
- Panr IL. § iv.] MICROSCOPIC CHARACTERS OF ROCKS. 95 
A. CrysTALts OR CRYSTALLINE SUBSTANCES.—Rock-forming 
_ minerals when not amorphous may be either crystallized in their 
proper crystallographic forms, or crystalline, that is, possessing a 
erystalline internal structure, but without definite external 
geometrical form. The latter condition is more prevalent, seeing 
that minerals have usually been developed round and against each 
other, thus mutually hindering the assumption of determinate 
erystallographic contours. Other causes of imperfection are fracture 
by movement in the original magma of the rock and partial solution 
in that magma, as in the corroded quartz of quartz-porphyries and 
rhyolites. In some rocks, such as granite, the thoroughly crystalline 
character of the component ingredients is well marked, yet they 
seldom present the definite isolated crystals so frequently to be 
observed in porphyries and in many old and modern volcanic rocks. 
Among thoroughly crystalline rocks good crystals of the component 
minerals may be obtained from fissures and cavities in which there 
has been room for their formation. It is in the “drusy” cavities of 
granite, for example, that the well-defined prisms of felspar, quartz, 
mica, topaz, beryl and other minerals are found. Successive stages 
in order of appearance or development can readily be observed 
among the crystals of rocks. Some appear as large but frequently 
broken or corroded forms. These have evidently been formed first. 
Others are smaller but abundant, usually unbroken, and often disposed 
in lines. Others have been developed by subsequent alteration within 
the rock.* 
A study of the internal structure of crystals throws light not 
merely on their own genesis, but on that of the rocks of which they 
consist, and is therefore well worthy of the attention of the geologist. 
That many apparently simple crystals are in reality compound, may 
not infrequently be detected by the different condition of weathering 
in the two opposite parts of a twin on an exposed face of rock, 
The internal structure of a crystal modifies the action of solvents on 
its exterior (¢.g. weathered surfaces of calcite, aragonite and felspars). 
Crystals may occasionally be observed built up of rudimentary 
“microliths,” as if these were the simplest forms in which the mole- 
cules of a mineral begin to appear (p. 100). 
Crystalline minerals are seldom free from extraneous inclusions, 
These are occasionally large enough to be readily seen by the naked 
eye. But the microscope reveals them in many minerals in almost 
incredible quantity. They are, a, gas cavities; 6, vesicles con- 
taining liquid; y, globules of glass or of some lithoid substance ; 
6, crystals; ¢, filaments or other indefinitely-shaped pieces, patches, 
or streaks of mineral matter. 
der K. K. Geologischen Reichsanstalt (Vienna), contain numerous papers on the microscopic 
structure of rocks. Rutley’s Study of Rocks, London, 1879, is a convenient little book. 
The manual of Rosenbusch and the work of Fouqué and Michel-Lévy, contain a tolerably 
ample bibliography of the subject, to which the student is referred. The titles of some 
of the more important memoirs which have recently appeared will be given in footnotes. 
1 Fouqué et Michel-Lévy. Min. Micrograph, p. 151. 
