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



considered that the mass of Dartmoor has suffered relatively little 

 denudation, and that it was probably first exposed only in Tertiary 

 times. 



Corundum and staurolite have both been observed in Devon and 

 Cornwall as produced by the alteration of argillaceous sediments by 

 dyke-rocks l ; but, while such occurrences would be sufficient to 

 account for the rare presence of corundum in the Cretaceous (and 

 also Eocene), the quantity of staurolite in the Cretaceous deposits 

 is far too great to be explained thus. A staurolite-bearing series of 

 rnetamorphic rocks such as those in Britanny could alone account 

 for the quantity observed in the Greensands. In short, as 

 Dr. H. H. Thomas deduced in the case of the staurolite in the New 

 Red rocks of Devon, 3 we must turn to Britanny or a part of the 

 old ' Armorica ' now submerged beneath the waters of the English 

 Channel for the source of this mineral. It is noteworthy that, 

 while clear, glassy, pleochroic andalusite is very abundant, the 

 variety dusky with inclusions which is so commonly found around 

 Dartmoor is only met with occasionally. A Cornish origin for 

 some, at any rate, of the clear grains is adumbrated. 



Kyanite also is not indigenous to the West Country. Although 

 the mineral is not abundant in the Upper Greensand of Haldon 

 (being by no means so plentiful as in the Lower Greensand over 

 its whole outcrop throughout England), its presence can be 

 explained only on the assumption of currents from an extra- 

 British land-area lying on the south-west. 



The absence of biotite, hornblende, pyroxene, etc., may be 

 correlated with the rarity of garnets ; or, as seems more probable, 

 may be due to the decomposition of these minerals either before or 

 after the deposition of the Cretaceous sediments. 



Despite the proximity of granite containing orthoclase and 

 oligoclase, felspars are usually absent from the Cretaceous deposits. 

 Only calc-alkali felspars of average refractive index /x D P540, 

 referable to oligoclase, are found, and those but rarely. 



(2) The Eocene Deposits. 



Marazion. — The evidence for the supposed Eocene age of the 

 gravels, clays, and sands occurring between Marazion and Penzance 

 (probably connected with the north-and-sotith through-valley) 

 has been given in detail by Clement Reid. 3 The gravelly material 

 occurs at about 20 to 30 feet above Ordnance-datum, and consists 

 of brown-weathered flints, fragments of Greensand chert, local 

 Palaeozoic rocks, etc., embedded in sand or loam. Reid hazarded 

 the opinion that the matrix (which he considered was not the 



1 H. G. Smith, Q. J. G. S. vol. lxxii (1916-17) p. 77; J. Parkinson, ibid. 

 vol. lix (1903) p. 408. 



2 H. H. Thomas, ibid. vol. lviii (1902) p. 630. 



3 Q.J.G.S. vol. lx (1904) p. 113, and 'Land's End (Sheets 351 & 358)' 

 Mem. Geol. Surv. 1907, p. 68. 



