360 ON MESSRS. BAKER AND TATe's NEW FLORA. 



In subdividing the Magnesian Limestone into five groups the 

 author has followed Professor King, and with him appears to 

 have erred in making one too many; for the third and fourth 

 groups of Mr. Tate, and the second and third of Professor King, 

 are undoubtedly but the same limestone under different aspects, 

 being termed fossiliferous or shell limestone when charged with 

 organic remains, and cellular or pseudo-breeciated when without 

 fossils. Sometimes the limestone under its fossiliferous form is 

 seen resting on the lower beds or compact limestone of Mr. Tate, 

 as at Humbleton Hill and Claxheugh ; and sometimes the lime- 

 stone under its cellular or pseudo-breeciated form is seen in 

 exactly the same position, as at the Trow Eocks, Down Hill, 

 Kyhope Pit, and Fulwell Water Works. Indeed there are more 

 localities showing the latter arrangement than the former. But 

 there is no section showing the one resting on the other ; though 

 there are sections showing the one passing from a shell limestone 

 into one that is pseudo-breeciated on the same geological horizon, 

 as at Claxheugh and Tunstall Hill. 



So far as my experience has gone the most natural method 

 of grouping the Magnesian Limestone strata of Durham is that 

 which arranges them in three subdivisions, without including 

 the Marl Slate. This method was first proposed by Mr. R, 

 Howse, in 1857. It is perhaps more strictly applicable to the 

 Magnesian Limestone of the north of the county than to that 

 of the south, where the middle portion of the series appears to 

 undergo considerable change, and the upper beds are not seen : 

 I here speak of the limestone in its range from Thrislington to 

 the Tees. 



In giving 100 feet as the thickness of the fifth group Mr. Tate 

 considerably understates it : 250 feet is nearer the average thick- 

 ness ; but it greatly exceeds this estimate on the coast between 

 Sunderland and Marsden. 



But as 300 feet as given for the third and fourth groups is 150 

 feet too much, the estimate of 600 feet as the total thickness of 

 the Magnesian Limestone is probably nearly correct. 



