CHAP. VI.] CARBONIFEROUS PERIOD. 99 



and uniform aspect, and where the plants and animals 

 which had established themselves would be likely to main- 

 tain their existence unchanged so long as the same con- 

 ditions prevailed. 



In this way it seems possible to explain the widespread 

 uniformity of Carboniferous deposits and the remarkable 

 persistence of Carboniferous forms of life, phenomena 

 which make this period a unique portion of geological time. 

 It is as if we were contemplating the close of one great 

 phase of the world's history, when the forces and causes 

 which had hitherto been operative were exhausted and 

 quiescent, when evolution was nearly at a standstill, and 

 the world was allowed a grand pause before entering on 

 the mighty changes which were to commence a new order 

 of things, and to give such a powerful impulse to the 

 development and differentiation of the earth's inhabitants. 



There now remains for consideration only the epoch of 

 the Upper Coal-measures, which exhibit certain important 

 points of difference from the measures below. The shales 

 and sandstones are generally of red, purple, brown, or 

 green tints ; the clays are often red, or mottled red and 

 green ; the coal-seams are thin and less numerous ; the 

 fossil remains are entirely freshwater, and there are occa- 

 sional beds of limestone which are not made up of organic 

 remains, but are either compact and creamy -looking when 

 freshly broken, or are porous and spongy in texture. All 

 these circumstances are indices of an important change 

 in the conditions of deposit. 



Red and mottled clays occur in other formations under 

 conditions which show them to be lacustrine deposits. 

 The colouring matter of the sandstones is ferric oxide, 

 every grain being coated with a thin pellicle of such 

 oxide, as if the colouring matter had been deposited upon 

 every grain as it came to rest at the bottom of a stagnant 

 body of water. Now such an amount of iron oxide is not 



