CONDITIONS OF ORIGIN OF IGNEOUS ROCKS. 207 



Moreover, the variations in chemical composition which result in 

 the elaboration of the different felspars and micas, in granites of cer- 

 tain localities, are precisely analogous to the differences in chemical 

 composition of clays, which result from the mechanical conditions of 

 washing, and transport at the time of deposition. These clays have 

 been experimentally analysed by washing, so as to be separated into 

 several parts, each with a chemical composition of its own. And 

 since the first washing removes all the larger particles of sand, it is 

 obvious that the percentage of silica in a clay is merely an expression 

 of the nearness of a deposit to shore, or indicates slope of sea-bottom, 

 or the presence or absence in the ocean of silicious organisms when 

 the deposit was forming. 



Evolution of Igneous Rocks. Stratified rocks present gradations 

 of chemical composition, which commencing with analogues of the most 

 extremely acidic portion of the igneous rocks, gradate into others which 

 represent the extremes of the basic series. And if these sediments 

 were successively melted up and cooled, we should expect the sequence 

 to begin with quartz rock in which but few accidental minerals are 

 scattered, and pass down through an intermediate series into crystal- 

 line limestones, in which earthy minerals are rare. The great central 

 part of the group has the chemical composition which is required to 

 produce all the known types of igneous rocks, if it were open to the 

 influx of sea-water. And the local conditions of more or less perfect 

 separation of the mechanical substances, and products of the growth 

 and decay of various groups of vegetable and animal organisms, may 

 explain the fact that almost every locality has its local varieties of 

 the rock families, to which its igneous rocks belong. In nature 

 a perfect geographical gradation of altered sediments can never 

 be expected, because it could only occur along an existing coast- 

 line, which was at right angles to some ancient shore-line, which 

 furnished sediments in horizontal sequence ; and then we should 

 require these sediments to have been melted up along their extent, 

 without mixture with other deposits. The vast thickness of ancient 

 clays, as exemplified in the older primary strata, may account in 

 part for the absence of liquefied silicious rocks ; but the greater fusi- 

 bility and capacity of the felspathic sediments for aqueo-igneous 

 changes, would influence the extrusion of rocks of this character, to 

 the exclusion of the extreme terms of the horizontal series of aqueous 

 rocks deposited. Geological sections show, however, that strata of 

 different mineral character are superimposed on each other. If, then, 

 such sediments are inferred to become so folded, compressed, and 

 heated by internal contraction of the earth's crust as to be melted up 

 in succession, according to the order in which they rest upon each 

 other, and to become partially mixed in the process of eruption, there 

 ceases to be any difficulty on chemical grounds in accounting for 

 either the normal trachytic or normal pyroxenic groups of Bunsen, 

 or for any of the intermediate or extreme types of eruptive rocks 

 succeeding each other from the same eruptive throat. And when the 

 chemical compositions of the several groups of volcanic rocks are 



