vi.] THE SLATES ON THE ROOF. 135 



a layer of fine mud, laid down at the bottom of the 

 ocean, several inches thick, eighty miles at least long, 

 and twenty miles perhaps broad, by a single eruption. 

 Suppose that hardened in long ages (as it would be 

 under pressure) into a bed of fine grained Felstone, 

 or volcanic ash; and we can understand how the ash- 

 beds of Snowdonia which may be traced some of 

 them for many square miles were laid down at the 

 bottom of an ancient sea. 



But now about the lavas or true volcanic rocks, 

 which are painted (as is usual in geological maps) red. 

 Let us go .down to the bottom of the sea, and build up 

 our volcano towards the surface. 



First, as I said, the subterranean steam would 

 blast a bore. The dust and stones, rasped and blasted 

 out of that hole would be spread about the sea- 

 bottom as an ash-bed sloping away round the hole ; 

 then the molten lava would rise in the bore, and 

 flow out over the ashes and the sea-bottom 

 perhaps in one direction, perhaps all round. Then, 

 usually, the volcano, having vented itself, would be 

 quieter for a time, till the heat accumulated below, 

 and more ash was blasted out, making a second ash- 

 bed ; and then would follow a second lava flow. Thus 

 are produced the alternate beds of lava and ash which 

 are so common. 



Now suppose that at this point the volcano was 

 exhausted, and lay quiet for a few hundred years, or 

 more. If there was any land near, from which mud 

 and sand were washed down, we might have layers on 

 layers of sediment deposited, with live shells, etc., 

 living in them, which would be converted into fossils 

 when they died ; and so we should have f ossilif erous 



