194 HISTORICAL PALEONTOLOGY. 



Trias. The division, therefore, between the Permian and Tri- . 

 assic rocks, and consequently between the Palaeozoic and Me- 

 sozoic series, is not founded upon any conspicuous or universal 

 physical break, but upon the difference in life which is ob- 

 served in comparing the marine animals of the Carboniferous 

 and Permian with those of the Trias. It is to be observed, how- 

 ever, that this difference can be solely due to the fact that the 

 Magnesian Limestone of the Permian series presents us with 

 only a small, and not a typical, portion of the marine deposits 

 which must have been accumulated in some area at present 

 unknown to us during the period which elapsed between the 

 formation of the great marine limestones of the Lower Carbon- 

 iferous and the open-sea and likewise calcareous sediments of 

 the Middle Trias. 



The Permian rocks exhibit their most typical features in 

 Russia and Germany, though they are very well developed in 

 parts of Britain, and they occur in North America. When 

 well developed, they exhibit three main divisions : a lower set 

 of sandstones, a middle group, generally calcareous, and an 

 upper series of sandstones, constituting respectively the Lower, 

 Middle, and Upper Permians. 



In Russia, Germany, and Britain, the Permian rocks con- 

 sist of the following members : — 



1. The Lower Permians, consisting mainly of a great series 

 of sandstones, of different colours, but usually red. The base 

 of this series is often constituted by massive breccias with 

 included fragments of the older rocks, upon which they may 

 happen to repose ; and similar breccias sometimes occur in 

 the upper portion of the series as well. The thickness of this 

 group varies a good deal, but may amount to 3000 or 4000 

 feet. 



2. The Middle Permians, consisting, in their typical de- 

 velopment, of laminated marls, or " marl-slate," surmounted 

 by beds of magnesian limestone (the ''Zechstein" of the Ger- 

 man geologists). Sometimes the limestones are degenerate or 

 wholly deficient, and the series may consist of sandy shales 

 and gypsiferous clays. The magnesian limestone, however, of 

 the Middle Permians is, as a rule, so well marked a feature 

 that it was long spoken of as the Magnesian Limestone. 



3. The Upper Pen?iia7is, consisting of a series of sandstones 

 and shales, or of red or mottled marls, often gypsiferous, and 

 sometimes including beds of hmestone. 



In North America, the Permian rocks appear to be confined 

 to the region west of the Mississippi, being especially well de- 

 veloped in Kansas. Their exact limits have not as yet been 



