254 AGRICULTURAL CHARACTERS OF LOWER SILURIAN ROCKS. 



timber. Wherever the sandy or quartzose members of the Lower Silurian rocks rise 

 from beneath this argillaceous cover, a sudden change is observable in the vegetation, 

 a wild heath or poor woodland replacing arable lands or luxuriant woods ; but where, 

 on the contrary, the Lower Silurian rocks happen to be calcareous, their points of pro- 

 trusion from beneath the beds of shale, as at Powis Castle for example, are very fertile. 

 Again, wherever trap rocks emerge, as in the Caradoc, Wrekin, Cornden, or Breidden 

 Hills, still greater diversity in the quality of the land is the result. Some of these va- 

 rieties will be adverted to hereafter, though in the mean time I would observe that in 

 general the decomposition of the trap rocks of this region is little favourable to vegeta- 

 tion, a circumstance which I attribute to the predominance of felspar 1 . 



We thus see, that the agricultural features not only vary with each change in the 

 substrata, but are further dependent on joints, dislocations, fissures, &c. ; so that it is 

 impossible to offer an opinion applicable to a wide space of country, each district having 

 its own modifications. Among various qualifying causes, none have acted more gene- 

 rally than those operations of water, by which the surface has in many cases been strewed 

 over with gravel and fine loam, in others with boulders and coarse gravel; thus fertilizing 

 some tracts, whilst others have been impoverished. The first of these causes has most 

 frequently operated in valleys adjacent to great rivers; the latter occurs chiefly on 

 mountain sides or slopes. Such phenomena, however, will be described in separate 

 chapters towards the end of this work, and they are now alluded to merely to explain, 

 why many large districts which appear in the map as occupied by particular rocks, 

 have agricultural features very unlike those assigned to such formations 2 . 



1 This remark is liable to exceptions, particularly in the spots where the trap contains much carbonate of lime. 

 In general trap rocks decompose to good soils. 



2 I had some intention of rendering this work more interesting to botanical readers by giving (in the Ap- 

 pendix) a list of plants indigenous to the Silurian rocks, but the attempt was abandoned as unconnected with 

 geological inquiry. The mineral composition of rocks are, it is true, generally indicated by the plants upon 

 their surface ; thus it is well known that limestone bears a vegetation very different from that growing upon 

 sandstone or clay ; but it matters not whether the limestone be Silurian or chalk ; whether the sand be Caradoc 

 or New Red; the clay "Wenlock shale or lias. In short, lithological structure alone determines the vegetation, 

 whatever may be the geological age of the rock. For example, the Pyrola minor delights in the Ludlow rocks 

 both near Ludlow and at Abberley, but the same little plant, with its congener P. uniflora was first discovered 

 on the primary rocks of the Highlands of Scotland : both have since been found on the carboniferous rocks of 

 the banks of the Tees, and one of them even amid the oolites of Oxfordshire. The beech flourishes on the 

 Silurian limestones and on the chalk, while the Old Red clays of Herefordshire rival, if they do not excel, the 

 Gault and Wealden clays in sturdy oaks. 



The Aconitum Napellus (common monkshood) has, I believe, been found native only on the banks of the 

 Ledwyche brook near Ludlow (Old Red Sandstone). But after all, substratal influence is constantly obliterated 

 by overlying detritus, and it is therefore difficult (except in naked and stony ridges) to trace any connection 

 between the subsoil and the plants. 



