R. A. PRTOR — BOTANICAL WOEK OF THE PAST SEASON-. 69 



"In general," indeed, "the apparent hills of Hertfordshire are not 

 ridges elevated above the general level of the surface ; but appear 

 to be such only when viewed from the valleys of the rivers, whose 

 waters have cut and furrowed deeply below the general level,"* 

 "for the wash of rain digs down where the ground is soft, and leaves 

 hills or ridges where it is hard. And as a stream cuts through a 

 hard stratum, the wash of rain is scooping out lateral valleys behind 

 it,"f as in the neighbourhood of Hitchin. " Now and then," to 

 use Mr. Geikie's words, " a valley has been cut completely across 

 the watershed, so as to draw its waters from the other side."| Thus 

 the Lea has cut a channel through the gorge of the chalk hills at 

 Luton, and now draws its head- waters from the Bedfordshire levels. 

 Thus, too, the Bulborne receives the drainage of the north-western 

 face of the Aldbury Nowers, and were it not for the disturbing 

 influence of the Grand Junction Canal, would probably encroach 

 still further upon the country now di-ained by the Thame. For it 

 must be confessed that the exact boundaries of our districts are not 

 altogether permanently fixed, but change with the slow changes 

 of the configuration of the earth's surface ; as from inequality of 

 denudation owing to the greater hardness or softness of the soil, 

 the basin of one river eats back into that of another, and gradually 

 interferes with the drainage of an entire district. The ridges, 

 then, " instead of being considered as barriers to the river, have 

 been actually formed by the river by the abstraction of intervening 

 masses," § and, except in that connexion, they cannot claim with 

 us any importance as limiting lines. 



Neither will a consideration of the geology of the county help 

 us to any convenient method of division. For as, with the excep- 

 tion of a very limited tract on our northern borders, "the Chalk 

 either forms the surface of all the county, or underlies it at no 

 great depth," || it would be necessary to fall back upon the super- 

 ficial beds, an arrangement that it would be almost impossible to 

 work out in practice. 



It is not intended, however, for a moment, to undervalue the 

 geological side of the question. M. Thurmann has pointed out, in 

 his ' Essai de Phytostatique,'^ that the distribution of plants is 

 influenced to a great extent by the mechanical properties of the 

 subjacent rocks, and to a much less degree by their chemical 

 composition. The Cretaceous rocks of England belong to his 

 dysgeogenous series : they are very permeable to fluids, and but 

 slightly absorbent, thus forming a dry shallow soil, whose chemical 

 condition however, it can hardly be doubted, is the cause of at 

 least some of the peculiarities of their flora. On the other hand, 

 a deep open soil with moistui-e at no great distance from the sur- 

 face is owing to the disintegration of eugeogenous rocks, which, as 

 they are very absorbent, and but slightly permeable, are readUy 



* Coleman, Flora Hertfordiensis, p. xxxi. 



t Greenwood, Rain and Rivers, p. 58. % Nature, vol. xiii. p. 2. 



\ Rain and Rivers, p. 59. || Coleman, Flora Hertfordiensis, p. xxx. 



H Cf . Flora of Middlesex, p. 357. 



