358 SUMMARY. 



permeable to fluids, ar: but slightly absorbent, and form a dry shallow 

 soil. The Jura has mostly these characters, which are also found in the 

 Permian, Oolite, and Cretaceous districts of England ; in Middlesex they only 

 appear in the very small portions of chalk in the N. and N.W. These rocks 

 are usually calcareous, and rain-water, in percolating through them, becomes 

 hard in consequence of the solvent action of the carbonic dioxide (carbonic 

 acid) which it contains. Eugeogenous rocks, on the contrary, are very 

 absorbent, and but slightly permeable; they readily disintegrate under 

 atmospheric influences, and form a deep open soil with moisture at no great 

 depth from the surface. Tracts in the Vosges and valley of the Saone 

 exhibit these conditions, and they are abundantly shown in England, 

 nearly the whole of Middlesex presenting them in a very characteristic 

 manner. In chemical nature they are either silicious or aluminous, and 

 their soluble constituents having generally disappeared in the process of 

 disintegration, rain-water has little solvent action upon them. 



M. Thurmann has classified soils derived from eugeogenous rocks into 

 'psammiqtces' and 'peliqms;' the latter differ from the former in having 

 their particles in a finer state of subdivision, the rocks from which they 

 are produced being softer, and therefore yielding more completely to the 

 disintegrating forces. These differences, however, result in reality from 

 chemical composition, the former including the silicious gravels and sands, 

 the latter the aluminoiis pure loams and clays.* 



The characteristic plants of dysgeogenous tracts M. Thurmann calls 

 ' xerophiles ; ' those of eugeogenous districts being termed * hygrophihs.' He 

 gives two contrasting lists, each containing 50 species typical respectively 

 of the floras of the Jura (xerophilous) and of the Vosges (hygrophilous). 

 A comparison of these lists with the flora of Grreat Britain demonstrates the 

 predominance of the hygrophilous character in the latter, and this character 

 is still more marked in the flora of Middlesex. 



Of the list of 50 typical ' xerophiles,' Great Britain possesses 23 ( = 46 

 per cent.), whilst it has as many as 41 ( = 82 per cent.) of the 50 ' hygro- 

 philes.' 



Middlesex has only 8 of the first list ( = 16 per cent.) ; — 



Orchis pyramidalis. Mercurialis perennis. 



Calamintha officinalis. Eagus sylvatica. 



Daphne Laureola. Eosa rubiginosa. 



Euphorbia amygdaloides. Orchic militaris. 



whilst it has 33 ( = 66 per cent.) of the * hygrophiles ' : — 

 Lathyrus macrorrhizus. Aira flexuosa. 



Sarothamnus scoparius. Ononis spinosa. 



Alnus glutinosa. Hypericum pulclirum. 



* Whilst the physical cliaractei-s of eugeogenous rocks almost exclusively determine 

 the character of the vegetation, this is probably not absolutely the case with dysgeogenous ; 

 the suitability of the soil they form to the growth of some at least of the plants peculiar 

 to them, is to be attributed partly to its chemical condition. 



