part 2] TERTIARY OUTLIERS OP THE WEST OE ENGLAND. 223 



(an amount actually too great, in view of the character of the 

 St. Erth mollusca) submerge many areas of garnetiferous rocks. 

 Moreover, it is only reasonable to suppose that in Pliocene times 

 a greater area of the granite would have been covered by contact- 

 rocks, for denudation had not then reduced the aureole to its present 

 extent ; and thus the area of garnetiferous rocks above sea-level 

 would have been greater still. Consequently, the problem of 

 the rarity or absence of garnets in the Cretaceous and Tertiary 

 sediments of Devon and Cornwall remains unsolved. 



(5) Deposits of Doubtful Age. 



Gravel near Kiddaford Water, Bovey. — In the course 

 •of discussing the results of the investigation detailed above, my 

 attention was drawn by Mr. H. Dewey to a patch of gravel, 

 lying about a quarter of a mile north of Riddaford Water, and 

 east of the small valley. 1 A plateau-like area occurs here at 

 about 400 to 430 feet above Ordnance-datum, and is of small 

 extent, the feature not being so well-marked as in Cornwall. 

 •On the 1-inch Geological Survey map the area is coloured as 

 Bovey Oligocene deposits, and is bounded on the west hy meta- 

 morphosed Culm. Gravel-pits expose about 10 feet of brown 

 and greyish gravelly sands. The pebbles include Culm chert, 

 killas, and granite, but no prolonged search for other rocks was 

 made. 



Examination of the finer sandy constituents shows that the 

 grey colour is due to enormous quantities of lustrous blue-black 

 schorl, which is seen under the microscope to occur in grains of all 

 sizes and tints. Beside zircon, rutile, and doubtful cassiterite, 

 andalusite is abundant. The last-named mineral is dirty with 

 numerous inclusions, possesses frayed edges, and displays faint 

 pink pleochroism. Grains as much as 0"7 mm. in diameter occur. 

 Chiastolite, with a well-marked cross, is also seen (fig. 6 b, 

 p. 220). Magnetite occurs in small grains, 0*1 mm. in diameter, 

 of irregular shape, and partly altered, usually to limonite, but 

 sometimes to more hematitic material. Ilmenite is found in 

 small amount in ii'regular grains about 0'5 mm. in diameter, 

 the grains being mostly altered to limonite and leucoxene. Many 

 kaolinized grains of calc-alkali felspars are found, but no alkali- 

 felspar. 



The mineral assemblage is thus purely local, requiring, how- 

 ever, the disintegration of aureole-rocks as well as granite for 

 its accumulation. The occurrence of magnetite is noteworthy, 

 although the mineral is not abundant. 



Grravel from above Lustleigh Cleave. — Gravelly deposits 

 occupy the bottom of a valley-like depression carved in the granite 

 of Dartmoor above Lustleigh Cleave, and situated about 2| miles 



1 H. Dewey, Q. J. G. S. vol. lxxii (1916-17) p. 63. 



