Parr IL. § iv.] MICROSCOPIC CHARACTERS OF ROCKS. 103 
Four types of rock-structure are revealed by the microscope. 
A, wholly crystalline ; B, semi-crystalline ; C, glassy ; D, clastic. 
A. WHOLLY CRYSTALLINE, consisting entirely of crystals or 
crystalline individuals, whether visible to the naked eye, or requiring 


Fic. 10.—Wuorty Crysrattine Srrucrure. GRanire (20 Diameters). 
The white portions are Quartz, the striped parts Felspar, the long, dark, finely striated 
stripes are Mica. (See p. 131.) 
_ the aid of a microscope, imbedded in each other without any intervening 
amorphous substance. Rocks of this type are exemplified by granite 
(Fig. 10) and by other igneous rocks. But they occur also among the 
crystalline limestones and schists, as in statuary marble, which 
consists entirely of crystalline granules of calcite (Fig. 16). 




Bye. 11.— SEMI-CRYSTALLINE SrRucTURE. DOLERITE, CONSISTING OF A TRICLINIC FELSPAR, 
AUvGITE, AND MAGNETITE IN A DEvITRIFIED Grounp-mass (20 Diameters). 
The numerous oblong Prisms are triclinic Felspar; the broader monoclinic forms, 
slightly shaded in the drawing, are Augite; the black specks are Magnetite; the 
neédle-shaped forms are Apatite. (See p. 148.) 
B, SEMI-CRYSTALLINE.—This division probably comprehends the 
majority of the massive eruptive or igneous rocks. It is dis- 
tinguished by the occurrence of what appears to the naked eye as a 
compact or finely granular ground-mass, through which more or less. 
