224 PROF. BOSWELL ON THE CRETACEOUS AND [vol. lxxix, 



north-west of Lustleigh village. The deposits were described by 

 Mr. H. J. Lowe l as evidence of river-capture since Eocene times ; 

 but his conclusions were not accepted by the officers of the Geo- 

 logical Survey when they resurvej r ed the area. The gravels occur' 

 at about 750 to 800 feet above Ordnance-datum, and have been 

 turned over for stream-tin. The workings are now mainly over- 

 grown. Pebbles of granite, elvan, schorl-rock, and vein-quartz, 

 were recorded. 



Examination of the sandy matrix reveals an abundance of : 

 tourmaline- and zircon-grains (0 - 2 mm. in diameter and of good 

 crystal form) together with a few grains of rutile, a single 

 fragment of epidote, and a single grain of garnet. Kaolinized 

 alkali- felspar and magnetite are abundant, in marked contrast to. 

 the occurrences at Riddaford. Ilmenite is exceedingly plentiful : 

 angular grains of all sizes up to 0"5 mm. in longest diameter- 

 occur, a few of them displaying alteration to leucoxene. The- 

 abundance and freshness of ilmenite, the occurrence of magnetite, 

 and the absence of aureole-minerals seem to indicate that only 

 granitic debris has gone to make up the deposit. 



I V. Comparison with other British Deposits 

 of Similar Age. 



Cretaceous. 



The petrology of the Upper Greensand generally has not yet 

 been investigated in detail, but Mr. G. Macdonald Davies has 

 published notes on the petrology of the formation in Surrey. 2 

 The Haldon Greensand differs mainly from the Upper Greensand 

 farther east in the abundance of its andalusite and topaz, minerals- 

 which would naturally break down to a greater or less extent 

 during transport. I have traced both andalusite and topaz in the 

 Upper Greensand of the Blackdown Hills, and as far east as. 

 Devizes. The limit of the eastward extension of these minerals 

 has yet to be proved. The proportion of blue tourmaline in the- 

 more easterly deposits is much smaller than in the Haldon sands, 

 but the brown variety is still abundant. Garnets occur much more 

 frequently in the Upper Greensand of Surrey than in that of the 

 West Country, yielding evidence, at any rate in part, of a dif- 

 ferent source of origin for the detritus. Staurolite does not seem 

 to be so abundant in the eastern as in the western deposits. 

 Kyanite, though always abundant in the Lower Greensand, has. 

 not been recorded in the Upper Greensand of Surrey. Over the 

 Blackdown area, however, it is abundant, and occurs in larger 

 grains than in the Haldon Cretaceous, where it is also less plentiful. 

 The Blackdown Greensand would therefore appear to contain 

 material brought by currents from the south-west or south-south- 

 west, as well as those from the west-south-west. 



1 Geol. Mag. 1902, p. 397. 



2 Proc. & Trans. Croydon Nat. Hist. & Sci. Soc. 1915, p. 84. 



