342 PEOr. J. PRESTWICH ON THE RAISED 



In the above remarks I do not overlook the effects of denudation 

 by ice-action, but this preceded by a long period of time the intro- 

 duction of the Kubble-drift, vrhich was the result of agencies inde- 

 pendent of, and subsequent to any visible exhibition of ice-action. 



That the disturbance indicated by the Rubble-drift was accom- 

 panied by a change of climate is shown by the circumstance that 

 whereas during the time of the Eaised Beaches subglacial con- 

 ditions obtained, and a northern fauna survived, the deposits im- 

 mediately overlying the Eubble-drift exhibit no. glacial characters, 

 and both fauna and flora are of species living at the present day. 

 The change may, however, have begun at the time of the Sand-zone. 



At the commencement of the subsidence whence the Rubble-drift 

 originated, there is reason to suppose that the land stood about 100 

 or 120 feet higher than at present, and it is certain that after the 

 emergence it regained approximately the same level, though since 

 then it has sunk to within 10 to 30 feet of the level it had during 

 the time of the Raised Beaches. 



In conclusion, the Rubble-drift embraces : — 



1. The mass of angular detritus and rubble forming the Head 

 overlying the Raised Beaches. 



2. The beds of angular gravel and loam on hill-slopes or projected 

 into the plains at their base, and not referable either to marine, 

 fluviatile, or glacial action. 



3. The trails of gravel — not referable to river-action — in subsi- 

 diary valleys, ending at their junction with the main valley in a 

 fan-shaped spread of gravel and brick-earth. 



4. The basement-gravels of most valleys and the stanniferous 

 gravel of Cornwall. 



5. Trails in some valleys of blocks of local origin. 



6. Slight, irregular scatterings of angular debris, clay and loam, 

 or brick-earth on the sides and^at the base of hills ; these can scarcely 

 be classified, although of frequent occurrence. 



7. The ossiferous breccias in fissures of the limestones of Devon- 

 shire and South Wales. 



That the submergence is of very late geological date is evident 

 from the fact that no beds intervene between the Rubble-drift and 

 the recent alluvium of our rivers, notwithstanding that the con- 

 ditions of land and sea were, in many instances, favourable for their 

 development. 



Nor, seeing how closely the Raised Beaches follow the present 

 lines of cliff, is it possible to conceive that any great length of time 

 can have elapsed since the formation of the old line of cliffs and 

 beaches. Still, it must not be forgotten that in the interval the 

 land stood for a time some 120 feet higher than at present, so that 

 the first erosion of the land, after the elevation of the beaches, took 

 place at a lowel level than that on which the cutting back of the 

 cliff's has since proceeded. But, allowing for that, the line of 

 20 fathoms (120 feet) is everywhere within so short a distance of 

 the shore that, had the whole space between that line and the shore 



