INTRODUCTION. 21 



the cynocephala-stage, 1 and our Upper Division the spinosa-stage. As a rule the 

 Lower Division is much the thickest in the neighbourhood of the great escarpments, 

 whilst in the West Midlands it fails altogether on the dip in many places, so 

 that the Upper Division rests directly on the Lias in the absence of the Lower 

 Division. In Dorsetshire it is sometimes the Upper and sometimes the Lower 

 Division which is best developed in a particular quarry or limited neighbourhood, 

 but I have failed hitherto in detecting any rule bearing on the point. 



Indeed there is very considerable irregularity and uncertainty in the Inferior 

 Oolite, though less, perhaps, than in beds of Corallian age. Yet the four great 

 districts display a marvellous difference both in their Gasteropod Fauna, due, 

 perhaps, to general difference of facies, and also in their composition. Two 

 features are fairly common to the Inferior Oolite throughout England, viz. that 

 the mechanical impurities of the limestones are sandy, and that there is a great 

 tendency to ferruginous deposits, especially in the Lower Division, which is much 

 more irony than the upper one. Fawn-coloured limestones prevail, but some beds 

 of the Cotteswolds, and also of the Lincolnshire Limestone, are exceptions, being 

 very pale in colour. The so-called "Oolite Marl" of the Cotteswolds is not 

 usually a marl in a lithological sense, but simply a calcareous paste, which has 

 very likely been derived from the waste of coral. The lithology of the beds of 

 Inferior Oolite age in Yorkshire, excepting the Whitwell and Cave Oolites, is very 

 exceptional in comparison with the usual types which prevail throughout the rest 

 of England. 



But if the lithology of the beds varies greatly, the difference in their develop- 

 ment is still more extraordinary. Where the beds are thinnest there, as a rule, 

 the fossils, especially the Gasteropoda, are the most numerous. We can easily 

 understand some of the causes which have produced this result. Given, for 

 instance, a certain length of time, during which a series of beds have been under- 

 going deposition, if the material of which they are constituted is thickened by 

 large additions of mineral debris we could almost imagine a time might arrive when 

 looking for a fossil would be like looking for a needle in a bundle of hay. In 

 most places outside the Yorkshire area, instead of mineral debris the beds are 

 thickened by calcareous debris, mainly secreted by organisms in the first instance. 

 Such debris may appear in the form of comminuted shelly matter, or as a calcareons 

 paste, or as an oolitic granule. It is thus that the beds of Inferior Oolite in the 

 Cotteswold escarpments are, for the most part, thickened. More rarely, and never 

 to any important extent, the beds are thickened by an actual growth of coral in 

 situ. There are three such bands — the name " reef " is scarcely applicable — in the 

 Cotteswolds, and there can be no doubt that such belts of coral have exercised 



1 The species of Gasteropoda in the cynocephala-st&ge of the Cotteswolds are so few in number that 

 it is hardly worth while making a separate division for these. 



