292 



DYNAMICAL GEOLOGY. 



origin is sometimes proved by the crystalline structure and minerals of the 

 rocks. It is probable that other islands of the Hawaiian group have suffered 

 from still greater explosions; for, as the accompanying map (Fig. 257) shows, 

 Oahu consists of portions of two mountain-cones. The larger part of the 

 eastern cone — the one of most recent extinction — must have been broken off 

 and sunk. A vertical wall over 20 miles long marks the course of the fract- 

 ure. Its highest point is over 3000 feet high after long exposure to denuda- 

 tion. Molokai bears evidence of like catastrophic experience. 



The conglomerates made by volcanic ejections contain ang^wtor fragments, 

 and never consist chiefly of rounded x)ebhles or stones. 



257. 



Explosive eruptions of another kind, which might be styled semi-volcanic, 

 are included among described volcanic phenomena. In such eruptions water 

 in large volumes gains sudden access to the heated depths beneath an extinct 

 or nearly extinct volcanic mountain through fractures or movements along 

 planes of weakness, as in other cases; but the heated depths are not hot 

 enough for fused rocks. The consequences are earth-shakings ; explosions 

 from the suddenly generated steam ; the rending of rocks in the deep- 

 seated region of the explosions ; projectile action throwing the stones and 

 great rock-masses so made, and the dust from abrasion, into the air and over 

 the adjoining region, attended by vast and violent effusions of steam, 

 making darkness and terrific storms about the mountain ; — and not outflows 

 of lava nor the projection of volcanic ashes and scoria from cooled lavas. 



