CHEMISTRY. 217 



visible, for entrance is as likely to have been effected above or below the 

 plane of a thin section as in it. If chlorite and epidote really occur as 

 results of the decomposition of feldspar, it should be easy to show the 

 parasitic growth of chlorite in feldspars, just as its development frotn horn- 

 blende has been shown in the present volume. 



Chemical analysis.— Tlic microscopc gives Hiainly negative results concerning 

 the decomposition of the feldspars of the Washoe rocks. Chemical analysis 

 of the decomposition products could lead to no definite results, because no 

 reasonably pure material could be obtained, and the only remaining source 

 of information is the analysis of the rocks. The diabase from the hanging 

 wall of the Lodf, which was analyzed, is a very slightly altered rock, and 

 has been described under slide 18. Its feldspars are transparent and have 

 undergone only an inappreciable amount of alteration; the rock nevertheless 

 contains a considerable quantity of water, as is shown by its loss in ignition, 

 2.47 per cent. Abundant fluid inclusions account for a part of this loss, 

 and the water of hydration of the small amount of chlorite it contains for 

 another portion. The ignition loss no doubt includes a small amount of 

 carbonic acid. The "propylite horse" analyzed by Prof. W. G. Mixter was 

 in all probability decomposed diabase. An inspection of the analysis shows 

 either that silica had been deposited in the rock, or what seems more likely, 

 that the bases had been in large part extracted. It contained 1 .83 per cent. 

 of water, or about two-thirds as much as the fresh rock. The bisilicates 

 must have been represented by chlorite, which contains about 12 per cent, 

 of water. The small quantity of aluminium not entering into the chlorite 

 may possibly have existed as kaolin, a supposition neither proved nor dis- 

 proved by the analysis, which, however, shows that the horse contained at 

 most a small percentage of that mineral. Four analyses of clays made for 

 the Exploration of the Fortieth Parallel by Professors Johnson and Mixter 

 are available. It is here, if anywhere, that kaolin must be indicated. On 

 comparison of these analyses with that of the fresh diabase, it appears that 

 they do not represent concentrations of any special mineral, but merely 

 highly altered rock masses. Barring the pyrite and water, the first three 

 show very nearly the same composition as the fresh rock, while a portion 

 of the silicic acid has apparently been abstracted from the Savage clay. 



