58 The American Geologist. July, 1895 
to an optic axis of tlie compound mineral. Assuming that 
the law of Tschermak is true, it is found that the general 
principle deduced holds true in its application to the special 
cases between andesine and anorthite. Between albite and 
andesine it is not possible to say as much. It appears that 
in the vicinity of oligoclase something produces such devia- 
tions that no conclusions can be drawn. So far as it goes 
this shows a substantial confirmation, by the methods of op- 
tics, of the formula? deduced theoretically for the composition 
of some of the plagioclases, but it is dependent on the as- 
sumption that the feldspars considered are of definite compo- 
sition, and represent ascertainable and fixed conditions in 
their physical structure. 
The studies of both Fouque and Michel-Levy tend, not so 
much to the denial of the law of Tschermak, as to its defini- 
tion, and to the correction of the popular interpretation of it. 
At the same time they tend to establish definite characters, at 
least physical characters, for several new intermediate feld- 
spars, and thus to eliminate the prevalent idea of vagueness 
and uncertainty which the law of Tschermak has propagated 
respecting those before known. If their chemical characters 
could be established with equal refinement there is great 
probability that the two methods would corroborate each 
other, but the mechanical separation of two or more of them 
when they are closely intergrown, which is almost always 
their natural condition, so that reliable chemical examination 
can be made, is as 3'^et a very difficult and almost impossible 
task. N. H. w. 
REVIEW OF RECENT GEOLOGICAL 
LITERATURE. 
Oni I)i(!i/)7io(jraptu,<<, Tetragraptus orh Phi/llograptus. Af CJerhard 
Holm. ((icol. Foren. i Stockholm l'Y)rhandl., Bd. 17, Hafte.'J, Sid. 319- 
359, Tatl. 11-1(5, 189").) Dr. Holm lias contiiuuHl his studies on srapto- 
lites pri'servt'd in limestone, which he began a decade ago, in describ- 
ing the intimate structure of a Climocofp-aptuK and n RetioUtes. The 
chitinous parts of these organisms were freed by means of acid and 
Ihoir special structure clearly revealed. 
