80 SCIENTIFIC AGRICULTURE. 



L YELL'S CLASSIFICATION. 



Mr. Lyell comprehends all the various rocks which compose 

 the crust of the earth in four great classes, depending for their 

 distinctive characters on their origin and age. These are 

 named as follows. 



f AQUEOUS, 

 J VOLCANIC, 

 j PLUTONIC, 

 ^ METAMORPHIC. 



Aqueous Rocks. This class, called also, the sedimentary 

 rocks, covers a larger portion of the earth's surface than any 

 of the other three classes. They are stratified, and supposed 

 to have been deposited by water, both running and quiescent: 

 they contain fossils, shells and coals. 



Volcanic Roclcs. This class of rocks has been produced, 

 both in ancient and modern times, by the action of volcanic 

 fires or subterranean heat. "They are for the most part 

 unstratified, and devoid of fossils:" they are more partially 

 distributed than aqueous formations, at least in respect to 

 horizontal extension. 



Plutonic Rocks. This class of rocks has been formed, " at 

 great depths in the earth, and they have cooled and crystalized 

 slowly, under enormous pressure, where the contained gases 

 could not expand." They are more crystaline than the others, 

 have no cavities, and contain no organic remains. They lie 

 below, and are older than all others. 



Metamorphic Rocks* These rocks, according to Mr. Lyell, 

 were originally deposited from water in regular strata, and 

 afterwards metamorphosed or changed by subterranean heat, 

 so as to assume a new and different texture. They contain no 

 pebbles, pieces of imbedded rock, nor organic remains, and are 

 often crystaline, as granite. They vary in color and compo- 



* This class is considered as merely a hypothetical division by many 

 of the best Geologists. 



