NOTE TO CHAPTER III. 



FELDSPAR DETERMINATIONS BY SZABO'S METHOD. 



In the present state of lithological science it is most desirable both to determine 

 the feldspathic constituents of rocks with accuracy aud to bring the evidence of inde- 

 pendent methods to bear for this purpose. Where rocks are extremely coarse-grained, 

 and at the same time carry feldspars the solidity of which is unimpaired by cracks, 

 the results of the examination of cleavage flakes under the microscope leaves little to 

 be desired ; but such rocks are exceptional. The determination of feldspars in rock- 

 slides is subject to two disadvantages. Crystals of above a millimeter in diameter are 

 very likely to be broken in grinding, and thus to escape examination ; and though the 

 microscopist may often infer the presence of two or more feldspars, he can be abso- 

 lutely certain only of the most basic species present. 



Szabo's method,' on the other hand, discriminates with great delicacy, not only 

 the well-established feldspar species, but also the mixed feldspars, perthite and loxo- 

 clase, and the intermediate feldspars, andesine and bytownite, as to the independence 

 of which mineralogists are not agreed. It is also particularly applicable to the larger 

 feldspars, so seldom obtained in perfect form in slides. The weakness of the method 

 lies iu the fact that it is not applicable to very fine-grained rocks, or to the minute feld- 

 spars of coarser rocks, unless a separation has first been effected by Thoulet's method ; 

 but this limitation does not prevent its being excellently adapted to confirm and sup- 

 plement the results of microscopic examination. 



At the time of writing Chapter III. I did not feel competent to apply Szabo's 

 method, never having had an opportuuit^y of seeing it carried into practice ; but while 

 the proofs of this volume were under correction, Professor Szabo visited the country 

 aud was good enough to illustrate his method experimentally to some of the members 

 of the Survey, including myself. After convincing myself of the accuracy of the 

 results obtainable and acquiring familiarity with the manipulation by repeatedly test- 

 ing a series of classical feldspars, such as anorthite from Monte Somma, labradorite 

 from Labrador, orthoclase from Baveno, etc., l' proceeded to an examination of the 

 feldspars of the Washoe rocks, the results of which are given below. From five to 

 ten crystals iu each specimen mentioned were tested, and no results obtained are 

 omitted. 



' Joseph SzaVi6, Ueber eine neue Methode tlie Feldspathe aucb in Gesteinen zu bestimmen. 

 Pestb, Fraukliu-Veiein, 1876. See, also, Fouqu^ et Ldvy, Mindialogie Micrograpbique, p. 108. 



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