OF THE CARBONIFEROUS SERIES. 645 the Yoredale-beds, Millstone Grit, and Gannister beds, are (as we have seen) palseontologically connected with the Mountain Limestone, while in composition and mineral character they are more nearly related to the Upper Carboniferous beds. It was in view of their mineral character exclusively that they were originally grouped with the Coal-measures into an Upper Carboniferous series ; for it is only recently that their palaeontological and stratigraphical relations to the Middle Coal-measures have been made out. The small develop- ment of coal in these earlier stages, as compared with the later, and certain minor differences in the arrangement of the strata, serve to distinguish the two sets of beds mineralogically. To this may be added evidence of a break in the continuity of the strata at the base of the Middle Coal-measures in Lancashire *, thus affording proof that the change of the fauna was coincident with, and was due to, changes in the physical conditions of land and water. It is true, however, that tripartite classifications have in several instances been proposed ; but they are open to similar objections as in the case of the binary divisions. Sir R. I. Murchison arranges the Carboniferous series of the British Isles and Europe into three stages, the lower of which only includes the Lower Limestone Shale and Calciferous Sandstone, while the middle includes all the strata up to the Millstone Grit ; the Lower, Middle, and Upper Coal-measures are grouped in the upper division f. Mr. H. B. Woodward, in his ' Geology of England and Wales ' (1876), adopts a binary classification, placing all the beds below the Millstone Grit in the " Lower Carboniferous," and all above in the " Upper Carboniferous." The objection to this arrangement is that it unites beds which are palseontologically different, and separates those which are palseontologically similar. Sir C. Lyell, in his ' Student's Manual,' has also adopted a binary division, placing all the beds above the Carboniferous Limestone in the upper, and those below this horizon in the lower division. Professor Geikie, in his edition of Jukes's ' Manual of Geology,' judiciously avoids the difficulty of the subject by omitting to make any classification at all ; yet, for general purposes of correspondence or description, some classification appears highly desirable. Mr. Lebour also adopts a binary classification, placing the beds below the Millstone Grit, including " the Bernician" and " Tuedian" groups, in the lower division, all above this in the upper. Prof. E. Renevier, in his ' Tableau des Terrains sedimentaires '£, gives the following classification : — Ages=Etages. English equivalents, Carbonif ere superieure : Houiller Coal-measures. „ moyen: Culm Millstone Grit, „ inferieur: Condrusien Mountain Limestone, „ ,, Ursien (Lower) Limestone Sbale, Carboniferous Slate, &c.
- The section showing the break occurs near Bury, and is figured and de«
scribed by myself in the memoir on the Geology of the Country around BoJton-le-Moors, p. 6 (1862). t ' Siluria,' 4th edit. p. 404.
Lausanne. 1874.
Q. J. G, S. No. 132. 2 1:
