the use of SZABO'S method, 11 ' or by the determination of the specific gravity. 

 For the latter purpose one of the heavy solutions must be used. A solution 

 must be obtained by careful dilution, in which several small grains of the 

 felspar will remain supended, and then the specific gravity of the solution 

 must be obtained by the ordinary methods. This will of course represent the 

 specific gravity of the felspar. The average specific gravities of the more 

 important felspars are as follows : albite, 2'62 ; oligoclase, 2'64 ; andesine, 

 2*65 ; labradorite, 2'69 ; bytownite, 2'71 ; aiiorthite, 2'75. 



So far we have been dealing only with those felspars which are 

 sufficiently large to be isolated, and separately examined, and we have seen 

 that various methods have been devised by which a fairly exact determination 

 of such felspars may be made. In the finer grained and compact rocks we 

 constantly find minute felspars which can only be examined in thin section. 

 The above methods are therefore quite unsuitable for the determination of 

 these, and we have therefore to fall back upon the statistical method of 

 M. LEVY. These small felspars occur as a rule in the form of columns, 

 elongated in the direction of the edge P/M, and twinned on the albite plan. 

 As a consequence of this it follows that the longitudinal, or lath- shaped 

 sections are those which are most marked under the microscope, and these 

 sections are of necessity taken approximately out of the zone P/M. Now 

 the extinctions in this zone vary within certain limits which are different for 

 the different felspars. M. LEVY gives these limits as follows : 



Extinction angle between two 



Extinction referred to the lamellse of a felspar twinned 



length of the microlites. on the albite plan. 



Albite ... to 19 ... to 12 



Oligoclase ... 0" to 2 ... to 3 



Labradorite to 17 or 27 ... to 18 



Anorthite ... to 30 (and over) ... 0' to 40 (and over) 



The data on which these figures are based, and the method adopted in 

 the calculation do not permit of absolute accuracy. (2) Nevertheless, the 

 results show that microlites of oligoclase must in all cases extinguish 

 approximately parallel with their length, and that felspars giving high 

 extinction angles must belong to the labradorite-anorthite group. The 

 application of this method requires of course the observation of a considerable 

 number of sections as the maximum extinction is the only one on which any 

 conclusion can be based. Moreover the existence of felspars intermediate 

 between the species recognised by M. LEVY complicates the matter very 

 considerably. Again, the presence of a felspar giving high extinction angles 

 does not necessarily exclude other felspars giving lower angles, and these of 



(1) It would be out of place here to describe this method. An account of it will be found in 

 the Proceedings of the American Association for the Advancement of Science, vol. XXXI. 

 Montreal Meeting, 1882. See also '' Uber eine neue methode die Felspathe auch in Gesteinen zu 

 bestimmen," Buda Pesth. 1876. It is a very simple and rapid method for the determination of 

 the felspars. Orthoclase, soda-orthoclase, albite, oligoclase, labradorite and anorthite may be 

 readily distinguished form each other ; the experiment in each case not lasting more than a 

 quarter of an hour. 



(2) See Schuster, T.M.M., Neue Folge, Band III., p. 273. 



