:228 PROF. BOSWELL ON THE CRETACEOUS AND [vol. lxxix, 



Cretaceous. 



(9) The Upper Greensand deposits are marine and glauconitic. 

 Their mechanical composition indicates that they were 

 deposited near land, and were in course of heing sorted, 

 but the process had not been completed. In view of the 

 occurrence of abundant small, clear, andalusite-grains, 

 and much kaolin and tourmaline (but very little chiasto- 

 lite), their origin may be sought in Cornwall rather than 

 in Dartmoor. The deposits offer no evidence that the 

 Dartmoor granite had been exposed by denudation. 

 .Rivers and currents brought heavy minerals, such as 

 kyanite and staurolite, into the Cretaceous basin from 

 the south-west. 



Eocene. 



i(10) The Eocene deposits represent the breaking down of an 

 enormous quantity of Chalk, as well as of much Creta- 

 ceous and granitic material and of aureolar rocks and 

 minerals. They are more clayey in character and more 

 poorly graded than the Cretaceous deposits. The me- 

 chanical composition and colour-mottling support the 

 view that they are river-deposits. The abundance, 

 angularity and large size of the topaz, and the frequent 

 occurrence of dusky andalusite, point to a source of 

 origin not far distant, such as might be provided by the 

 Dartmoor granite. The abundance of the topaz, a vein 

 and marginal mineral belonging to a late stage of crystal- 

 lization of the granitic magma, suggests that the Dart- 

 moor intrusion was being newly exposed. The Greensand 

 was also laid under contribution, but there is no evidence 

 that far-travelled minerals arrived directly in the Eocene 

 deposits. The drainage-area that yielded the sediments 

 was much more restricted than in Cretaceous times. 



The petrography of the Marazion deposits provides no 

 clue to their age. The constituents of the outlier near 

 Buckland Brewer are similar to those of the Haldon 

 Eocene. The presence of flints links the two deposits, 

 and is in contrast with its absence in the Pliocene deposits 

 of Cornwall. 



Oligocene. 



i ( 11) The restriction of drainage-area noted in the Eocene was 

 maintained in the Oligocene, resulting in two isolated 

 lake-basins in North and South Devon respectively. The 

 petro graphical character of the rocks in both basins is 

 similar, the deposits being poorly graded and obviously 

 a result of the sudden arrest of streams of detritus- 

 laden water, bearing clayey and coarser material from 

 Dartmoor. The minerals are all of local origin, and the 

 presence of chiastolite (characteristic of the aureole) is 

 noteworthy. 



