Classification of Rocks. — Spiirr. 215 
oligoclase feldspar families. Evidently a similar transition 
family is necessary between the dioritic and the diabasic fami- 
lies, and for this place the belugitic family is proposed by the 
writer, belug}'te being derived from the locality where rocks 
belonging to this family are described by him. 
The principal criterion of the classification of the feldspar 
rocks, therefore, lies in the determination of the feldspars, 
which as Michel-Levy* has said, is indispensable. Methods 
which have been worked out by Michel-Levy and Fouque in 
Paris permit easy determination of feldspars in thin section, 
according to their optical properties. Both the methods of 
Michel-Levy and of Fouque depend upon the fact that the 
position of the plane of the optic axes varies in feldspars ac- 
cording to their chemical and crystallographic variations, and 
the novelty of both methods consists in establishing criteria 
by which certain definite sections of the feldspars may be 
recognized in a thin rock section. Michel-Levy's method 
is useful, but its practical application demands the presence of 
feldspars which are twinned, both according to the albite and 
to the Carlsbad laws ; while the system of Fouque may be used 
on twinned or untwinned feldspars and even on orthoclase. 
The methods of determining feldspars according to single and 
double refraction, which have been perfected by Becke and 
Michel-Levy , are also useful, but can only be employed in 
certain cases. The Fouque method is, however, nearly al- 
ways applicable, and if necessary a dozen different determina- 
tions may usually be made in one thin section. By this method 
sections are sought which are quite perpendicular to a bisec- 
trix, as shown by the optical figure, and on these sections 
Fouque worked out for the diiiferent feldspars the different 
angles which the plane of the optic axes makes with the trace 
of the edge between the base and the brachypinacoid. These 
methods enable one to accurately determine the feldspars in 
nearly every section, and this permits a more accurate rock 
classification. 
Within the families as defined by the characteristic feld- 
spars the groups differ according to the other minerals pres- 
ent. In the scheme of grouping cognizance is taken, first, of 
*Principes a suivre pour Line Clas.sification universelle des Roches 
pp. 155-156. 
