THE PRODROMUS 231 



The first process is the violent thrusting up of the strata, 

 whether this be due to a sudden burning of subterranean gases, 

 or be brought about through the violent explosion of air due to 

 other great downfalls near by. This thrusting up of the strata 

 is followed by a scattering of the earthy matter as dust and the 

 breaking up of rocky matter into lapilli and rough fragments. 



The second process is the spontaneous slipping or downfall 

 of the upper strata after they have begun to form cracks, in con- 

 sequence of the withdrawal of the underlying substance, or 

 foundation. Hence by reason of the diversity of the cavities 

 P. 32. and cracks the broken strata assume different positions ; while 

 some remain parallel to the horizon, others become perpendic- 

 ular to it, many form oblique angles with it, and not a few are 

 twisted into curves because their substance is tenacious. This 

 change can take place either in all the strata overlying a cavity, 

 or in certain lower strata only, the upper strata being left unbroken. 



The altered position of the strata affords an easy explanation 

 of a variety of matters otherwise obscure. Herein may be 

 found a reason for that unevenness in the surface of the earth 

 which furnishes occasion for so many controversies ; an un- 

 evenness manifest in mountains, valleys, elevated bodies of 

 water, elevated plains, and low plains. But passing over the 

 rest, I shall now treat briefly certain points concerning moun- 

 tains. 



THE ORIGIN OF MOUNTAINS 



That alteration in the position of strata is the chief cause of 

 mountain formation is clear from the fact that in any given 

 range of mountains there may be seen : 



1 . Large level spaces on the summits of some mountains. 



2. Many strata parallel to the horizon. 



3. Various strata on the sides of the mountains inclined at 

 different angles to the horizon. 



4. Broken strata on the opposite sides of hills, showing abso- 

 lute agreement in form and material. 



5. Exposed edges of strata. 



6. Fragments of broken strata at the foot of the same range, 

 P. 33. partly piled into hills, and partly scattered over the adjoining 



country. 



