Page:The Zoologist, 3rd series, vol 1 (1877).djvu/176

150 general over the county, but, unlike its equivalent near Glasgow, it is a loose agglomeration of sand and hunch-backed pebbles and large stones. At Bloodyforeland it forms a cliff 110 feet in height, the matrix being a drab-coloured clay. Overlying the boulder clay in many places is a coarse gravel, highly charged with peroxide of iron; and where this is overlaid by the turf the stratum of gravel in contact with it is seen to be white instead of a reddish colour: this seems to be caused by the deoxidation of the iron by the organic matter in the turf. Bog-iron is to be found wherever the land is bleak and barren, and hundreds of tons of it are shipped annually to England, where it is used instead of lime for the purification of gas. Along the shores of Lough Swilly a light blue clay is found about forty feet above the present sea-level: it contains numerous fragments of shells, and seems to mark the old sea-beach. Bog is the surface deposit on a large portion of the county, and in some places it is as much as twenty feet deep, with trees occurring abundantly in it. In an island in the Rosses district the sea beats against a seven-feet cliff of bog, and in different places trees can be seen submerged, and the structures known as "smelting-pots," which were used some centuries ago for reducing iron, have been seen in three fathoms water. These facts point to a comparatively recent depression of the land, and this is apparently going on, although it is in no place so well marked as in the Rosses district.

The influence of plant-life in modifying geological conditions is not so often referred to nor so well understood as its influence in modifying climate. An instance of the former may be noticed. At Glassagh, in the district of Faunett, thirteen miles from Ramelton, the shores at one time were very sandy, and the kelp made from the seaweed cast in there brought only a low price; but some years ago the Earl of Leitrim planted all the bare sand above high-water mark with bent, which has held the sand together, so that now enough vegetable soil has been produced as permits of the growth of a sward composed of Viola tricolor, Anthyllis vulveraria, Erodium cicutarium, some of the coarse grasses and arenaceous mosses. The sand has been gradually disappearing, and the shore consists now of granite rock and beautiful pebbly strands, enabling the cottars to secure the weed free from sand, and to get the highest price for their produce. On the same Fannet coast the marine Algæ may be studied without much effort,