THE FIRE-HARDENED ROCKS 



mentary strata, although they were composed of soft 

 materials to begin with, have become very hard since, 

 in some cases owing to the enormous pressure of the 

 accumulated deposits above them, in other cases because 

 of chemical action. In a few cases they have become 

 hardened not so much by losing their water, as by direct 

 heat. But the hardest of them is not so hard as another 

 class of rocks with which we are all acquainted rocks 

 like granite, or quartz, or basalt. And it will be evident 

 to any one who thinks about the subject for a moment 

 that no amount of pressure would make a rock as hard 

 as a diamond. Now how have these rocks been made? 

 The answer is that they have been made in some interior 

 furnace of heat deep down in the earth. Sometimes they 

 have boiled up, and we can trace them bursting their way 

 through the sedimentary strata above them. We do not 

 know very much about the furnaces or cauldrons whence 

 they have come; in fact, we know very little about the 

 depths of the earth. The deepest mine-shaft known is 

 near Lake Superior, and is only 5000 feet in depth. In 

 Silesia a bore-hole has been made by the Austrian 

 Government of a mile and a quarter in depth. It would 

 be by no means an easy task to sink a great boring. 

 The Hon. Charles Parsons has described some of the 

 difficulties. 



The shaft would have to be sunk in a neighbourhood 

 where it would not be likely to encounter water on its 

 way down, because otherwise there would be the necessity 

 of pumping operations. In order to be of value for 

 purposes of observation, the shaft would be of the size 



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