VOLCANIC ROCKS. 235 
III. ALLUVIAL DEPOSITIONS. 
269. These are described to comprehend all such sub- 
stances as have been formed from previously existing rocks, 
of which the materials have been worn down, by long expo- 
sure to the agency of water and air, and afterwards de- 
posited in nearly horizontal beds on the surface of the land. 
Alluvial deposits have been formed, and are still forming 
in every quarter of the globe. They occur both in 
mountainous regions and in flat countries, filling up the 
valleys or hollows in the one ; and often forming vast 
and extended plains in the other. 
They consist of peat, sand 9 gravely loam, clay, and 
other substances. 
IV. VOLCANIC ROCKS. 
270. Volcanic rocJcs are composed of such mineral sub- 
stances as have been ejected from volcanoes, or have been 
formed by the agency of subterraneous fires, and have 
undergone certain changes in such fires. 
They are of two kinds ; the one called pseudo volca- 
nic, such as burnt clay, porcelain jasper, and earth- 
slag, which have been altered in consequence of the 
burning of beds of coal in their neighbourhood ; and 
the other, called true volcanic minerals, such as stones, 
ashes, and lava, which have been thrown out of real 
volcanoes. 
271. It will somewhat tend to illustrate the history of 
the mineral kingdom, to state, in conclusion, under a 
tabular form, the relative heights of the principal moun- 
tains, or masses of rocks, which occur in the different 
countries of the world ; previously remarking, that the 
most lofty and magnificent of these, respecting which 
any account sufficiently authentic has hitherto been 
obtained, are the mountains of Nepaul and Thibet, in 
Asia, one of the former being 27,667, and the highest 
of the latter measuring at least 23/262 feet, or from 4- J 
miles to 5| miles in perpendicular height above the level 
