118 



SUPPOSED FORMER INTENSITY 



[On. VII. 



me 



And 



masses is more minutely examined 

 ) products of a great many successive 



by which scoriae and lava were again and again emitted, and 

 afterwards consolidated, then fissured, and finally traversed 



matter 



As 



Ave 



enlarge, therefore, our knowledge of the ancient rocks 



formed 



them as the ag^re^ate effects of innumerable 



may 



those now experienced in volcanic regions. 



may 



data for 



volume of matter 



state of fusion at two given periods, as if we were to compare 

 the columnar basalt of Staffa and its environs with the lava 

 poured out in Iceland in 1783 ; but for this very reason it 

 would be rash and unphilosophical to assume an excess of 

 ancient as contrasted with 



modern 



of melted 



matter at particular periods of time.f It would be still more 

 presumptuous to take for granted that the more deep-seated 

 effects of subterranean heat surpassed at remote eras the 

 corresponding effects of internal heat in our own times. 

 Certain porphyries and granites, and all the rocks commonly- 

 called plutonic, are now generally supposed to have resulted 

 from the slow cooling of materials fused and solidified under 

 great pressure ; and we cannot doubt that beneath existing 

 volcanos there are large spaces filled with melted stone, 

 which must for centuries remain in an incandescent state, 



cool and become hard and crystalline when the 

 subterranean heat shall be exhausted. That lakes of lava are 

 continuous for hundreds of miles beneath the Chilian Andes, 

 seems established by observations made in the year 1835. J 



dierever the fluid contents of such reservoirs are 



and then 



Now 



from 



at 



the bottom of the sea, the matter so ejected may aftord 

 evidence by its arrangement of having originated at difleren 



* See Elements of Geology, 6th ed. 

 chap. 30 to 32, inclusive. 



t See below, Icelandic eruptions. 



ch. 27. , 



t See below, Chilian earthquake, en. 



28. 





