Page:A Treatise on Geology, volume 1.djvu/193

 CHAP. VI. and grits below ledges of limestone, over which the stream flings itself, in a free and lofty leap, into a dark and precipitous glen. (Hardrow force, in Wensley Dale; Ashgill force, in Aldstone Moor.)



Another thing worthy of notice in the scenery of the limestone districts in the north of England, especially Derbyshire, is the difference of herbage on the millstone grit, limestone shale, and limestone. On the latter (l), a fine green turf—on the shale (s), bluish green sedgy pastures—on the grit rocks (m), brown or purple heath, enable a geologist to mark out the leading features of districts with great facility, suggest to the botanist many interesting inquiries, and demonstrate to the agriculturist the dependence of the quality of soils on the rocks which they cover.

The surface of country occupied by the rocks of the carboniferous system is proportionably much larger in the British islands than in other parts of the globe. In Ireland the greater part of the plains and broadly undulated interior consists of the mountain limestone, in places covered by coal measures, and in other parts supported by the old red sandstone. In fact, excluding the parts previously described as gneiss, mica schisf, clay slate, and grauwacke slate, and a large tract of later strata (red sandstone, green sand, chalk, &c., capped by basalt) extending from Lough Neagh to Lough Foyle, and to the sea-coast of Antrim, much of the rest of Ireland belongs to the carboniferous system. But the quantity of coal yielded by the coal fields about