[6] Reach their maximum in the Trias, but pass up to newer rocks.
[7] Hitchcock, Mem. of Amer. Acad., New Series, vol. iii, p. 129, 1848.
PRIMARY OR PALÆOZOIC SERIES
CHAPTER XXII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Line of Separation between Mesozoic and Palæozoic Rocks. — Distinctness of Triassic and Permian Fossils. — Term Permian. — Thickness of calcareous and sedimentary Rocks in North of England. — Upper, Middle, and Lower Permian. — Marine Shells and Corals of the English Magnesian Limestone. — Reptiles and Fish of Permian Marl-slate. — Foot-prints of Reptiles. — Angular Breccias in Lower Permian. — Permian Rocks of the Continent. — Zechstein and Rothliegendes of Thuringia. — Permian Flora. — Its generic Affinity to the Carboniferous.
In pursuing our examination of the strata in descending order, we have next to pass from the base of the Secondary or Mesozoic to the uppermost or newest of the Primary or Palæozoic formations. As this point has been selected as a line of demarkation for one of the three great divisions of the fossiliferous series, the student might naturally expect that by aid of lithological and palæontological characters he would be able to recognise without difficulty a distinct break between the newer and older group. But so far is this from being the case in Great Britain, that nowhere have geologists found more difficulty in drawing the line of separation than between the Secondary and Primary series. The obscurity has arisen from the great resemblance in colour and mineral character of the Triassic and Permian red marls and sandstones, and the scarcity and often total absence in them of organic remains. The thickness of the strata belonging to each group amounts in some places to several thousand feet; and by dint of a careful examination of their geological position, and of those fossil, animal, and vegetable forms which are occasionally met with in some members of each series, it has at length been made clear that the older or Permian rocks are more connected with the Primary or Palæozoic than with the Secondary or Mesozoic strata already described.
The term Permian has been proposed for this group by Sir R. Murchison, from Perm, a Russian province, where it occupies an area twice the size of France, and contains a great abundance and variety of fossils, both vertebrate and invertebrate. Professor Sedgwick in 1832[[1]] described what is now recognised as the central member of this group, the Magnesian limestone, showing that it attained a thickness of 600 feet along the north-east of England, in the counties of Durham, Yorkshire, and Nottinghamshire, its lower part often passing into a fossiliferous marl-slate and resting on an inferior Red Sandstone, the equivalent of the Rothliegendes of Germany. It has since been shown that some of the Red Sandstones of newer date also belong to the Permian group; and it appears from the observations of Mr. Binney, Sir R. Murchison, Mr. Harkness, and others, that it is in the region where the limestone is most largely developed, as, for example, in the county of Durham, that the associated red sandstones or sedimentary rocks are thinnest, whereas in the country where the latter are thickest the calcareous member is reduced to thirty, or even sometimes to ten feet. It is clear, therefore, says Mr. Hull, that the sedimentary region in the north of England area has been to the westward, and the calcareous area to the eastward; and that in this group there has been a development from opposite directions of the two types of strata.
In illustration of this he has given us the following table:
THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND.
| N.W. of England | N.E. of England | |
| Feet | Feet | |
| Upper Permian (Sedimentary) | 600 | 50–100 |
| Middle Permian (Calcareous) | 10–30 | 600 |
| Lower Permian (Sedimentary) | 3000 | 100–250[[2]] |