222 



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



before or after deposition, and (2) derivation of the sediment 

 from non-garnetiferous rocks. In the event of garnetiferous rocks 

 occurring locally, a third explanation might he added — the sub- 

 mergence of such rocks beneath the waters in which the sediments 

 in question were being laid down. 



With regard to (1), if decomposition Avere the cause of absence 

 •or rarity, intermediate stages of partly-decomposed garnet should 

 be found. However, such examples are notably absent, and the 

 rare occurrences are those of clear fresh mineral fragments. Since 

 rocks containing abundant garnets occur locally, we are left with 



Fig. 7. — Graphical representation of typical mechanical 

 analyses of the Pliocene deposits. 



100% 



90 



to 



70- 



o 















-60 ■- 



C7i 













o 

 u 



50 . 







a 





c 









> 



40 . 







d 













2 



30 . 







■•J 





1 



20 . 



.1 



10- 



St. AgB£f •- 



40 2 



1-0 05 025 0-01 005 

 >- Grade -Sizes (diameter in millimetres) 



001 



the alternatives, either that they were submerged, or that the 

 detritus-bearing currents avoided them. In view of the fact that 

 the Pliocene materials lying on the 400-foot platform are of 

 shallow-water and even shore-line and wind-drifted character, 1 

 it is improbable that a subsidence of more than 500 feet below 

 the present level took place. Such a subsidence would be even 

 now insufficient by hundreds of feet to submerge the garnet- 

 bearing metamoi-phic aureoles and granite-masses of, for example, 

 Bodmin Moor or Dartmoor. Nor would a subsidence of 600 feet 



1 Many of the minerals are well-rounded at St. Erth, and especially so at 

 St. Agnes. 



