Reviews — A. Strahan — Geology of Purheck ^ Wet/mouth. 169 



The author is considerably exercised as to the question on what 

 horizon the boundary should be drawn between the Jurassic and 

 Cretaceous systems. " Without attempting," says the Director- 

 General, " to decide this stratigraphical question in what is 

 intended to be a local memoir, Mr. Strahan has laid stress on 

 the classification which, though local, represents most naturally 

 the relations of the rock-groups to each other in the South of 

 England." We perceive that, whilst the base of the Purbecks 

 is entirely distinct from the underlying Portland Beds, there 

 is a difficulty in separating the former from the Wealden Series, 

 whether palseontologically or stratigraphically ; moreover, in this 

 area the Wealden folds with the Juiassics. On the other hand, 

 as fresh-water conditions were continued, no matter what the 

 period may have been, the Wealden waters had no chance of 

 receiving any accession of the neighbouring marine fauna, which 

 in other regions was already assuming a Cretaceous type. Although 

 there may be a difficulty in separating them, yet a very marked 

 difference exists between the Lower and Middle Purbecks and 

 the Wealden taken as a whole. The Purbecks consist largely 

 of calcareous beds, and the area was liable to sudden incursions 

 of the sea, which restored for a time a semi-marine fauna : the 

 Wealden, on the contrary, is very poor in calcareous matter, 

 and the deposits are more essentially fluviatile. 



Mr. Strahan expresses his views in the following table (p. 76) : — 



Formations, 



Conditions of 





Equivalents in the 



South of England. 



Deposition. 



Affinities. 



North of England. 



Chalk. ) 







( Chalk. 



Upper Greensand and ' 

 Gault. j 



Wholly marine. 



Cretaceous. 



Red Chalk and Car. 







( stone. 



Break. 



— 





Break. 



Lower Greensand. 



"Wholly marine. 



Cretaceous. 



Tealby Series and 



Falceontological break 







Spilsby Sandstone ; 



due to sudden change 







a stratigraphically 



of conditions. 







continuous series 

 of marine deposits 



( 



Estuarine marine 



Oolitic as re- 



Wealden. 



alternating with 



lates to the 



with Oolitic affini- 



' 



and passing down 



plants, rep- 



ties in the lower 



Pui-beck. 



into fresh-water 



tiles, & marine 



part. 





and terrestrial. 



molluscs. 



/ 





— 



— , 



Break. 



Falceontological break 









due to sudden change 









of conditions. 









Portlandian. I 

 Eammeridgian, etc. j 



Wholly marine. 



Oolitic. 



Kimmeridgian, etc. 



