202 DE RANGE : UNDERGROUND WATER-SUPPLY AND RIVER FLOODS. 
and an absorption of probably another third in areas containing porous 
rocks, which quantity is partly recovered by natural springs, and in 
districts of artesian wells, the loss from this cause is inappreciable. 
From these considerations it would appear that at least one-third of 
the annual rainfall was available for running waters, to be used in the 
maintenance of life, and utilized for the purposes of trade, but this is 
not the case, a large amount has to be ''written off," for "depreciation" 
caused by floods. It is often stated that England and Wales offers 
an epitome of the Geology of Europe, it is equally true that the large 
County of Yorkshire offers an epitome of the Geology of South Britain, 
with one important exception as regards its hydrogeological condition, 
the absence of any extensi ve area, of wholly impermeable strata, like 
the Silurian and Cambrian rocks of Wales and the Englisli Lake 
District, where the whole of the rainfall is either run off in floods or 
caught and arrested in lakes, some of which are rock-basins, others 
produced by old glacier moraine damj^, others by dams of masonry, 
like the Vyrnwy Lake of the Liverpool Corporation Waterworks. In 
these districts the water is either caught or lost. The dry weather 
flow of the streams averaging only one-fourth to three-fourths of a 
cubic foot per second per 1000 acres drained, due to water arrested 
by peat-mosses in the high uplands, which slowly give off their 
moisture like a sponge, while the volume of these streams in periods 
of floods is often 500 cubic feet per second per 1000 acres drained. 
The whole of the Yorkshire Rocks, of all geological ages, form a 
series of alternating porous permeable material, alternating with 
impermeable shales and clays, which, succeeding each other in regular 
order, dip with the slope of the ground from the western water-shed 
towards the sea. Thus waters absorbed at high levels are often 
returned to the streams by natural springs at lower levels, and could 
be further made available by artesian wells, carried through the 
impermeable material to the water-charged zone beneath . 
The older Carboniferous Rocks of North-western Yorkshire, called 
the Yoredale series by Professor Phillips, after the Dale of the Yore 
or Ure, constitute the area of waterfalls of South Britain, alternations 
of shale, limestone, and sandstone, repeat each other, and water is 
absorbed by " swallow holes," from the peat-covered uplands, which let 
