GEOLOGY. 397 



Of composite rocks, granite is an aggregate of quartz, feldspar, 

 and mica ; with which hornblende is sometimes intermingled ; but it 

 is then more properly called sienitic granite. The name sienite, is 

 applied to an aggregate of feldspar and hornblende, sometimes mingled 

 with quartz ; and when fine grained, and of a greenish color it has 

 also been termed greenstone. Protogine, is a granite in which the 

 mica is replaced by talc, steatite, or chlorite ; rendering it softer. 

 When feldspar is found intermingled with augite, hypersthene, or 

 diallage, the rock thence resulting is named from one of the latter 

 three minerals : and diallage rock is often found associated with ser- 

 pentine. Gneiss differs from the preceding granitic rocks only in 

 being stratified, or found in layers ; and mica slate, differs from gneiss 

 chiefly in being stratified in thinner layers, caused by the mica being 

 diffused in parallel directions through the quartz. 



The name of trap, or trappean rocks, is applied to those which 

 contain a considerable portion of feldspar, hornblende, or augite, and 

 are supposed to have been ejected from the interior of the earth in a 

 melted state. Indeed, the trappean rocks pass by insensible grada- 

 tions, on the one hand into granite and gneiss, and on the other into 

 basalt, and similar rocks of known volcanic origin. Basalt, is now 

 considered as a trappean rock ; analogous to greenstone, or augite 

 rock, in its composition : and wacke is a softer kind of basalt. Por- 

 phyry, is an aggregate of large crystals, usually of feldspar, imbedded 

 in a cement or basis of feldspar, or some allied mineral. Graywacke, 

 is an aggregate of various small mineral fragments, angular, flat, or 

 rounded, united by a silicious cement ; and conglomerate or pudding- 

 stone, is an aggregate of larger pebbles, or boulders, united by a 

 silicious, argillaceous, or ferruginous cement. Amygdaloid, is a 

 rock usually of basalt, wacke, or greenstone, in which are vesicular 

 cavities, more or less filled up with various minerals, apparently 

 formed there by infiltration. Breccia, is an aggregate of angular 

 fragments, of one or more minerals, united by some cement. Sand- 

 stone, is chiefly composed of grains of quartz, united by a cement, 

 which may be calcareous, argillaceous, or silicious, and is often 

 colored red by the oxide of iron. *flrgillite, or clay slate, consists 

 of indurated clay, often interspersed with particles of other minerals ; 

 and passing, by imperceptible gradations, into silicious slate, contain- 

 ing a large portion of silex, or sand. 



2. Under the head of Systematic Geology, we would comprise 

 a general analysis of the structure of the earth ; reserving for Descrip- 

 tive Geology, the application of this analysis, to the description of the 

 various countries and regions which have yet been explored. The 

 study of Paleontology, so far as it relates to the characters of ancient 

 plants and animals, belongs to the branches of Botany and Zoology ; 

 but in so far as it examines what particular species are found in par- 

 ticular strata or formations, we would include it in the present section. 

 On penetrating deeply into the earth, at various contiguous places, 

 we find its mass to be composed of numerous strata, or layers, usu- 

 ally nearly horizontal, though sometimes much inclined. The strata 

 which are inclined, are said to dip; and the direction in which they 

 descend most rapidly, is called the direction of the dip. When, from 



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