508 ON THE FORMATION OF R0CKJ5. [Mfj 



CLASS IL 



Order i. 

 l^oclcs of Volcanic origin. 



Thrown out of active volcanoes, tlie origin of wliose 

 formation rests on the evidence of our senses. 



Tliere are two modes of examining rocks ; one, tlie 

 investigation of their external appearance, and internal 

 structure, which can be accomplished M'ith hand spe? 

 cimens in a cabinet, and belongs properly to mineralor 

 gy ; the otlicr is the tracing, upon an extensive scale^ 

 the relative position of their beds, whether stratified or 

 flivided by vertical fissures ; if stratified, whether hori- 

 zontal or at a dip from the liorizori, whether the strati- 

 fication is regular, occupying large fields of extensive 

 countries; or (Louslsting of detached insulated masses, 

 with vertical fissures, partially scattered oji the sur- 

 face of all the otlier formations, &c. : this perhaps be- 

 longs to geology, and cannot be studied in the cabinet^ 

 but requires much practice in the mountains, liammer 

 in hand. 



Lavas thrown out from recent volcanoes are va- 

 rious in their fractures ; they contain a number of 

 insulated crystals ; and are composed of a variety of 

 different substances; but have one general distinguish- 

 ing mark, which runs tlu-ough the wiiole, and sepa- 

 rates thei.: from rocks of Neptunian origin : tliis mark 



