150 MINERALS AND GEOLOGY 



Metamorphic strata are regarded as consisting wholly or in great part 

 of sedimentary deposits that have been altered or rendered crystalline 

 by heat or chemical agencies. Eruptive rocks are known in many 

 instances to have cooled down from a state of fusion, and are thought 

 in others to have been consolidated from a plastic condition due to 

 aqueo-igneous agencies. They have been formed, or have been brought 

 into this condition, beneath, or deeply within, the Earth's crust, and 

 have been forced upwards from time to time through fissures in the 

 overlying rocks. In each of these divisions Sedimentary, Metamor- 

 phic, and Eruptive the included rocks belong to various periods of 

 formation. 



II. SEDIMENTARY ROCKS. 



The rocks of this division make up by far the greater portion of 

 the Earth's surface. Having been formed by the agency of water, 

 they are often called Aqueous Rocks. They consist for the greater 

 part of muddy, sandy, -and other detrital sediments, collected by the 

 mechanical action of water, and subsequently consolidated by natural 

 processes, as described a few pages further on. Various limestones, 

 however, and certain other rock matters of this division, have heen 

 deposited from waters in which their materials were chemically 

 dissolved. 



These sedimentary or aqueous rocks are characterized essentially 

 by occurring in beds or strata ; secondly, by exhibiting in many 

 instances, a more or less clearly-marked detrital or sedimentary 

 structure ; and thirdly, by often containing organic remains. The 

 latter, comprising shells, bones, leaf-impressions, &c. (see Part IV.), 

 are the fossilized parts of animals and plants which lived upon the 

 Earth, or in its waters, during the periods in which these rocks were 

 under process of formation, as described below. 



The sedimentary rocks may be conveniently discussed under the 

 following heads : (1) Composition or mineral characters ; (2) Modes 

 of formation ; (3) Subsequent changes and effects produced by geolo- 

 gical agencies. 



(1) COMPOSITION OF SEDIMENTARY ROCKS. 



As regards composition, these rocks fall mainly under the following 

 sub-divisions : 



