358 GEOLOGICAL SURVEY OF THE TERRITOKIES. 



growth other kinds of vej>etables, possessing the same properties with 

 the same mode of life. We do not know them as yet, for tbe vegetable 

 remains fossilized in sand and clay beds seem to represent species of 

 plants bordering the swamps, rather than the species which have con- 

 tributed to their composition. 



The evaporating and absorbing power of the plants of the bogs play 

 another remarkable part in tbe economy of nature in moderating the 

 extreme of temperature, especially greatly reducing the excess of cold. 

 Everybody knows how even a thin fog i)revents frost. The flora of our 

 Lignitic, like that of the Coal epoch, has a number of species, whose 

 affinity is with plants now considered as characteristic of a tropical or 

 subtropical climate. It. has been generally argued therefrom that at 

 the time when our combustible minerals were in progress of formation 

 the climate. of our country was much warmer than it is now. From the 

 examination of the 'first specimens of Tertiary fossil-p'uints found on 

 this continent I was inclined to admit a same opinion; but the more I 

 have studied the distribution of the plant of former epochs, comparing 

 it with that of ours, the more I have been led to believe that the differ- 

 ences in the general characters of the vegetation, as indicated by fossil 

 remains, result essentially from atmospheric humidity, rather than from 

 temperature. In Ireland and in Scotland, near the mouth of the Firth 

 and of the Clyde, as high as 57° of latitude, a limit corresponding on 

 the American continent with North Labrador, the vegetation under a 

 high degree of atmospheric humidity, already presents a tropical aspect. 

 Tropical ferns, species of HymcnophyUiim and Triclwmancs^ co\qt the. 

 rocks and the mossy trunks of the trees, mixed with European forms of 

 ferns, which, by their luxuriance and size, recall the characters of the 

 vegetation of the Southern Islands.* It has been remarked already 

 that the coal and lignite beds pro\ed for the thueof their formation a far 

 greater degree of atmospheric humidity than we have now. If a mere 

 ditterence in the proportion of humidity can produce at our time a 

 change in the character of the vegetation corresponding to that indi- 

 cated by temperature in 25° of latitude, or the difference between the 

 southern swamps of oars and those of Scotland, we can easily admit, as 

 resulting from the same cause, the facies of the vegetation of Greenland 

 at the Tertiary epoch. This flora, as we know it already, has a general 

 relation and some identical species wiith that of our Lignitic deposits of 

 the West. 



These details, which indirectly throw light on the productive causes, 

 the distribution, and the original composition of the lignite-beds, are 

 sanctioned by the importance of a formation, justly considered as a 

 most essential series of our American geology. 



THE LIGNITIC CONSIDERED IN ITS APPLICABILITY.^ 

 AREAL DISTRIBUTION AND THICKNESS OF THE STRATA. 



§ 1. The Northern Lignitic Basin. 



The formation to which the name of Great Lignitici^ fittingly applied 

 is for the first time noticed in Lewis's and Clarke's expedition to the Rocky 

 Mountains, 1804:: 



The coal or lignite was first observed twenty miles above tbe Mandan Village. The 

 bluffs on each side of the Missouri are npward of 100 feet high, composed of sand and 

 clay, with many horizontal strata of carbonated wood, resembling pit-coal, from 1 to 



* Schimper, Vegetable Paleontology, vol. 1, p. 358. 



