AND TRIAS OF CENTRAL EUROPE. 399 
In the Russian area the Magnesian-limestone series constitutes 
an important formation of considerable dimensions as the represen- 
tative of marine conditions. In North Germany it is paleontolo- 
gically by far the most important member of the Dyas Group ; but 
as we proceed southwards in the country about the Riesengebirge, in 
Bohemia and in Bavaria, the Zechstein member of the Dyas gradually 
disappears, so that there the Dyassic Group is represented only by 
rocks indicative of shallow-water conditions. The same holds good, 
as is well known from the ordinary text-books, for the Permian 
series of England, the Magnesian-limestone strata becoming gra- 
dually attenuated from north to south, so that they dwindle from 
a thickness of 600 feet in the county of Durham to a series of thin- 
bedded, unfossiliferous, and more or less gritty coarse limestones, 
until about the latitude of Nottingham they disappear altogether. 
Again, in addition to the vast areal extension of the Russian Per- 
mian series, it is worthy of remark that in Courland and Lithuania 
the Zechstein type is developed *, though not over a very large area; 
and Mr. T'welvetrees + has recently shown how complete is the de- 
velopment, in the Orenburg country on the west of the Urals, of 
the Zechstein formation, as distinct from the subordinated limestone 
strata which occur there interbedded with the lower series of sand- 
stones, grits, and conglomerates, just as occasional limestone-bands 
occur in the Lower Rothliegende of Germany. 
Upon the whole therefore we seem justified in saying that in 
Permian or post-Carboniferous times anything like deep-sea conditions 
prevailed chiefly towards the northern and north-eastern portions of 
the European area, in which the rocks of the period were deposited, 
and that as we proceed southwards we find signs of the gradual dis- 
appearance of conditions favourable to their typical development. 
This fact may help to explainthe stunted and dwarfed condition of the 
Paleozoic types of life as they are presented in the fauna of the 
Magnesian-limestone or Zechstein series. It was not merely that 
the latitude of the marine area (or areas) in which these remnants 
of a Paleozoic fauna continued to exist, was rather high, but con- 
ditions as to temperature, more unfavourable to organic life (in its 
fuller development) than could be accounted for by latitude alone, 
- would seem to have prevailed. Differences in the physical geography 
of this part of the surface of the globe, and a different relative 
distribution of land and oceanic waters, may have been, and probably 
were, such as to exclude the free oceanic circulation of warmer waters 
from the south, while free access was permitted to colder currents 
from high arctic regions. As the north-eastern area of Britain was 
shut off from the midland counties, and perhaps from what is now 
Lancashire and the vale of Eden, so on a much grander scale it 
would appear that there was an extensive region about the middle 
of Germany and Bohemia which shut off the area of the post-Carbon- 
iferous rocks of Europe from free communication with more southern 
* Credner, ‘Hlemente der Geologie,’ p. 484. 
t Geol. Mag. Dee. ii. vol. ix. p. 409; also Quart. Journ. Geol. Soc. vol. xxxviii. 
p. 490. 
252 
