CHEMISTRY. 215 



many cases. On the other hand, it seems certain that the black border of 

 many hornblendes has been attacked, and has given place to a transparent 

 mineral, which is more or less diffused in and obscured by the groundmass. 

 The natural supposition is that it is ferrous carbonate. 



iimenite. — Tltauic iron ore may often be observed in slides from the Dis- 

 trict passing into leucoxene. The nature of tin's substance is doubtful, 

 and no occurrence in the District is conclusive as to its nature, yet many 

 cases have been observed the character of which would be very satisfactoiily 

 accounted for if the supposition of Messrs. Fouqn(i & L^vy, that leucoxene 

 and titanite are identical, were accepted. 



Decomposition of the feldspars. — Thc fcldspars of tlic Washoe rcglou have offered 

 a far more effectual resistance to decomposing agencies than the bisilicates, 

 much more, too, than would be supposed from a macroscopical examination 

 of the rocks. In the mines it is very rarely that a particle of augite, 

 hornblende, or mica, can be found, these minerals being nearly always 

 wholly replaced by alteration products ; but it is the exception when a 

 moderately hard rock does not show under the microscope well defined and 

 fairly fresh feldspars. When wholly unattacked the feldspars of diabase, 

 of some diorites, and of the older andesites are transparent, and the rocks 

 then show only the tints due to the presence of magnetite and the bisilicates. 

 They are then dark, somewhat basaltic-looking masses. But when only a 

 very minute amount of change has taken place in the feldspars, they become 

 opaque through irregular reflection, and form the most prominent feature 

 of the rock. Rough estimates, made with the help of the microscoj)e, indi- 

 cate that the decomposition of much less than one per cent, of the feldspar 

 substance suffices to destroy the transparency of the crystals. 



The nature of the decomposition of the feldspars is still very obscure. 

 It is usually considered that the triclinic feldspars as well as orthoclase are 

 sometimes converted into kaolin, though Professor Tschermak maintains, 

 as an analytical result, that the hydrated aluminium silicate resulting from 

 the alteration of plagioclase contains but a single molecule of water, and 

 not two, as is the case with kaolin. Saussurite and pinitoid are the names 

 given to complex silicates, or mixtures of silicates and other substances, 



