264 THE CAUSES AVHICH 



Pernij the plain situated between the Volga and Ural^ they are 

 conformable. Thus^ according to Professor Dawson,^ we perceive 

 at the western side of Europe and eastern side of North America 

 great disturbances inaugurating the secondaiy period ; and in 

 the interior of both^ over the plains between the Volga and 

 Ural in one^ and between the Mississippi and Rocky Mountains 

 in the other, entire absence of these disturbances. During the 

 Permian period there remained in each of our Continental areas a 

 somewhat extensive inland sea. That of Western America was 

 a northward extension of the Gulf of Mexico ; that of Eastern 

 Europe a northward extension of the Euxine and Caspian. In 

 Europe the land was interrupted by considerable water areas, not 

 lakes, but inland sea basins, sometimes probably connected with 

 the open sea, sometimes isolated. In England the thick yellow 

 magnesian limestone, the outcrop of which crosses in a nearly 

 straight line Durham, Yorkshire, and Nottingham, marks the 

 edges of one great Permian sea extending far to the eastward. 

 Yet while this age of land depression and change in character 

 of the European flora lasted, when much of our nascent northern 

 continents were plunged beneath the sea, we have yet evidence 

 to show that insects resembling large Hemiptera, and cockroaches, 

 enlivened the forest stretch, where as in Saxony and Cassel they 

 remained unsubmerged, although such recovered casts are few, 

 till in the thin, clay-parted marine and fresh-water limestones 

 of the Lias age, deposited along a sinking shore line in 

 Gloucester, Warwick, and Somersetshire, or in the later litho- 

 graphic limestones formed in Bavaria during the Oolitic, or 

 those deposited at Swanage during succeeding Purbeek time, 

 we are transported to other tranquil marine marsh-lands in 

 Europe, and to river banks, fringed with their feathery equiseta 

 and crowned with landscapes of stunted palm, Cycads, and ' 

 Zamias, or to fern-carpeted woods of tree-ferns and palms. 

 Here we hear the surging summery sound of gnats, see 

 the larger dragon-flies of the family yEsehnina multiply and 

 hawk to and fro, or mount high over boughs of arbor vitse, 

 while smaller brilliant Jgrionina sun on the watery mirror and 

 Givide the prey with their sub-aqueous larvae and diving spiders. 



* The disturbance would seem to be owing to the general depression of these 

 areas. 



