W. H. Hudleston—The Yorkshire Oolite. 245 
The first question we should ask ourselves is, what are the 
relations of the Corallian Gasteropoda to the general fauna of the 
formation ? and secondly, but more especially, what are its relations 
as a group, to the fauna of analogous beds above and below? The . 
first part of the question is easily disposed of, as a reference to the 
table of fossils will show the relative proportions of Gasteropods 
to Lamellibranchs, etc., whilst it may also be noted how rare the 
former are in the lower beds, and how, as the calcareous deposits 
begin to predominate, the Gasteropods increase in numbers both 
individually and specifically. It is worthy of remark that an 
abundance of Gasteropods is not always the accompaniment of a 
coralline facies, as is proved by their extreme rarity in the Lower 
Coral Rag of Hackness. 
The second part of the question opens up an interesting field for 
speculation as to whence the “species” were derived. To those 
who know the present insulated position of the Yorkshire Corallian 
area this is a subject of considerable difficulty. This insulation is 
both lateral and vertical. The lateral insulation on the west and 
north is to a certain extent, though we cannot say how far, due to 
denudation along the outcrops. On the south the insulation must 
be ascribed, in our present state of knowledge, to non-deposition, 
whilst on the §.E. the formation passes under higher beds, and is 
then lost to view, though there are some reasons for thinking that 
the formation gets thinner in this direction. 
Jn a vertical direction its insulation is still more complete. Thus 
when we regard the formations which immediately preceded those 
beds in time within the area, we find a considerable thickness of 
clay, with few or no univalves. This clay gradually becomes more 
sandy, and finally passes, in most instances without any marked 
petrological break, into the Lower Calcareous Grit, which may be 
regarded as the foundation of the Corallian beds, and as the connecting 
link with the Oxfordian, of which a considerable portion of these beds 
in Yorkshire may be considered as an exceptional development. A 
reference to the table of fossils will show that there are but few 
Gasteropods in the Lr. Cale. Grit, even in the Lower Corallian Lime- 
stones, which are included in the same column. ‘This may be capable 
of two explanations, firstly, that the conditions were not favourable ; 
secondly, that, having been driven away by “Oxford Clay” con- 
ditions, they had not had time to re-occupy the ground. This latter 
view seems to me by far the most probable, though it may well be 
that the conditions became more favourable for Gasteropods as the 
Corallian accumulations increased. 
Most geologists are aware that the so-called Yorkshire Oolites 
consist of great masses of sandy and argillaceous sediment, in part 
estuarine, and, as a rule, poorly furnished with fossils; but that on 
certain horizons there are rich shell-beds of limited vertical extent, 
which give us passing glimpses, as it were, of the fauna of the 
period. Such are the Dogger, Millepore Bed, Scarborough Lime- 
stone, Cornbrash, and shell-bed at the top of the Kelloway Rock. 
It will therefore be an interesting subject for inquiry as to the 
