Vol. 6S.^ KEUPER MARLS AROtTlSrD CHARlSrWOOD. 287 



but has been bleached by water percolating along the porous seams. 

 Often ' pendents of discoloration '' extend down into the underlying 

 red beds. 



Gypsum. 



The diflPerent kinds of gypsum are described, and it is shown 

 that the original beds have been greatly affected by solution and 

 redistribution and recrystallization of the mineral. 



The Bedding. 



Continuity and lenticularity both in red and in grey beds are 

 described, as also the current-bedding in the sandstones and in 

 certain types of marl. 



The stratigraphy of the area is discussed, and it is shown 

 that certain horizons have distinctive characteristics. 



It is urged that stratigraphical sequence should be sought for 

 in other areas, in the hope of proving more certain relations or 

 differences between the red deposits in different parts of England. 



Mineral Composition of the Rocks. 



Detailed investigations on this subject were carried out. 

 By treatment with acids and by decantation processes, etc., it 

 was found that the marls consist of : 



(1) A large proportion of a certain aluminous mineral. 



(2) A smaller proportion of fine quartz-sand. 



(3) A considerable proportion of dolomite, in the form of minute rhombs. 



They are indeed well described as 'marls.' Nearly all the rocks, 

 both red and grey, contained these three constituents. 



The carbonate. — Almost every rock, red or grey, was found 

 to contain a considerable amount of carbonate. 



Samples of red marl from near the top of the series contained 

 13 per cent, of carbon dioxide, corresponding to 28 per cent, of 

 dolomite, and some contained considerably more. 



The dolomite obtained from the fine marl, when viewed with a 1/12 

 oil-immersion lens, proved to be in the form of minute rhombs, the 

 largest of which measured only about "006 mm. across, and most 

 of them were still smaller — say "0015 mm. 



The crystals appear well-shaped and sharp. Probably they were 

 formed by precipitation during accumulation of the sediment. 



In the grey rocks the proportion of dolomite is greater, and the 

 rhombs are often considerably larger, as, for instance, in the 

 quartzose-dolomite seams. 



The argillaceous mineral. (The results of the observations 

 on the argillaceous material are so unexpected that I feel some 

 diffidence in putting them forward, and only do so because it 

 may be long before I have an opportunity of carrying out' the 



