66 LITHOLOGICAL GEOLOGY. 



water and air being the chief means through which decomposition is 

 carried forward. The best slate, when free from pyrite, is exceeded 

 by no other rOck in durability. Compact pure limestone is next to it 

 in this respect ; but this yields superficially to the dissolving powers of 

 atmospheric waters. Want of durability may come (l)from openness 

 of texture ; (2) from the composition of the constituent minerals ; and 

 (3) largely from the composition of accessory minerals, like pyrite. 

 See p. 687. 



7. A Study of the Accessory Minerals, or those not Essential Constitu- 

 ents of the Hock. — All accessory minerals have prominent interest, and 

 none more so than those revealed by the microscope. Some of the 

 microscopic minerals occur inside of other crystals, and are hence 

 called inclusions. Among the inclusions there are sometimes globules 

 of liquid carbonic acid inside of crystals of quartz, topaz, and certain 

 other hard and firm minerals ; crystals of apatite or calcium phosphate, 

 a mineral occurring microscopically in nearly all crystalline rocks ; 

 also cubes of common salt, and other crystallizations, part original to 

 the rock, and part a result of changes that have taken place within it. 



The following are some terms not explained above : — 



Quartzose. Consisting of quartz ; containing much quartz. 



Calcareous. Consisting of limestone (calcite) ; containing much 

 calcite. 



Ferruginous. Containing much iron oxide. 



Argillaceous (from argilla, clay). Made of more or less hardened 

 clay or fine mud ; containing clayey material. 



Granitoid. Like granite in crystalline-granular condition. 



Granitic. Made of granite, or of comminuted granite. 



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, or doleryte, 

 and related basic eruptive rocks, are often amygdaloidal. 



Scoriaceous. Slag-like, very open cellular, or inflated, like the 

 scoria of a volcano or slag of a furnace. 



Pyritiferous. Containing pyrite. 



Cryptonrystalline. Having no grains visible in the texture. Same 

 as aphanitic. The term microscopic is used for the same ; and macro- 

 scopic for coarse-granular, or when the grains are large enough for a 

 more or less perfect study of the rock without the use of the micro- 

 scope. A rock may be studied microscopically, or macroscopically. 



Acidic. Basic. Crystalline rocks contain usually — and the igneous, 

 almost always — a feldspar as one of the ingredients, In a feldspar, the 

 silica is the acidic constituent ; and those feldspars which contain the 

 larger percentage of silica — exceeding 60 per cent. — like orthoclase, 



