1862.] HULL — CARBONIFEKOTJS STRATA. 135 



We may consider a group of rocks as primarily representing three 

 periods : the first of movement, the second of quiescence, the third 

 of movement again. We have already seen that the formation of 

 calcareous strata depends mainly on the absence of sandy or muddy 

 matter in the sea, which we may believe would be most likely to 

 occur duiing a long period of repose from oscillations of land, as 

 every movement of that kind would tend to increase the quantity of 

 sediment poured into the sea. Hence we have the following paral- 

 lelism in the three stages : — 



Upper stage ^presenting movement resulting in sedimentary strata. 

 Middle stage „ quiescence „ calcareous strata. 



Lower stage „ movement „ sedimentary strata. 



The movements of the introductory stage have generally been 

 more powerful than those of the closing stage ; and thus, while we 

 seldom or never find a Geological Epoch introduced with the forma- 

 tion of limestones, we sometimes find limestones maintaining their 

 position to the close, as in the case of the Clymenia-limestone of the 

 Upper Devonian of the Rhine, and in the tipper Silurian Group of 

 North America. The earliest stage is generally formed of sandstones 

 and conglomerates, representing those physical changes which intro- 

 duced the new epoch. 



(i.) Iso-diametric Lines. — We may regard aU formations composed 

 of sedimentary materials as exhibiting in cross-section a figure in- 

 cluded by the arc of a curve and its chord (fig. 4). The end of the 

 figure which tapers the more rapidly will represent the shore, the 

 other the sea-deep ; and the form of the figure wiU be variously mo- 

 dified by circumstances. The thickest or deepest portion wHl be not 

 at the centre, but between the centre and the shore. 



Fig. 4. — Diagram representing the Primary Section of a Formation. 



Shore. 



e d c Bea-surface. The Deep. 



Now, if we divide this figure by a series of lines (A, B, C, &c., and 

 a, b, c, &c.), each decreasing by the same amount, and trace these 

 lines over the region occupied by the formation, each will be a 

 sort of stratigraphical contour ; but, instead of representing equal 

 altitudes, will show equal thicknesses. As such, these lines should 

 properly be called iso-picthic ; but this word is so difiicult of pro- 

 nunciation that I prefer the term iso-diametric, or simply isometric. 

 Such lines are not intended to show the present or actual thickness 

 of the strata, which may have been in part denuded, but the original 

 development before denudation, and may thus be traced over areas 

 where the whole has been swept away. In tracing out such lines, 

 it is necessary to make accurate comparisons of sections scattered 

 over the entire area, and of the original thickness of the strata which 



