124 Dr. R. L. Sherlock — Datum-lines in English Keuper. 



so, the depth of the gypsum below the Ehsetic might be much the 

 same as at Gotham. In Warwickshire gypsum occurs at Spernall 

 Park, 7| miles north-west of Stratford-upon-Avon, at about 150 to 

 160 feet below the Ebsetic beds, but not in workable quantities. 

 Owing to the scarcity of measured sections we cannot be certain that 

 this second horizon occurs at a constant depth below the top of the 

 Keuper — it is only probable. 



The question of the mode of origin of gypsum is a big one and 

 cannot be gone into here. It suffices that the gypsiferous deposits 

 we are dealing with are clearly, in the main, primary strata, although 

 secondary gypsum is also present, and the deposits represent some 

 special condition, occurring at a definite period over a wide area and 

 therefore of chronological value. 



The occurrence of these two belts of gypsiferous strata at 

 approximately constant depths below the Rhaetic beds points 

 strongly to the conformability of the Rhsetic to the Keuper. The 

 sharp line of demarcation between them is therefore no more than 

 the result of the waves of the open sea entering the Caspian-like 

 sea in which the Keuper was deposited and washing up the 

 Keuper mud. 



The highest beds of the Keuper are the Tea-green Marls, at one 

 time considered to be part of the Rhsetic beds. If the Rhfetic were 

 unconformable to the Keuper it would follow that the Tea-green Marls 

 occurred at varying positions in the Keuper Series, and it would be 

 highly probable that the green colour is the result of alteration of 

 red beds by secondary changes. But if, as the evidence given above 

 seems to show, the Rhsetic is conformable to the Keuper, then the 

 Tea-green Marls everywhere occur at about the same horizon, so far 

 as the upper boundary is concerned, and there is a probability that 

 their colour is original. The green ^strata, however, vary greatly in 

 thickness, for instance at Colston Bassett, Nottinghamshire,^ they 

 are only about 15 feet thick, whereas near Watchet ^ they are 115 feet 

 in thickness. Also the base is often indefinite, sometimes dying 'Out 

 downwards raggedly, sometimes ending in alternate beds of green 

 and red marl. Hence, the top being fixed, the base must occur at 

 somewhat different horizons in different places. 



One result of the wide variations in thickness of the Tea-green 

 Marls is that the higher gypsum horizon is sometimes below it, in 

 red strata, as in jSTottinghamshire, and sometimes well within the 

 green beds, as at Watchet. It appears tliat the conditions of 

 formation of green marl were neither inimical nor helpful to the 

 formation of gypsum. 



The green strata seem to indicate the coming of the open-sea 

 conditions. The occasional presence of fossils such as Ostrea hristovi, 

 Richardson, recorded by Mr. L. Richardson,^ indicates a change in 



■^ B. Smith in Geology of the Melton Mowbray District and South-East 

 Nottinghamshire (Mem. Geol. Surv.), 1909, p. 16. 



^ L. Richardson, " The Ehsetic and Contiguous Deposits of West, Mid, and 

 part of East Somerset " : Quart. Journ. Geol. Soc, vol. Ixvii, pp. 19-20, 1911. 



^ L.Richardson, " The Rhsetic and Contiguous Deposits of Glamorganshire " : 

 Quart. Journ. Geol. Soc., vol. Ixi, p. 399, 1905. 



