Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/55

Rh oppositifolium, , and  grow inter-mixed, and in the swamp by the stream side are ,, , , and , , and yellowish-green soft glossy tufts of. The drier and more grassy parts of the hill-side yield ,, , , , and abundance of . The highest ridge is formed of large rounded crags of a coarser-grained rock than the scattered masses below, often variegated with lichens, obscure-crusted black-pointed Lecidece, interspersed sometimes with the brighter hues of ', wide-spreading silvery-grey patches of ' and ', and darker ones of the omphalodes variety of the latter, coral-like cushions of Sphaerophoron, and the ink-coloured leathery thallus of '. The crags are too dry to produce many mosses, the principal kinds which occur being ' and ' upon their exposed faces, and in the shady crevices ' and '. In the sheltered hollows ' may sometimes be seen and Luzula syhatica amongst the scattered ling bushes, and casually ' and  under the shade of the heather.

This, then, is the kind of vegetation which, with extremely little variety, covers a large proportion of the uncultivated and especially the upland parts of our two counties. The most distinctly marked influence upon the topography of the species produced by the distribution of the subjacent rocks, is the more or less absolute restriction of a certain number to the limestone tracts. Out of our eight hundred and forty-four native species there are forty-four which show clearly this lithological restriction. In North Yorkshire, as might be expected from the greater predominance of the calcareous element in the strata, the number of these Xerophilous, or dry-loving species, as Thurmann calls them, is considerably greater. There are three tracts of limestone in the North Riding, the Carboniferous, Permian, and Oolitic, which are surrounded by, and separated from, one another by wide tracts underlaid by arenaceous and argillaceous beds, so that it is a district exceedingly well -adapted for showing