KOCKS. 



CHAP. VIII.— ROCKS OfUtylNEKAL AGGREGATES. 



General Nature of Rocks. In the early part of this vol. 

 ume it is stated that the rocks of the globe are mineral in 

 their nature, and consist either of a single mineral in a mas 

 sive state, or of intimate combinations of different minerals. 

 Limestone, when pure, is a single mineral, — it is the spe- 

 cies calcite or carbonate of lime ; common granite is a com- 

 pound or aggregate of three minerals, quartz, feldspar, and 

 mica. Sandstones may consist of grains of quartz alone, like 

 the sands of many sea-coasts, being such a rock as these 

 sands would make if agglutinated ; it is common to find along 

 with the quartz, grains of feldspar, and sometimes mica. 

 Clay slates consist of quartz and feldspar or clay, with some- 

 times mica, all so finely comminuted, that often the grains can- 

 not be observed. Conglomerates or puddingstones, may be 

 aggregates of pebbles of any kind : of granite pebbles, of 

 quartz pebbles, of limestone pebbles, or of mixtures of differ- 

 ent kinds, cemented together by some cementing material, 

 such as silica, oxyd of iron, or carbonate of lime. 



Texture or structure of Rocks. — Rocks differ also in tex- 

 ture. In some, as granite, or syenite, the texture is crys- 

 talline : that is, the grains are more or less angular, and 

 show faces of cleavage ; the aggregation was the result 

 of a cotemporaneous crystallization of the several ingredi- 

 ents. Common statuary or white building marble, consists 

 of angular grains, and is crystalline in the same manner. 

 But a pudding-stone is evidently not a result of crystalliza- 

 tion ; it consists only of adhering pebbles of other rocks with 

 a cementing material which is often not apparent. Sand- 

 stones also are an agglutination of grains of sand, — just such 

 rocks as would be made from ordinary sand by compacting 

 it together ; and clay slates are often just what would result 

 from solidifying a bed of clay. There are therefore crystal- 

 line and uncrystalline rocks. It should be remembered, 

 however, that in each kind of rock, the grains themselves 

 are crystalline, as all solid matter becomes solid by crystal- 

 lization. But the former kind is a crystalline aggregation 

 of grains, the latter a mechanical aggregation. 



In crystalline rocks it is not always possible to distinguish 

 the grains, as they may be so minute, or the rock so com 



