72 



LITHOLOGICAL GEOLOGY. 



e. As to the number and kinds of mineral constituents, as explained 

 beyond. 

 /. As to the kind of crystalline aggregation or structure : 



Granular (phanero-crystalline, or distinctly crystalline), which may be either 

 coarse granular, as in granite and much architectural marble, or fine granular, 

 as in some statuary-marble. 



Cryptocrystalline, or concealed crystalline, as in flint, no particles being 

 distinct. 



Granitoid, having each of the mineral constituents separately crystallized 

 and distinct, as in granite, syenite, diorite. 



Other terms bearing on structure are 

 as follow : — 



Porphyritic. — Having the feldspar in 

 distinct crystals through the mass of 

 the rock, or speckling it with spots 

 of white or a light color that are often 

 rectangular or nearly so (fig. 58). 



The term porphyritic is sometimes applied 

 also where hornblende or pyroxene is in dis- 

 tinct crystals in the rock-mass, the rock in this case being described as por- 

 phyritic with hornblende or with pyroxene. 



The feldspar crystals are often double or twin crystals, as shown by a line of 

 division through the middle (see fig. 58). Granite, diorite, dolerite, and lavas, 

 as well as porphyry, are sometimes porphyritic, and the feldspar crystals might 

 be very large or very small. 



Homogeneous, having the mineral ingredients not separately dis- 

 tinguishable, but forming a homogeneous mass, granular or other- 

 wise, like most trap, or dolerite and argillite. 



Amygdaloidal (from amygdalum, an almond). Having numerous sphe- 

 roidal or almond-shaped cavities, like some trap, dolerite, basalt, the cavities 

 filled with minerals foreign to the rock, such as quartz, calcite, and the 

 zeolites. 



Scoriaceous. — Slag-like, very open, cellular, or inflated, like the scoria of a 

 volcano or slag of a furnace. 



It should further be observed that a rock — 



When Quartz predominates, is hard and often gritty. G. — 2.5-2.8. 



Feldspar — hard, usually light-colored. G. =2.5-2.8. Either cleavably 

 crystalline or cryptocrystalline. 



Hornblende and Pyroxene — hard, usually dark-green to black; heavy. G. = 

 2.8-3.6. Often tough. 



Mica — slaty, glistening with mica scales, not very hard, not greasy to the 

 touch. G. =r 2.5-2.8. 



Talc — often slaty, somewhat glistening, a greenish color, grayish, brownish, 

 not very hard; & greasy feel. G. = 2.4-2.7. 



