VEGETATION OF COAL PERIOD. 185 



perhaps, as the mangrove-swamps of the present day. In 

 the tropics great quantities of trees and shrubs grow down 

 not only to the very edge of the salt water, but actually in 

 it. Many of the coal-bearing strata accordingly contain 

 marine shells, and there are in Russia coal-beds which 

 alternate with limestone, showing that they must have been 

 buried in the sea. 



During the Coal Period, when the best coal and all our 

 English coal was formed, the climate greatly differed from 

 what it is now, and tropical plants flourished all over the 

 world. Vegetation was far more luxuriant and its character 

 was very different, for the English jungles were filled chiefly 

 with ferns, reeds, horsetails, club-mosses, of gigantic size, 

 but with no solid trunks ; and, indeed, there was but little 

 wood, properly so called, in these old forests. 



Another change, too, has taken place since man came 

 upon the scene, for the river-banks are now covered with 

 grass, corn, &c., which are carefully harvested, whereas, 

 when the greater part of the land was clothed with a rank 

 vegetation which was left to grow and decay as it would, 

 every little brook and stream would be loaded with dead 

 leaves, and in time of flood would tear up and carry 

 away many a tree. The state of things must, indeed, have 

 been somewhat similar to that now existing in the vast un- 

 broken forests through which flow the mighty Amazons and 

 its tributaries. 



The Rio Negro and many other large tributaries are 

 quite brown or even black, like bog- water, from the dissolved 

 vegetable matter which they contain, while the little brooks 

 are half choked with dead leaves, rotten branches, &c., and 



