Volcanos. 143 



alternation, of precipitated, sedimentary, and fragmentary mat- 

 ter, giving rise to the transition formations. 



" Ln this manner was formed the first crust or solid envelope of 

 the globe. But beneath this crust a new process had now com- 

 menced, occasioned by the increase of temperature and of ex- 

 pansive force of the upper granite beds; which, having been 

 greatly reduced in temperature by the dilatation it had endured, 

 and the partial vaporization of the water it contained, now be- 

 gan to receive an accession of caloric from the more intensely 

 heated nucleus. The first effect of such an increase in the ex- 

 pansive force of this zone, opposed as it was by the increasing 

 pressure of the strata, whose progressive deposition was going 

 on above, would be to consolidate the intermediate bed of gneiss ; 

 the next, to produce, sooner or later, the disruption of the solid 

 crust, which impeded its actual expansion. This result took 

 place on those parts where accidents of texture or composition 

 in the oceanic deposits led to them to yield most readily ; and in 

 this manner were formed, in the primeval crust of the earth, 

 those original and deep fractures, through some of which (fis- 

 sures of elevation) were protruded portions in a more or less 

 solid state of the inferior granite, together with replications of 

 the foliated rocks, (as described in a former chapter;) while 

 others (fissures of eruption) gave rise to local extravasations of 

 the heated crystalline matter in form of lavas, that is, still far- 

 ther liquefied by the greater comparative reduction of the press- 

 ure they supported. By these partial elevations of the superfi- 

 cial strata, violent movements were at times, as has been men- 

 tioned before, communicated to the waters of the ocean which 

 broke up the projecting eminences, and distributed their frag- 

 ments in conglomerate or sedimentary strata. At first, the sur- 

 face of the globe consisted chiefly of mica-schist ; and hence mica 

 and granular quartz predominate in the earlier conglomerates 

 and sedimentary strata, (grey-wacke, grey-wacke slate, quartz- 

 rock.) Precipitations of silex and carbonate of lime, continued 

 to mix with the sediments of this period, and Mr. Scrope supposes 

 quartz rock and transition limestone to owe their dark colors to 

 admixture with the finest particles of mica. For a long time it 

 is probable that local developements of subterranean expan- 

 sion, producing partial elevations of the earth's crust, local ex- 

 travasations of crystalline rocks in the form of dykes, beds, &c. 

 and local deposits of conglomerate beds, alternated with periods 

 of comparative tranquillity, during which the finer sedimenta- 

 ry deposits and precipitations took place, and hence the al- 

 ternations of the various sedimentary and arenaceous strata 

 which compose the secondary formations. Meantime, as the 

 temperature of the ocean decreased, it began to be thickly 



