360 On a Ternary Oeological Classijication. [JiJy, 



out to sea to the southward, and beyond the region to which the 

 sediment was carried, and of this phenomenon we have several ex- 

 amples over the British area, the most striking of which are furnished 

 by the Carboniferous, Permian, and Lower Jui'assic formations. 



As I have on a former occasion treated this branch of the 

 subject at length, it will only be necessary here briefly to refer to 

 the illustrations afforded by two of these groups of strata.* 



Carhoniferous Series. — The calcareous central member, or Car- 

 boniferous limestone, attains in central England a development 

 nowhere else reached in Britain. Its base is never exposed, 

 although the beds are over many parts of Derbyshire thrown into 

 highly inclined positions with numerous faults and flexures. 

 Several sections measured with the utmost care by the geological 

 surveyors combine to give a thickness of more than 4000 feet for 

 this great calcareous formation in this part of the country, where it 

 is also in composition remarkably free from the intermixture of 

 shales, sandstones, or other sedimentary materials. It is in this 

 district also that its organic origin is strikingly brought to light, 

 for it abounds in corals, shells, and crinoidal remains. To the 

 region of Derbyshire we may emphatically point as a former ocean- 

 bed, where during the era of the Carboniferous limestone, a hmpid 

 sea offered full scope for the development of the marine animals 

 which were the limestone builders of that geologic period. 



If, from the Derbyshire district, we trace the range of the 

 Carboniferous limestone northwards into Scotland, we find this 

 formation gradually deteriorating both in quality and thickness. 

 In North Lancashire, Cumberland, and Yorkshire, it has nowhere 

 a thickness exceeding 2000 feet, frequently less ; and, as shown by 

 Professor Phillips, is spht up into several distinct bands by the 

 intercalation of beds of shale and sandstone with coal. This 

 deterioration is still more strikingly exhibited when we pass into 

 Scotland, for in the Carboniferous thstricts of Lanarkshire, Kenfrew- 

 shire, and the Lothians, the formation is represented by several 

 thousand feet vertical of sandstones and shales, with coal and kon- 

 stone, and a few beds of limestone only attaining a combined thick- 

 ness of about 150 feet. Here, then, we have a clear illustration of 

 the effects produced in the calcareous strata by the introduction of 

 muddy or sandy sediment. The same waters which, being free 

 from impurities in the region of central England, gave scope for the 

 maximum development of the limestone, were charged with sand 

 or clay towards the north, and in proportion as this was the case 

 interfered with the growth of the limestone, and in the Scottish 

 area well nigh prevented its formation. 



Carhoniferous Sedimentary Strata. — But not only did the 



Great 



* See my paper, " On the Relative Distribution of tlie Curboniforous Strata of 

 sat Britain." ' Jonrn. Gool. Soc. Lomlon,' vol. xviii.. p. 127 (with ninp\ 



