368 



SUPPLEMENT TO CHAPTER VIIL 



ON THE GREAT COAL FORMATION. 



Beside what has been already stated respecting the freshwater 

 origin of the coal strata, I wish the reader to bear in mind, that the 

 marine or freshwater character of formations, must be determined by 

 the general assemblage of the organic remains, and not by a few in- 

 dividuals, or a few species of shells, which differ from the general 

 character of the fossils ; particularly as we now know, that several 

 species supposed to be marine, are capable of living in fresh water. 



In the great coal formation of England, the upper and middle beds, 

 for many hundred feet or yards in thickness, abound in the remains 

 of terrestrial or of marsh plants with some freshwater shells, but 

 without any admixture of marine species. The whole mass of the 

 coal measures, however, rest on marine limestone ; and in some 

 parts of England, the lowest measures alternate with beds of lime- 

 stone, and also contain some shells supposed to be marine. Though 

 the upper and middle parts of all the English coal fields are freshwa- 

 ter formations, we can have no doubt, that the limestone on which 

 all the coal fields rest was deposited under the ocean ; but many 

 circumstances tend to prove, that its elevation was a slow and long- 

 continued process, and that the lower part of the coal measures was 

 deposited when nearly on a level with the sea, or in situations subject 

 to interruptions of marine water ; or else the ground was subject to 

 oscillations, which brought it, at different times, below the level of 

 high tides. 



The great valley of the Mississippi contains the largest coal field 

 in the known world ; and what is now annually taking place in some 

 parts of that valley, appears to confirm in a remarkable manner, the 

 opinions I have advanced (pages 113 to 118) on the formation of coal 

 and the cause of the frequent recurrence of the same series of strata 

 at different depths, in the same mine ; which I attribute to the peri- 

 odical filling and desiccation of lakes. In the second volume of 

 Mr. Stuart's interesting " Travels in the United States," there is a 

 very instructive account of the Valley of the Mississippi, quoted from 

 an American review. 1 shall here insert the parts immediately con- 

 nected with the present subject ; — 



" What is called the Valley of the Mississippi is not in reality a 

 valley, but an extensive elevated plain, without hills or inequalities 

 deserving notice. It extends west from the western slope of the Al- 

 leghany Mountains, to the sand plains near the Missouri, a distance 

 of about 1500 miles, and south from the valley of the northern lakes, 

 to the mouth of the Ohio, about 600 miles. No part of the globe 

 possesses such an extent of uniform fertillity. The difference in ele- 



