6 STRATIGRAPHICAL GEOLOGY 



Carboniferous and Cretaceous are used in a far wider sense than 

 their mere lithological signification. These names were originally 

 given to certain British formations, in which the most conspicuous 

 elements were respectively beds of coal and chalk ; but they have 

 since acquired a chronological meaning, and are used as systematic 

 names for the periods of time during which the rocks were formed. 

 They have also been adopted in other countries for rock-groups, 

 which occupy the same relative place in the geological succession. 



The Cretaceous System then includes, not only the Chalk, but 

 also the underlying beds of sand and clay, which contain a similar 

 assemblage of genera and species. Moreover, the Cretaceous System 

 includes not only the Chalk and its associated beds in Europe, but 

 all the rocks which were formed during the same period of the 

 earth's history. Elsewhere, therefore, as in North America, the 

 system may consist of sandstones, shales, and coal-seams without a 

 bed of real chalk in it. 



So the Carboniferous System means that system of strata of 

 which the British Coal-measures form an integral part, but which 

 includes several other rock -groups, all united by a community of 

 fossils. It is these fossils, and not the occurrence of coal-seams, 

 which are the criterion of the so-called Carboniferous period, for, 

 as above mentioned, in America there are coals of Cretaceous age. 

 In the same way, when we speak of the Carboniferous Limestone 

 Series, we do not mean a group which everywhere consists of lime- 

 stone, but one to which the great Carboniferous limestone belongs ; 

 when traced northward through England, this limestone is found 

 to split up into separate beds, which finally become subordinate to 

 thick masses of sandstone and shale. 



The Devonian System is now considered to include the great 

 series of rocks which are still called the Old Red Sandstone. 

 This is a particularly awkward and unsatisfactory name for an 

 important group of rocks. It was called old because it lay beneath 

 the Carboniferous rocks, above which came another group of 

 similar red sandstones. The latter were formerly termed New Red 

 Sandstone, but this has been divided into two distinct systems 

 which have received appropriate names, and it is to be hoped that 

 "Old Red Sandstone" also will soon be replaced by a better name. 

 As Jukes observed fifty years ago, 1 "it is avowedly a provisional 

 designation, just exactly as the names of all the great groups of 

 stratified rocks are provisional. They are temporary names adopted 

 for present purposes^ and have grown into use, and will continue to 

 be used until they art\ superseded by more appropriate terms, which 

 increasing knowledge ciily can show to be more appropriate." 

 1 Manual of geology, second edition, 1862, p. 429. 



