Spencee: Geology op the Halifax Haed Bed Coal. 183 



Phillips made tlie attempt. The least mimber of years that this ex» 

 tremely cautious geologist could arrive at was 5,000 years. To those 

 who have not studied the question in all its bearings this may seem a 

 very large estimate for these few feet of strata. But when we come 

 to consider that the Cephalopoda, to which class our Goniatites, 

 Nautili, and Ortlioceras belong, enjoy long life, and take into considera» 

 tion their enoni:ous numbers of all sizes, from microscopic forms to 

 others large as the pearly Nautilus, representing the spreading of the 

 family over a great part of England, and the birth, growth, and decay 

 of untold generations, I think we must come to the conclusion that 

 5,000 years is within the mark. All the species which occur in these 

 beds are found also in the millstone grit, the Yoredale and the 

 mountain limestone strata. The mountain limestone is undoubtedly 

 a deep sea deposit^ and all the animal remains found in it belonged to 

 species which lived in the sea in which that rock was deposited. 

 In the Yoredale rocks there is a succession of beds of limestone 

 having the same kind of fossils, but intercalated amongst them there 

 are other beds having a somewhat different class of fossils which 

 indicate estuary deposits. The change is still farther carried on in 

 the millstone grit rocks where the limestone deposits become both 

 rarer and thinner, and at length- in those marine deposits overlying 

 the hard bed coal, we have the last appearance of those calcareous 

 deposits which form such a conspicuous feature of the early 

 carboniferous rocks. Now the question might be reasonably asked, 

 " How is it that these marine strata are not grouped with the 

 millstone grit strata, rather then with the coal-measures, seeing that 

 they are so naturally tied together by their fossils ?" Well, in the 

 first place, the rough rock is regarded by the universal consent of 

 geologists as the uppermost member of the millstone grit rocks, this 

 conclusion having been arrived at by a thorough examination of all 

 the facts bearing on the case. And in the next place, as the late 

 Professor Pnillips long ago observed, " An examination of the 

 neighbourhood of Halifax has shown another order of phenomena, 

 and another set of shells which connect this same series with the 

 upper or true coal-measures. In the upper coal series of Northumber- 

 land, Derbyshire, and Yorkshire, are several layers of bivalve shells, 

 commonly referred to the genus Unio, from which the freshwater 

 origin of these coal deposits has been inferred. No shells of this 

 kind have ever been met with in the mountain limestone group, which 

 there is every reason to consider as of decidedly marine origin ; not 



