KOCKS, 



CHAP. VIIL— ROCKS OR MINERAL 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 iri 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 



