332 Mr. J. A. Phillips's Notes on the Chemical Geology 



neighbourhood of Lincoln in Placer county, had a specific gravity 



of 255, and yielded the following results : — 



Water lost at 212° F 031 



Water lost by subsequent ignition . 1*88 



Silica 72-05 



Alumina 17*87 



Peroxide of iron T98 



Lime 1*65 



Magnesia 038 



Potash 1-71 



Soda 2-40 



100-23 

 When examined under a low power, this slate is seen to be 

 made up of angular fragments of quartz connected together by 

 clayey matter, their largest planes being parallel to the lines of 

 cleavage. It also contains numerous spots of peroxide of iron, 

 which in many instances still retain the cubical form of iron 

 pyrites. By the aid of a high power the quartz fragments are 

 seen to enclose a few small gas-cavities ; and one or two fluid- 

 cavities, with distinctly visible vacuities, were observed. 



Examination of decomposed Auriferous Slate from Lincoln. — 

 The decomposed siliceous matter before referred to, somewhat 

 resembling chalk in appearance, was found to have a specific 

 gravity of 2*50, and on being analyzed gave the following 

 results : — 



Water lost at 212° F 0-11 



Water lost by subsequent ignition . traces 



Silica 93-33 



Alumina 4*46 



Peroxide of iron traces 



Lime 0'75 



Magnesia 0'43 



Potash . 0-52 



Soda traces 



99-60 

 When examined under the microscope, this substance appears 

 to consist of the siliceous skeleton of the auriferous slate from 

 which the aluminous and ferruginous matters have been almost 

 entiiely removed. The angular fragments of quartz, being thus 

 deprived of the cementing material which originally bound them 

 together and gave solidity to the mass, remain in a disintegrated 

 and friable state, although still retaining distinct indications of 

 stratification. 



The number of well-defined fluid-cavities in the veinstones of 

 the auriferous lodes of California is exceedingly limited; and in 



