Earthquakes and Volcanoes 



obtained, the rate of augmentation being affected 

 by the character of the rocks bored through and 

 by the position which the strata occupy ; whether 

 for example, they are inclined or horizontal. 

 Thus at the Astley pit at Dunkenfield in Cheshire 

 the rate was found to be i for every seventy- 

 seven feet, but this appears to be unusually low. 

 Perhaps it will not be far wrong to assume that 

 the average increase is i for every sixty feet: 

 such at least is the rate which was adopted a 

 few years ago by the Royal Coal Commission in 

 their calculations. 



Even the deep sinking at the Rosebridge 

 Colliery is but the veriest dent in the earth's 

 surface compared with the actual radius of the 

 globe. It gives therefore but scant information 

 respecting the temperature of the deep-seated 

 portions of the interior; but, assuming such a 

 rate of increase to continue, it is evident that at 

 the depth of only a few miles the heat would be 

 sufficient to fuse any known rock. It is true 

 that the melting point of a solid body may be 

 greatly modified by pressure; and it is obvious 

 that, at great depths, the pressure must be pro- 

 digious. Nevertheless, the eruption of lava 

 from volcanic vents sufficiently shows that, 

 whatever the general state of the earth's interior, 

 there must be at least local masses of molten 

 rock. 



Additional evidence of the existence of heat 

 at great depths is furnished by the temperature 

 of the water yielded by certain springs. Some 

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