268 The American Geologist. April, 1892 
Notes to accompany a Tabulation of the Igneous Rocks based on the Sys- 
tem of Prof. H. Rosenbusch. By Frank D. Apams. (Can. Ree. Sci., Vol. 
tv, No. 9, Dec., 1891, pp. 465-469, with table.) 
In the present transition stage of petrographical classification, it is 
a difficult matter to lay before the student a scheme for the determina- 
tion and classification of rocks in which their relations to each other as 
brought out by their mineral and chemical composition are shown clearly 
and concisely. Such schemes or tabulations are usually too much bur- 
dened with unimportant details and doubtful sub-divisions which only 
serve to confuse and bewilder those using them. In the table accom- 
panying these notes, Mr. Adams has succeeded admirably in presenting 
a classification of the igneous rocks based on that of Prof. H. Rosen- 
busch of Heidelberg, which is very simple, and may be readily compre- 
hended by beginners in petrographical work, whilst at the same time 
nearly all rock names of real importance are included in it. 
The author points out that although there has recently been a strong 
tendency among petrographers to consider rocks from a chemical stand- 
point, yet a purely chemical classification presents many grave difticul- 
ties, and therefore that mineralogical composition, and structure must 
still play an important part in any scheme which is to be generally 
adopted. ‘ 
In his table the igneous rocks are first classified in three horizontal 
columns, headed “Abyssal (Plutonic) Rocks,” “Dyke Rocks,” and “Effu- 
sive (Volcanic) Rocks,” the characteristic structures of each of these 
groups being given. 
The table is also divided into eight vertical columns according to the 
mineralogical and chemical composition of the rocks, headed, “Alkali 
Feldspar Rocks,” “Alkali Feldspar—Nepheline (or Leucite) Rocks,” 
“Leucite Rocks,” ‘“Nepheline Rocks,” ‘“Melilite Rocks,” ‘“Lime-Soda 
Feldspar-Nepheline (or Leucite) Rocks,’ “Lime-Soda Feldspar Rocks,” 
and “Rocks containing no Feldspathic constituent.” 
The rocks are then subdivided according to their bisilicates and micas, 
further subdivisions being made in the more acid ones by the presence or 
absence of quartz, and in the basic ones, the presence or absence of olivine. 
Briefly summarized the more important points to be noted in connec- 
tion with the table are: 
1. Ina general way the classification is a chemical one, the rocks 
decreasing in acidity from left to right of the table. The prin- 
cipal exception is the Nepheline, Leucite and Melilite rocks. 
2. Several of the rock groups are given positions which differ 
from those which they occupy in Prof. Rosenbusch’s book, the 
object being to more clearly bring out their chemical relation- 
ships. As examples may be mentioned the Nepheline, Leucite 
and Melilita rocks, placed immediately after the Orthoclase 
Nepheline (or Leucite) rocks; the Diabases, here classed with 
the volcanic rocks; the Finguaites, Alnoites, and some of tlie 
Acmite Trachytes which will be found among the Dyke rocks. 
The Pyroxene rocks are separated from the Olivine rocks, and 
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