26 THE EARTH FORMED : 



of these, sometimes three, associated in various proportions, 

 compose the rock, which thus appears in many varieties, 

 passing under different names. 



Where granite does not appear upon the surface, or else 

 some other igneous rock, such as will presently be adverted 

 to, we find that great flooring overlaid with rocks of a diffe- 

 rent character and history namely, what are called Aqueous 

 or Sedimentary Rocks. These are in the form of strata or 

 beds, and have evidently been for the most part produced as 

 a 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 exist- 

 ence, the earliest from the original granite, and so on in suc- 

 cession ; and this, by means or 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 de- 

 rived from the felspar and the addition of mica, Micaceous 

 Schist has been formed. Sometimes the quartz forms a se- 

 dimentary rock by itself. For such elections of materials, 

 as they may be called, we see natural means of accounting, 

 when we reflect that the lighter particles of any substance 

 suspended in moving water are liable to be carried most ra- 

 pidly, and to the greatest distance. ( 10 ) It is also to be re- 

 marked 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 crystaline texture, 

 are strangely waved and contorted, and often cannot be 



