
* 
[Boor It. 4 
90 - GHOGNOSY.” * ">? ae 
approximately uniform in size, as in granite. ‘This structure is cha- 
racteristic of many eruptive rocks. Though usually distinctly 
recognizable by the naked eye (“macromerite” of Vogelsang’), it 
sometimes becomes very fine (“micromerite”), and may be only 
recognizable as thoroughly crystalline with the microscope ; at other 
times it passes into a porphyritic or porphyroid character by the ap- 
pearance of large crystals dispersed through a general:ground-mass, 
Porphyritic, composed of a compact or finely crystalline 
eround-mass, through which distinct larger crystals, generally of some 
felspar, are dispersed. This and the granitic structure are the two 
great structure-types of the eruptive rocks. By far the largest 
number of these rocks belong to the porphyritic type. Vogelsang 
las proposed to classify this type in three divisions: Ist, Grano- 
phyre, where the ground-mass is a microscopic crystalline mix- 
ture of the component minerals with a sparing development of an 
imperfectly individualized magma (see p. 103); 2nd, Felsophyre 
having usually an imperfectly individualized or felsitic magma for 
the ground-mass (p. 104); 3rd, Vitrophyre, where the ground-mass 
is a glassy magma. ‘The second sub-division embraces most of the 
porphyries, and a very large number of eruptive rocks of all ages.” 
Segregated.—In granite and other crystalline massive rocks, 
vein-like portions, coarser (or finer) in texture than the rest of the 
mass, may be observed. These “contemporaneous veins,” as they 
have been called, belong to the last phase of consolidation, when 
segregations from the original molten or viscous magma took place 
along certain lines where from fracture or otherwise the individual 
minerals could crystallize out from the general mass. They have 
been sometimes termed “segregation,” or “ exudation ” veins. 
2. Composition.— Before having recourse to chemical or micro- 
scopic analysis, the geologist can often pronounce as to the 
general chemical or mineralogical nature of a rock. Most of the 
terms which he employs to express his opinion are derived from 
the names of minerals, and in almost all cases are self-explanatory. 
The following examples may suffice. Calcareous, consisting of or 
containing carbonate of lime. Argillaceous, consisting of or 
containing clay. Felspathic, having some form of felspar as a 
constituent. Siliceous, formed of or containing silica; usually 
applied to the colloid or caleedonic forms of this oxide. Quartzose, 

containing or consisting entirely of some form of quartz; used more ~ 
particularly of the crystalline forms of silica. Carbonaceous, con- 
taining coaly matter, and hence usually associated with a dark colour. 
Pyritous, containing diffused disulphide of iron. Gypseous, con- 
taining layers, nodules, or scattered crystals of calcium sulphate. 
Saliferous, containing beds of, or impregnated with, rock galt. 
As rocks are not definite chemical compounds, but mixtures of 
' Z. Deutsch. Geol. Ges. xxiv. p. 584, 
* Vogelsang, loc. cit. Compare the classification as granitoid and trachytoid, 
poste (, Pp. 130. 
