166 THE STOEY OF THE EARTH AND MAN. 



thus have two causes, either of which seems sufficient 

 to produce the effect. 



Viewed in this way, the great disturbances at the 

 close of the Palgeozoic period constitute one of the 

 most instructive examples in the whole history of the 

 earth of that process of collapse to which the crust 

 was subject after long intervals, and of which no 

 equally great instance occurs except at the close of 

 the Laurentian and the close of the Mesozoic. The 

 mineral peculiarities of the Permian are also accounted 

 for by the above considerations. Let us now notice 

 some of these. In nearly all parts of the world the 

 Permian presents thick beds of red sandstone and 

 conglomerate as marked ingredients. These, as we 

 have already seen, are indications of rapid deposition 

 accompanying changes of level. In the Permian, as 

 elsewhere, these beds are accompanied by volcanic 

 rocks, indicating the subterranean causes of the dis- 

 turbances. Again, these rocks are chiefly abundant in 

 those regions, like Western Europe, where the physical 

 changes were at a maximum. Another remarkable 

 feature of the Permian rocks is the occurrence of great 

 beds of magnesian limestone, or dolomite. In England, 

 the thick yellow magnesian limestone, the outcrop of 

 which crosses in nearly a straight line through Dur- 

 ham, Yorkshire, and Nottingham, marks the edge o! 

 a great Permian sea extending far to the eastward. 

 In the marls and sandstones of the Permian period 

 there is also much gypsum. Now, chemistry shows us 

 that magnesian limestones and gypsums are likely to 



