The Jurassic Rocks of Britain. 417 
north, whilst that other great divider of the Estuarine series, the 
Gray or Scarborough Limestone, behaves in a manner the reverse of 
this. In the Howardians the equivalents of most of the marine 
beds, except the Cornbrash, may be recognized, but under different 
conditions of development. 
A glance at plate iv. which illustrates the variations in thickness 
and mineral character of the Lower Oolites in Yorkshire, is very 
instructive in this connection. One cannot help noting that the 30 
or 40 feet of “‘ Estuarines” and Oolite at Kirkby Underdale, where 
deposit is at a minimum, practically mark the southern margin of 
what we may regard as the Yorkshire basin of deposition. From 
this point we perceive a somewhat irregular increase in a northerly 
direction until at least 700 feet of beds are recorded, exclusive of 
the Blea Wyke Beds, which would add some 70 feet more to the 
score of the Lower Oolites. It is almost needless to add that the 
details are well worked out in the text, and present many features 
of great interest alike to those who are acquainted with the district 
and to those desirous of becoming so. 
The Middle Oolites, from the Kellaways Rock to the uppermost 
Corallian inclusive, are disposed of in three chapters (98 pages). 
Of course it is well known to students of Jurassic geology that 
“ Kellaways Rock” and “Oxford Clay” in Yorkshire mean some- 
thing very different from what these names imply in the south of 
England. This discrepancy arises from the necessity of adopting 
a lithological rather than a paleontological basis of subdivision in 
mapping the country. A glance at table v. of this memoir will 
show what a very subordinate part the ‘Oxford Clay” plays in 
many portions of the interior of the county. Throughout the 
Tabular range, in fact, until we approach the coast, it is nothing 
more than a few feet of sandy clay between two powerful arenaceous 
formations, the Kellaways Rock below, and the Lower Calcareous 
Grit above. Here also, as in the case of the Lower Oolites, we 
perceive a minimum of deposition as the southern margin of the 
Yorkshire Basin is approached at Acklam. 
The Corallian Series, which constitutes the upper member of the 
Middle Oolites, is exceptionally well developed in Yorkshire. Mr. 
Strangways recognizes the two palzontological divisions of this 
series, which had been indicated by Messrs Blake and Hudleston, 
in their paper on the “Oorallian Rocks of England” (Q.J.G.S. 
vol. xxxii.). The lower division, which may be regarded as Upper 
Oxfordian, is the zone of Am. Perarmatus and includes the Lower 
Calcareous Grit. the Passage Beds, and the great series known as 
the Lower Limestone, although the top shell-bed of that formation 
gives indications of the change in the fauna which obtains in the 
higher beds. This may generally be distinguished from the Upper 
Limestone (Coralline Oolite and Coral Rag) by the prevalence of 
Gervillia instead of Chemnitzia (Pseudomelania). The two lime- 
Stones are not easy to separate in the Howardian Hills, although at 
Appleton the fauna of the Passage Beds is clearly to be recognized. 
The solidarity of the two limestones in the Howardians is certainly in 
DECADE III.—VOL. X.—NO. IX. 27 
