Vol. 68.] KETTPEE MAELS AROTJN-D CHAENWOOD. 289 



The chemical composition indicates that the Keuper Marls 

 accumulated under conditions unfavourable to the abstraction oi 

 alkalies from silicates, a process which under present conditions 

 probably takes place readily enough. To-day these red marls form 

 rich wheat land. 



The fine quartz -sand comprises less than ]0 per cent, of 

 the normal marl. The grains are very angular and ill-assorterl. 

 The majority measure less than -05 mm. across ; but there are 

 plenty of larger grains up to, say, "2 mm. 



Lower down in the Keuper the proportion of quartz in the 

 marls is often much greater. In the Tea-Green Marl the amount 

 of quartz is very small. 



The same heavy minerals occur in the sand present in the marl 

 as in the Upper Keuper Sandstones. 



Quite unweathered fragments of plagioclase arc fairly plentiful 

 in the impervious bands of marl and quartzose dolomite, etc. 



Heavy Minerals in the Upper Keuper. 



The same heavy minerals are present in almost every rock, and 

 indicate a distant origin for the bulk of the sediment. 



The grains are intensely vt^orn, and exactly resemble heavy 

 mineral grains extracted from seolian sands for comparison. Their 

 rounding and smoothing is probably far more significant than the 

 occasional seams of spherical quartz-grains. 



Compari-?on is also made with a series of heav}' mineral grains 

 taken from Carboniferous sand. The contrast is remarkable. 



Garnets. — This is one of the most abundant minerals, and must 

 have come from afar. 



The garnets from a number of different deposits are compared, 

 and it is shown that the garnets in deposits such as the Carboni- 

 ferous have angular re-entrant shapes due to development of the 

 (1, 1, 1) cleavage ; but the garnets from the Trias and from recent 

 desert sand have an entirely different appearance, being smooth and 

 round. 



The surface exhibits a very fine granulation which, under a 

 quarter-inch objective, is seen to be due to pyramidal depressions 

 bounded by the (1, 1, 1) cleavage-plane. 



The amount of garnet in the basal beds resting upon the South 

 Leicestershire syenites is remarkably great. 



Zircon. — This mineral is very abundant, and the majority of 

 grains are small, due to smallness of the original crystals. 



There are intensely worn zircons from afar and a variable pro- 

 portion of unworn zircons derived locally from the Charnian rocks, 

 especially around the Mountsorrel Granite, which contains a large 

 amount. 



St aur elite. — The proportion of this mineral varies greatly, and 

 gometimes it is very abundant. 



