OF THE PLEISTOCENE DEPOSITS OF DEVON. 453 



forward a very hypothetical explanation of the production of the 

 clay with flints. Mr. Whitaker's theory * of simple decomposition of 

 the Chalk seems to me to be insufficient to explain the fracture of 

 the flints to depths of 30 feet below the surface, and to account for 

 the lie of the substance on the Greensand, and for the absence of 

 the clay in Cretaceous districts not forming parts of the tableland 

 area. My solution is only a tentative hypothesis, for which I have 

 not got sufficient corroboration to be a warm advocate. 



The gravels of Orleigh Court and Colford, resulting in part from 

 the redistribution of Cretaceous materials, were deposited during 

 the earlier stages of the excavation of the present drainage-system. 

 The sands flanking the Bovey valley (e), and conforming to its slopes, 

 form another Posttertiary problem. Owing to the high incline on 

 which they lie, it does not appear probable that the adjacent country 

 during their deposition presented its present relative levels, but that 

 movements of upheaval and depression of local irregularity, aided, 

 perhaps, by such dislocations as the fault pointed out by Mr. Pen- 

 gelly in the lignitiferous beds of Bovey (producing a downthrow of 

 100 feet), may have materially altered the contour of the ground. 



The pebble-gravel on Straightway Hill (£), if not merely the result 

 of the decomposition of the subjacent Triassic pebble-bed, appears to 

 have been formed after the breaking-up of the Cretaceous plain and 

 the initiation of the larger valleys, but before the lines of present 

 drainage had been elaborated. How far other gravelly beds of the 

 Trias may have been thus redistributed it is impossible to say. 



"We now come to the river-gravel period, after the selection of 

 the present lines of drainage, but before all the valleys had been 

 excavated to their present depth. 



in speaking of the excavation of the valleys I would not be un- 

 derstood to mean that that operation was absolutely synchronous 

 in different parts of the area, nor that it was uniform in progress : 

 some valleys may have been initiated in very early Pleistocene 

 times; others, in Palaeozoic districts, may have been of Triassic 

 or Cretaceous age, and merely reexcavated. The relative magni- 

 tudes and velocities of the rivers, and the constitution of the rocks 

 through which they flowed, would always exercise a material influ- 

 ence on the production of the contour. From the beginning of this 

 gravel-period the process of fluviatile erosion seems to have gone on 

 uninterruptedly. The rivers would appear to have exhibited much 

 greater force and volume than those of the present day ; for their 

 gravels exist in a fragmentary condition, capping hills or mixed with 

 soils on slopes parallel with the present lines of drainage, but sel- 

 dom exhibiting distinct terrace-like characters, which they would 

 have done had the water- channels been as impotent to carry off 

 large masses of material as those of the present day. 



* Memoir illustrating Sheet 13 of the Geological-Survev Map, pp. 54, 55; 

 also Sheet 7, pp. 64, 65. 



