THE 
AMERICAN GEOLOGIST 
Vol. VII. JUNE, 1891. No. G. 
PETROGRAPHICAL TABLES. 
Alfred C. Lane, Houghton, Mich. 
The two tables that are given herewith are for the use of the 
students of the Michigan Mining School. I have asked for their 
publication in the Geologist, not only because they may be 
useful to its readers, but because in tables of the kind errors 
are liable to creep in, in spite of all feasible care. I hope for 
corrections and suggestions, and shall be obliged to those who 
send me them. I will try to acquaint them with the corrections 
of others. 
1. The first table is to enable tyros to quickly recognize the 
minerals of a rock section with a good degree of probability. It 
is- based on Levy and Lacroix's tables and contains the substance 
of them with slight additions and corrections. The minerals are 
arranged according to their optical properties. These are gener- 
ally well defined. As in other such tables there may at times be 
doubt between alternatives, where we must impale our mineral on 
both horns of the dilemma. 
In dividing minerals according to the index of refraction I have 
thrown them into three groups according as they have practically 
the same refraction as Canada Balsam and rio relief, or are more 
in relief yet do not ordinarily appear rough in surface, or are 
commonly rough in surface (•'riinzeligchagiiiirt"). One can always. 
I think, throw out one of these three groups in studying a given 
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