Petrographical tables. — Lane. 339 
In uniaxial minerals 2y=0, i. e. , b = t or n, and the double 
refraction is written directly preceded by the sign of the mineral, 
which is at the same time that of (e — o). 
When the dispersion of the optic axes is strong that is noted ; 
\\ < () means, for example, that the hyperbolas are bluish toward 
the center. If the mineral is colored that is also noted, and some 
other briefly put characters. But it should not for a moment be 
forgotten that such tables serve merely for probable recognition. 
Not onhy are there many yet undescribed minerals, but the prop- 
erties vary through isomorphic or morphotropic groups, and in 
some cases optical diagnosis will not serve to distinguish well 
known and defined minerals (e.g.), zircon and cassiterite. It ordi- 
narily suffices, however, for the common minerals. (The difficult 
and not very common group of zeolites are bracketed. ) 
2. The second table illustrates Rosenbusch's classification of 
the igneous (massive "massige"') rocks, and is founded on his 
last edition. The rocks are arranged as far as practicable in hori- 
zontal rows according to like mineral composition. Like struc- 
tures, and names when confined to a determined structure, are in 
vertical columns. 
I have thought it best to add numerous references to page and 
line. Each page is supposed to have forty lines, and the line is 
added as a decimal to the number referring to a page. The table 
may be taken, then, as an analytical index to Rosenbusch's second 
volume. For the benefit of those who know neither the author 
nor the book, it should be added that he regards petrographic 
classification as in a transition stage, and has purposely omitted 
to make anj* general table himself. I do not know if one is 
forthcoming in his shorter petrography. The classification is not 
exactly in the order given by the book, although it finds its justi- 
fication there, nor will it be accepted by him as anything final. 
It may, however, also be useful as a guide to the present use of 
rock names b}' those who follow the book, to those who do not 
rend German fluently. Levy's critique and tables have already 
been referred to in this magazine. In the table some constituents 
whose names are embodied in that of the rock are printed in 
italics and not repeated. 
My obligations to Doctors H. B. Patton and L. L. Hubbard for 
suggestions and help in proof-reading are great. 
Houghton, Dec, 1890. 
