PERPETUAL MOTION. 79 



besides this, streams of lava from a submarine volcano 

 flowed over them, one being a great mass of black basalt, 

 1,000 feet thick. After being thus buried, the trees were 

 once more raised, and this time hoisted up 7,000 feet 

 above the sea, and now, by the wear and tear of time, they 

 have been exposed to view and stand high up in the 

 mountains and hundreds of miles inland, like ghosts of 

 their former selves. 



Similar risings and sinkings of the land have gone on, 

 and are still going on, more or less, all over the world, so 

 that it is quite true, as has been said, that " nothing, not 

 even the wind, is so unstable as the crust of this earth." 



As to the causes of this perpetual motion scientific 

 men are at present by no means agreed. On whatever 

 part of the earth's surface we may be standing we are 

 something less than 4,000 miles from its centre; but the 

 deepest mine in this country has not quite been carried to 

 the depth of even half a mile, which is a mere scratch 

 in comparison. What would any one know of a cocoanut 

 if he had but scratched its shell ? And yet a scratch on a 

 cocoanut shell would be proportionately far deeper than 

 the deepest cutting we have yet made in the crust of the 

 earth. 



We may argue, indeed, that because the mercury in 

 a thermometer is found to rise on an average one degree 

 for each sixty feet that it is carried below the earth's 

 surface, therefore, at the depth of but a few miles the 

 heat must be so intense as to melt any rock with which 

 we are acquainted ; and if we can also prove that the 

 heat goes on increasing at the same rate throughout, our 



