4 FLOWERS OF THE HILLS AND DRY PLACES 



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 con- 

 veyed 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 undu- 

 lations of no altitude, though tilted originally, it may be, an equal 

 amount. 



The contrast between such types is well seen in the marked escarp- 

 ment 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 fore- 

 going, be called Lithophytes and not merely surface plants, such as the 



