64 LITHOLOGICAL GEOLOGY. 



Argillaceous, having a clayey nature or constitution, or containing much clay, as 

 shale is argillaceous, a sandstone may be argillaceous. 



Ferruginous, containing oxyd of iron; sometimes having a red, brownish-red, or 

 brownish-yellow color, in consequence of the disseminated oxyd of iron; sometimes con- 

 taining the ore in plates or masses of a metallic lustre. 



Pyritiferous, containing pyrite (p. 59) disseminated through the mass, either in 

 cubic crystals, or in grains or masses. 



Basaltic, made of material derived from basalt; also like basalt. 



Pumiceous, made of pumice. 



Garnetiferous, containing garnets. 



So, also, staurolitic, containing staurolite; anihophyllitic, containing acicular horn- 

 blende of the variety anthophyllite. 



Sedimentary rocks differ, further (d), as to the mechanical condition 

 of the constituents : whether — 



(a) Rounded stones or pebbles ; or (b) angular stones; or (c) sand ; or (d) clay. 



Crystalline rocks differ, further, — 



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

 beyond. 



f. As to the kind of crystalline aggregation or structure : — 



Granular (phanerocrystalline, 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, syenyte, dioryte. 



Flg> _ 58- 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 

 distinct crystals in the rock-mass, the rock in this case being described as porphyritic 

 with hornblende or with pj'roxene. 



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

 through the middle (see Fig. 58), and by the difference in lustre of the two halves. 

 Granite, dioryte, doleryte, and lavas, as well as porphyry, are sometimes porphyritic, 

 and the feldspar crystals may be very large or very small. 



Homogeneous, having the mineral ingredients not separately distinguishable, but 

 forming a homogeneous mass, granular or otherwise, like argillyte and most trap or 

 doleryte. 



Amygdaloidal (from amygdalum, an almond). Having numerous spheroidal or 

 almond-shaped cavities filled with minerals foreign to the rock, such as quartz, calcite, 

 and the zeolites. Trap (doleryte) and basalt are often amygdaloidal. 



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

 or slag of a furnace. 



It should be further observed that a rock — 



When Quarts predominates, is hard an d often gritty. G. =2.5-2.8 



