THE CARBONIFEROUS-LIMESTONE SERIES AT CUfTON. 187 



are the beds referred to by Sir H. de la Beche as " Upper Mixture 

 of Sandstones, Marls, and Limestones." 



Hitherto the limestones known as the " Black Rock " have been 

 regarded as the base of the Middle Series of Limestones, and it is 

 with reluctance that I propose to alter the existing arrangement. 

 I am, however, compelled to remove the " Black Eock " from the 

 Middle Limestones and include it with the Lower Limestone Shales 

 under the term of Lower Limestones. My reason for doing so is 

 that the Black- Rock beds have no claim to be a part of the Middle 

 Limestones, on the ground that the organisms, chiefly Encrinites, 

 which contribute to the structure of the rock are more allied to 

 those in the Lower Limestone Shales than to the fossils which occur 

 in the Middle Limestones. At the top of the Black-Rock series 

 there is clear evidence of an alteration of conditions under which 

 the limestone was being deposited ; this is indicated by a thickness 

 of about 100 feet of oolitic limestone which closed what may be 

 termed the "local Encrinite Period," and preceded conditions ex- 

 tending over a length of time difficult to estimate, during which a 

 great thickness of strata was deposited in which crinoidal remains 

 are few in number and small in size. 



Professor Hull has certainly underestimated the thickness of the 

 Lower Limestone Shales ; and as my measurement corresponds closely 

 with that of Sir H. de la Beche, I accept his statement of 500 

 feet. 



In this paper, then, I shall refer to the Carboniferous-Limestone 

 series at Clifton in the following divisions and estimated thicknesses. 



■•o 



feet. 



Stage C. Upper Limestones 100 



. Stage B. Middle Limestones 1620 



Stage A. Lower Limestones, including (i.) Black 

 Rock, 490 feet, (ii.) Lower Limestone Shales 

 500 feet 990 



Total 2710 



It will be observed that, at Stage B, I have put down ^' Middle 

 Limestones " in the place of " Carboniferous Limestone." I do this 

 because the whole should be classed as Carboniferous Limestone, an 

 opinion apparently held by Sir H. de la Beche, for he speaks of that 

 portion of the series marked by Stage B as " the central portion of 

 the series. 



The LiMESTONE-roEMiNG Organisms. 



Lower Limestones. 



(i.) Loiver Limestone Shales. — These beds rest conformably on 

 sandy strata which in turn lie conformably on the Old Red Con- 

 glomerate. The lowest beds of true limestone are made up largely 

 of Ostracoda and Corals, among which latter Monticulipora tumicla, 

 PhiU., is conspicuous. Small Crinoids are numerous in some beds, 



o2 



