THE EARTH FORMED. 25 



sediment of sand, clay, or other materials, at the bottom of 

 seas, the matter being hardened by heat and pressure after its 

 deposition. Whence the materials of these rocks'? With 

 some peculiar exceptions, each group of them has been derived 

 from the substance of such rocks as were previously in existence, 

 the earliest from the original granite, and so on in succession ; 

 and this, by means of processes which continue in operation 

 at the present day. That is to say, the atmosphere, by the 

 chemical action of its materials, and the vapours with which 

 it is charged, wears down whatever rocks are exposed to it ; 

 rivers carry the particles into the sea ; the sea also erodes the 

 rocks against which it impinges, and strews the matter along 

 its bottom ; thus are sediments laid down, to be in time 

 formed into rocks. Many of the earliest or lowest strata are 

 obviously composed of material but slightly changed from the 

 original granite ; such are all the rocks bearing the name of 

 Gneiss. Others present the component materials in different 

 combinations ; as, for instance, where, with clay derived from 

 the felspar and the addition of mica, Micaceous Schist has been 

 formed. Sometimes the quartz forms a sedimentary rock by 

 itself. For such elections of materials, as they may be called, 

 we see natural means of accounting, when we reflect that the 

 lightest particles of any substance suspended in moving water 

 are liable to be carried most rapidly, and to the greatest distance. 1 

 It is also to be remarked of all these early rocks, that they have 

 evidently been subjected to an extraordinary degree of heat, 

 insomuch that they generally have acquired a new crystalline 

 texture, are strangely waved and contorted, and often cannot 

 be distinguished from the underlying granite, the adjacent 

 parts of which may sometimes be detected as having been 

 placed there after the deposition of the aqueous rocks. Now 

 the lowest stratified rocks are sometimes found lying in a 

 nearly horizontal position, as they would be originally formed ; 

 but more generally they are tilted up in high inclinations, with 

 the broken edges directed towards granitic mountains ; indi- 

 cating that the rise of these mountains from below was the 

 cause of the change of position in the stratified rocks. Thus 

 the earliest strata were in their turn exposed to the wearing 

 influences of sea and atmosphere, and the materials appro- 

 priated to form new rocks. And, precisely as might be ex- 

 pected, these new rocks are laid down unconformaUy to the 



1 De la Beche's Geological Researches, 1834. 



