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THE POPULAR SCIENCE MONTHLY. 



largely restricted to the valleys, while in the Lowlands they spread in 

 broad sheets, continuous over wide tracts. The very bottom earth is 

 a strong clay or till, much like our hard-pan, which is therefore older 

 than any of the overlying deposits. Their relations are shown in the 

 accompanying figure. 



Fig. 2. 



Diagrammatic Section, showing Relative Position op Till tt x and overlying Sand and 

 Gravel Series S S x . W. = River Valley. 



This till is so tough that engineers would much rather excavate 

 the most obdurate rocks than attempt to remove it from their path. 

 Hard rocks are more or less easily assailable with gunpowder, and the 

 numerous joints and fissures by which they are traversed enable the 

 workmen to wedge them out often in considerable lumps. But till has 

 neither crack nor joint ; it will not blast, and to pick it to pieces is a 



Fig. 3. 



Scratched Stone (Black Shale), prom the Till. 



very slow and laborious process. Should streaks of sand penetrate it, 

 water will readily soak through, and large masses will then run or col 

 lapse, as soon as an opening is made into it. 



