1864.] T. STERRY HUNT ON LITHOLOQY, 179 



Silica 37.13 37.17 = Oxygen 19.82 



Magnesia 39.36 39.68= " 15.87 



Protoxyd of iron 22.57 22.54= " 5.10 



99.06 99.39 

 The augite of thisolivinitic doierite appears in the form of small 

 (irystallinj grains, and also in short thick and terminated prisms, 

 which are re;idily detached from their matrix. They are often aa 

 inch in length by half an inch in diameter, and are sometimes 

 partially coated by a film of brown mica. These crystals cleave 

 readily, presenting brilliant surfaces, and are black in color, with 

 tn ash-gray streak. Their hardness is 6.0, and their specific gravity 

 3.34. Analysis gave as follows : 



Silica 49.40 



Alumina 6.70 



Lime 21.88 



Magnesia 13.06 



Protoxyd of iron 7.83 



Soda and traces of potash 74 



Volatile 50 



100.11 



The augite which abounds in the non-olivinitic doierite that 

 forms the greater part of Montarville, does not appear to differ 

 from that just described. 



An average specimen of this olivinitic doierite, or peridotite, was 

 reduced to powder : it did not effervesce with nitric acid, and whea 

 ignited lost only 0.5 per cent. When gently warmed with sulphurie 

 acid, the olivine was readily decomposed, with the separation of 

 flocculent silica ; and by the subsequent use of a dilute solution of 

 loda, followed by chlorhydric acid, and a second treatment with 

 the alkaline ley, 55.0 per cent of the whole were dissolved. This 

 portion consisted of silica 37.30, magnesia 33.50, protoxyd of iron 

 26.20, alumina 3.00 = 100.00 : being equal to 18.4 of magnesia 

 for the entire mass. In another experiment, 18.0 per cent were 

 obtained. Taking the mean of the two analyses of olivine above 

 referred to, which gives 39.5 per cent of magnesia, 18.0 parts of 

 this base correspond to 45.5 parts of olivine. The remaining 9.5 

 parts of dissolved matter represent alumina and silica from th« 

 feldspar, and oxyd of iron from the magnetite ; both of which were 

 •omewhat attacked by the acids. The undissolved portion of the 

 rock equalled 44.7 per cent., and appeared to consist of a feldspar, 



