DETAILED DESCEIPTION OF SLIDES. 91 



Section 4 . (Chapter III.) 

 DETAILED DESCRIPTION OF SLIDES. 



Reasons for this section. — 111 vIcw of tliG Considerable alterations proposed in 

 the classification of the Washoe rocks, it appears proper to submit detailed 

 descriptions of a sufficient number of slides to enable Hthologists to judge 

 whether the methods employed in the determinations are correct and the 

 o-rounds upon which distinctions have been drawn sufficient. Nearly but 

 not quite all the statements made in the foregoing sections of this chapter 

 concerning the microscopical character of the rocks may be substantiated 

 from these sHdes. It was considered that further descriptions were need- 

 less and would be burdensome. 



Determination of feldspar. — Thc fcldspars have bceu determined optically ac- 

 cording to the rules laid down by Messrs. Fouqu^ and Levy.^ This method 

 is very tedious, and is, properly speaking, applicable only to the determina- 

 tion of the most basic feldspar present; but by applying it to a great num- 

 ber of cases the microscopist is able to satisfy himself of the prevailing- 

 feldspars as well, and in this respect it appears to me more satisfactory than 

 the determination of isolated feldspar fragments by their specific gravity. 

 In two cases M. Thoulet's method has been employed. Professor Szab6's 

 method has not been attempted.- 



An explanation of the method of reference to the slides by a system of 

 coordinates in millimeters, referred to the upper left-hand corner of the glass, 

 will be found in the description of the lithological illustrations, page 145. 



GRANITE. 

 Slide 460. Close to Bed Jacket mine. 



Typical granite. — This is a moderately fine-grained gray micaceous granite. 

 The slide shows besides orthoclase, quartz, and mica, a few plagioclases, 



' Miueralogie Micrographique, 1879. So far as I know this method was first snggested by Prot; 

 R. Pmiipclly, Proc. Amer. Acad., Vol. XIII., p. 258. 



2 Tests by this method, subsequently made, are described on p. 405, et aeq. 



