130 , GEOGNOSY. -[Boox II. 
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petrographers have proposed to divide the rocks into an acid group, : 
including such rocks as granite, quartz-porphyry and quartz-trachyte, - 
where the percentage of silica ranges from 60 to 75 or more, and a 
basic group, typified by such rocks as leucite-lava and basalt, where 
the proportion of silica is only about 50 per cent. 
In the vast majority of igneous rocks the chief silicate is a felspar 
—the number of rocks where the felspar is represented by another 
silicate (as leucite or nepheline) being comparatively few and unim- 
portant. As the felspars group themselves into two divisions, the 
monoclinic or orthoclase, and the triclinic or plagioclase, the former 
with, on the whole, a preponderance of silica; and as these minerals 
occur under tolerably distinct and definite conditions, it is cus- 
tomary to divide the felspar-bearing massive rocks into two series, 
—(1) the orthoclase rocks, having orthoclase as their chief silicate, 
and often with free silica in excess, and (2) the plagioclase rocks, 
where the chief silicate is some species of triclinic felspar. The 
former series corresponds generally to the acid group above 
mentioned, while the plagioclase rocks are on the whole decidedly 
basic. It has been objected to this arrangement that the so-called 
plagioclase felspars are in reality very distinct minerals, with 
proportions of silica, ranging from 43 to 69 per cent.; soda from 0 to 
12; and lime from 0 to 20.1. But the state of minute subdivision in 
which the minerals occur in most massive rocks, makes the deter- 
mination of the species of felspar so difficult that the term plagio- 
clase is of great service as at least a provisional term under which to 
unite the felspars that crystallize in triclinic forms. In addition to 
the felspar-rocks, there must be noted those in which felspar is either 
wholly absent or sparingly present, and where the chief part in rock- 
making has been taken by nepheline, leucite, olivine, or serpentine. 
From the point of view of internal structure a classification based 
upon microscopic research has recently been proposed by MM. 
Fouqué and Michel-Lévy. These writers, pointing out that most 
eruptive rocks are the result of successive stages of crystallization 
each recognizable by its own characters, affirm that two phases of 
consolidation are specially to be observed, the first marked by the 
formation of large crystals which were often broken and corroded 
by mechanical and chemical action within the still unsolidified 
magma; the second by the formation of smaller crystals, crystal- 
lites, &c., which are moulded round the older series. In some rocks 
the former, in others the latter of these two phases is alone present. 
Two leading types of structure are recognized among the eruptive 
rocks. 1. Granitoid, where the constituents are mainly those of 
the second epoch of consolidation, but where neither amorphous 
magma, nor crystallites are to be seen. ‘This structure includes 
three varieties, (a) the granitoid proper, haying crystals of ap- 
1 Dana, Amer. Jour. Sci. 1878, p. 432. This article contains a trenchant criticism 
of modern lithological classification. See on the subject of the retention of the term 
“‘ nlagioclase,”’ Bonney, Geol. Mag. 1879, p. 200. 
