1857.] 



GEOLOGICAL. RELATIONS OF PLANTS. 



175 



6. In proportion as we advance from an austral to a boreal, 

 and from a continental to an insular climate, the proportion in 

 number which the restricted [i.e. dysgeogenous and eugeogenous) 

 bear to the ubiquitous species lessens, principally through reason 

 of many of the eugeogenous species being able, under more hu- 

 mid conditions of climate, to adapt themselves also to dysgeoge- 

 nous situations. 



II. 77*6 field of study lithologically viewed. 



For phytostatic purposes the field of study may be conveni- 

 ently considered as subdivided into six lithological zones, viz. : — 



1. Psammo-eugeogenous ; including the endogenous and meta- 

 morphic rocks of the Scotch Highlands, and sedimentary strata 

 that surround them. 



2. Mixed ; including the Silurian, Devonian, and accompany- 

 ing strata of the southern part of Scotland and of Wales, and 

 the west of England. 



3. Primary dysgeogenous ; including the carboniferous forma- 

 tions of the Penine chain and Permian limestones, enclosing the 

 coal-pits of Durham and West Yorkshire. 



4. Eugeogenous ; including the new red sandstone strata of 

 the centre of England. 



5. Secondary dysgeogenous ; including the liassic, oolitic, weal- 

 den, and cretaceous strata of the south-eastern half of England. 



6. Subeugeogenous ; including the fen-country and London 

 and Hampshire tertiary basins. 



These are occasionally interrupted by intervals of less typical 

 or exceptional nature. 



