part 2] TERTIARY OUTLIERS OF THE WEST OF ENGLAND. 219 



Petrockstow Basin. — These deposits occur in a basin-like 

 area about 300 feet above Ordnance-datum, round which the 

 River Torridge at present follows a curious course. The basin 

 has been proved to a depth of 100 feet, but it has not been 

 bottomed. 



The deposits are similar in general characters to those at Bovey, 

 but clays are more prevalent. 1 Remarkable superfine sands also 

 occur, and the lignite impregnates the clay to a greater extent 

 than in the Bovey Basin, and is not found in such well-defined 

 beds. The two basins were clearly filled in a similar manner 



100% 



Fig. 5. — Graphical representation of typical mechanical 

 analyses of the Oligocene deposits. 



•40 20 10 05 0-25 0-01 0-05 0-01 



— ^*- Grade -Sizes (diameter in millimetres) 



with detritus resulting from the rapid denudation of granite- 

 country. Although the Petrockstow basin rests upon Devonian 

 rocks, and is farther from Dartmoor, its materials are not 

 essentially different from those of Bovey. They are equally 

 poorly graded, as the graphical representation of mechanical 

 .analyses in fig. 5 indicates. 



The minerals identified in the sands and clays are included in 

 the table on p. 226. 



The heavy residues are both small in amount (less than O'Ol per 



1 Chemical analyses of three of these clays are given in the ' Summary of 

 Progress ' of the Geological Survey for 1909 (1910) p. 59 ; see also W. A. E. 

 Ussher, Q. J. G-. S. vol. xxxiv (1878) p. 457, and Trans. Devon. Assoc, vol. xi 

 ;(1879) p. 422. 



