^818.^ ON THE FORMATION OF ROCKS. 231 



ORDER II. 



Where the fire has not existed witliiii the reach of 

 Jjustory, but where the nature and component parts, the 

 relative situation, &c. is little different from the active 

 volcanoes, having the remains of craters, cements, 

 scuria, &c. &c. placed in the same relative situation ; 

 the currents of Lava radiating from the crater, and 

 covering all the classes of rocks, and filling up all the 

 inequalities the currents meet with. Between this ar- 

 rangement, and that of an active volcano^ there is a 

 direct and perfect analogy. 



In comparing old Lavas with those that h^ve re 

 cently been thrown out of a crater, considerable al- 

 lowance ought to be made for the great change that 

 has taken place in the former by the action of air and 

 water, and the substances that accompany them. The 

 constant filtering of the water, through all the pores of 

 the Lava, takes oli its asperity and roughness, while 

 the pores themselves a^e filled by depositions of the 

 various substances held in solution by the water ; at the 

 same time the water oxides tlie iron in the Lava, and 

 changes it into a dull eariliy fracture; ail w hich changes 

 disguise and mask the true character of the ixick, and 

 are liable to deceive observation, if it is partial and li- 

 mited to a small extent of country. 



It is tlie nature of volcanic rocks to be in detached 

 pieces, and particuiariy after time and decompositioijj 

 have worn away ail the scoria, cinders, porous La- 

 vas, &c. when the most solid part of a current of La^ 

 ^a becomes insulated at a considerable disiancc from 



