkalic feldspar and quartz. 

 The riebeckite occurs in irregular poikilitic patches from - 2 mm 

 down to minute microscopic specks ; it is somewhat intergrown 

 with quartz and feldspar : it sometimes shows a tendency to 

 elongation parallel to the vertical axis ; crystal boundaries are 

 entirely wanting. The refractive index is high; the birefrin- 

 gence very low indeed. It is strongly pleochroic and c and b 

 are a deep bluish green sometimes passing into a greenish blue, 

 while a is a light gray-brown sometimes almost colorless : the 

 absorption c and b>a is therefore very strong and combined 



