456 



W. A. E. USSHER ON THE CHRONOLOGICAL VALUE 



cumscription of the belt of forest-land, produced swamps and morasses 

 near the sea-board, which, with the decaying timber, were finally 

 entombed as the sea regained its old cliff-bounds. 



In inland localities the forest growth died out, leaving few traces 

 except in vegetable mould, peaty soil, or tree-trunks, as at Bolven- 

 tor, in Bodmin Moor, Cornwall, and in some of the river-valley 

 alluvia of Devon. 



During, and probably for some time previous to, the growth of the 

 forests, the broad alluvial tracts in many of the valley-bottoms 

 were being formed ; but the decreasing rainfall no longer supplying 

 an adequate quantity of water, the dwindling streams expended 

 their energies in the excavation of their present courses in their old 

 deserted beds, whilst the sea was cutting back the soft Secondary 

 cliff-line to its present bounds. 



Such, so far as I can read them, are the changes indicated by the 

 Pleistocene deposits of Devon. The life-history of that period, as 

 furnished by the caves, would be well worthy of an independent 

 perusal ; owing to the impossibility of correlating cavern -deposits 

 with those sub coelo, and to an imperfect knowledge of the subject, 

 I have not attempted any absolute correlation. 



The following Table embodies the results arrived at : — 



Period 



Deposits. 



Changes indicated. 



Correspond- 

 ing to 



If 



SI 



? Gravel patch on north 

 summit of the Blackdowns. 



? Old bone - breccia of 

 Kent's Cavern in part. 



f Probable extension of subsidence 

 J to south-western counties during 

 1 late Tertiary times. 

 [_ Approach of cold. 



I Preglacial. 



( 



■< 



1-1 



Clay with flint and chert 

 considered as portions of a 

 metamorphosed ice-bed. 



? Part of the Bovey- valley 

 clays. 



?Clay of Petrockstow in 

 part. 



? Old bone-breccia of 

 Kent's Cavern (in part) and 

 crystalline stalagmite. 



/ 



Snow accumulating on the high 

 lands. 



Formation and descent of glaciers 

 as the cold increased, finally be- 

 coming confluent and debouching 

 in an ice-field upon a district chiefly 

 composed of Cretaceous and pro- 

 bably Tertiary strata, planing its 

 surface. 



Gradual amelioration of climate, 

 and disappearance of ice. 

 ■{ Liberation of large quantities of 

 surface-water from melting ice &c. 



Thorough redistribution and re- 

 moval of then-existing Tertiary 

 outliers. 



Partial destruction of upper part 

 of old ice-bed, and removal of its 

 moraine rubbish. 



Eemoval of much Secondary 

 strata from Palaeozoic districts. 

 1 



1 



J 



> 



i 



Ph 



.2 > 



• o 



to 

 0- 



5 



/ 



I. 



Old gravel patches of 

 Colford and Orleigh Court ; 

 waterworn materials on the 

 Blackdowns and Haldon ; 

 sands flanking the Bovey 

 valley. 



? Bedistributed Tri assic 

 pebble-beds of Straightway 

 Hill &c. 



? Cave-earth of Kent's 

 Cavern (in part). 



