286 THE HISTORY OF 



and clay, and gradually decomposed into a substance 

 resembling peat, and which is forming a layer of coal for 

 future ages. In a similar way, did the great streams of 

 this period float into masses and deposit countless trunks, 

 especially of Conifers, in estuaries and fresh- water basins, 

 which perhaps were brought, through later depressions, yet 

 deeper beneath the surface of the ocean, covered with 

 deposits of sand, lime or clay, and then upheaved again. 

 These are the layers of brown coal, often so very extensive, 

 which are always a valuable gift of the soil, but still a 

 poor substitute for the true coal withheld. 



The new elevation of some important rock systems, 

 in particular those of the Himalayas, appears to have, in 

 great part, put an end to all this life, through the alteration 

 of the level of the ocean which it caused, and since, at the 

 same time, the earth attained the limit of its possible 

 cooling, it thus appears to have produced the present 

 structure of the solid land and its organisms. 



All the succeeding changes which now occurred, eleva- 

 tions and depressions of the land, were dependant merely 

 on local actions of subordinate importance. 



We may abridge the preceding sketch into the following 

 main points. The gradual development of the vegetable 

 world commenced with the simplest plants, and advanced 

 gradually through the succeeding periods to the most 

 perfect plants of our existing vegetation. The structures 

 of the first period correspond to a tropical climate contem- 

 poraneously extended all over the globe, which passed by 

 degrees from the poles towards the equator into the present 

 climatal conditions ; and keeping pace with this, appeared 

 another change, for the plants of the oldest period which 

 seem to have been equally distributed over the whole 

 earth, by degrees were confined into regions of dis- 



