1869.] On a Ternary Geological Classification. 361 



antagonistic influence of the mnddy waters pervade the Carboni- 

 ferous area during the period of the central calcareous member, but 

 it held sway and gained ground throughout the succeeding stages 

 so as almost to annihilate the limestone builders during the jjeriods 

 of the millstone grit and coal-measures, or cause them to migi'ate 

 into other regions of the ocean. But the point we have specially 

 to deal with is the direction from which this sedimentary matter 

 was drifted. Now, the very accurate measurements which the 

 Carboniferous series enable us to obtain, point clearly to a northerly 

 source, for in this direction the sedimentary strata tend to attain 

 their greatest development, and this, be it observed, is the direction 

 in which the calcareous strata tend to thin away and disappear. 

 Thus if we take a series of vertical sections along a line from 

 Leicestershire and Warwickshire into North Lancashire, we find 

 the proportion of the Yoredale series, millstone grit, and coal- 

 measures as follows :* — 



Here, then, we see a steady augmentation in the thickness of the 

 sedimentary materials within a distance of about 120 miles to the 

 extent of 16,000 feet.t In Scotland and the north of England, it 

 is true, owing to a shallower sea bed, and the nearer approach to 

 the parent land, the increase is not maintained, but the northern 

 source of the sedimentary materials is no less clear. 



Now, if it be inquired, " What relationship as regards the mutual 

 development of the calcareous and sedimentary materials can there 

 be between the period of the Carboniferous limestone and that of the 

 succeeding strata ? " the reply is this : that these relations were 

 maintained, though in different proportions, throughoid all the 

 stages of a natural group of strata represerding a geological cycle 

 (such as the Carhomferous) , and were only reversed or considerably 

 changed with the introduction of a new natural group. 



Permian Series. — My illustration from this group of the 

 English strata will be stated in briefer terms than the foregoing. 



The central calcareous member of this group is clearly repre- 

 sented by the magnesian limestone of Durham, and Yorkshire and 

 Nottinghamshire along the north-east of England. Along this 

 region it attains a maximum thickness of about 600 feet, as shown 

 by Professor Sedgwick ; while the overlying sedimentary strata are 

 but feebly represented, and the lower red sandstone (Bothe-todte- 

 liegende) does not exceed 250 feet in thickness. Now, if we com- 



* These measurements are taken chiefly from sections of the Geological Survey 

 of Great Britain. 



t See my paper " On the Thickness of the Carboniferous Strata of Lancashire, 

 &c.," ' Jour. Geol. Soc. London,' vol. xviii. 



