Mountains and hills are essentially caused by the major folds in the crust. They exhibit, exposed at the surface more usually than not, the rocks themselves, upon which in the same way are based the subsoils and soils of more lowland districts, whose derivation is not so obvious. A mountain or hill being based upon a physical character, though in itself independent of questions of soil, essentially tends to cause the plants growing on it to be adapted to dry - soil conditions as xero-philes, for the drainage is thereby at once modified. So that this group consists largely of xerophiles, with others that grow in moist hollows and are not xerophilous.

A natural classification divides such plants into Lithophytes, or plants growing on bare rock surfaces themselves (chiefly Cryptogams), and Chomophytes, which grow on hills, etc, or rocks with a covering of detritus or subsoil. Of the last are those that grow on the surface (Exochomophytes), and Chasmophytes (crevice plants), which grow in the crevices of rocks, vertical or horizontal. Those here considered, surface plants, are members of the Mesophytic associations driven to higher ground for one reason or another, which by virtue of their position are mainly xerophilous.

A change can be often noticed in the character of the common plants that have a wide range geographically and also altitudinally as we study them in different habitats. For in the hollows a ubiquitous plant like Dandelion is luxuriant in growth with broad leaves, and in wet meadows the leaves are still more broad, but upon the hills the foliage is much more divided and the whole plant adapted to a xerophilous habit, though not provided so definitely with those characters that stamp Xerophytes. Here a physiographical cause may be seen to act in such a way as to bring about a difference in vegetative characteristics. This is only one of the features that are induced by a retreat to a highland habitat, and it must be remembered that the glacial plants were driven to high ground on the retreat of the ice-sheets.

The porosity or degree of saturation and the structure of the rock itself greatly influence the nature of the habitat or subsoil in hilly regions, where soil is continually being worn down by rain and conveyed to the valleys. Some rocks are hard, such as sandstone and grits, and disintegrate little, so that there is a little soil formed, as in the case of syenitic or schistose rocks. While granitic rocks decompose so that alkaline constituents are set free, yet they again are less easy to break down than the chalk or even many types of limestones.

These main types continue to retain a mountainous character, or more or less the original position in which they were tilted, whereas others, such as carboniferous clays and shales, or triassic clays and liassic limestones and shales, are worn down into inconspicuous undulations of no altitude, though tilted originally, it may be, an equal amount.

The contrast between such types is well seen in the marked escarpment between the lower and middle Lias formed by the latter. The marlstone escarpment affords a habitat for many truly rupestral plants that grow on the bare rock, there being little soil formed above it. This difference between the resistance of rocks to weathering is again well seen in the alternation of soft shales and dykes of diorite in the Cambrian series near Nuneaton, where they give rise to a series of dykes and troughs which diversify the country and lend extra charm to an otherwise beautiful district.

The hills which are built up of older rocks, such as granite rocks with little soil, furnish a habitat for Ploughman's Spikenard, which is fond of stony places, growing in little or no soil, and Clary, which is found in such stations as well as in woodland situations on sand soil. Almost bare sand rock is a support for Musk Mallow, Wild Thyme, Sheep's Fescue. On sand soil on hills Cotton Thistle may be found, and on sand, on high as well as low ground, Sheep's Sorrel. Bare stony ground is a special requirement of Kidney Vetch, Rest Harrow, and Hare's Foot Trefoil. High clayey ground is suited to Field Scabious and Dropwort.

On limestone the Oak-leaved Mountain Avens grows luxuriantly, and Salad Burnet is found on hills where a lime soil is provided in which some proportion of clay occurs, which it may derive from chalk or even a calcareous sandstone. The chalk is a soft rock, which affords a soil derived from the rock itself on which a characteristic flora is to be found.

Some we include here which may almost, like some of the foregoing, be called Lithophytes and not merely surface plants, such as the View of Whernside (2414 feet) from summit of Ingleborough graceful Dyer's Weed and the spreading- Rock Rose, which closes its golden cup-like flowers as soon as the sun is obscured. Here grow Hairy Violet, too, Silky Mountain Vetch, Sainfoin, Box, and Musk Orchis. Wet hills are the favourite habitat of Yellow Balsam, Gentians, Felwort, and the fragrant Orchis.

Mountain Vegetation

Photo. L. R. J. Horn - Mountain Vegetation