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Page:Popular Science Monthly Volume 4.djvu/664

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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.
PSM V04 D664 River valley basin composition.jpg
Diagrammatic Section, showing Relative Position of 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.
PSM V04 D664 Scratched stone from the till.jpg
Scratched Stone (Black Shale), from 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 collapse, as soon as an opening is made into it.