GEOLOGY. 211 



just as the carboniferous series, with its coal measures, over- 

 lies the Silurian. 



" From wliat I have so far said of the geology of tlic West, 

 it will be perceived that I have adopted the classification of 

 the older fossiliferous rocks, as laid down by its illustrious 

 author, Mr. Murchison. But I must confess that I hesitated 

 about it a long time ; notwithstanding the opinions of my 

 friends MM. Vanuxcm and Conrad, both distinguished geolo- 

 gists and conchologists, who had recognized among my fossils 

 irrecusable evidences of the occurrence of a silurian group 

 in the West. Having attached too nmch importance to the 

 term ' old red sandstone ;' seeking, in vain, over the country 

 that I was exploring, an equivalent for it, either mineralogical 

 or pakeontological, which would enable me to separate the 

 carboniferous from the silurian system, unless I chose to find 

 it in the sandstone on the St. Louis of Lake Superior, or 

 that of the environs of Little Rock, in the Arkansas ; and 

 not feeling authorized to do so, from the absence of fossils — 

 fearing, moreover, that these rocks were actually beyond the 

 limits of the system under consideration, as I said before, I 

 could not but hesitate. However, having recently become 

 acquainted with the learned papers read in 1840 before the 

 Geological Society of France, by MM. Murchison and De 

 Vemeuil — one ' on the Devonian rocks of the Boulonnais ;' 

 the other ' on the importance of determining the limits be- 

 tween the mountain limestone and the inferior formations' — 

 a new light was afforded me ; all my doubts were dissipated ; 

 and I then saw the necessity, in identifying the relative ages 

 of rocks, and especially those separated from each other by 

 long intervals of country, to attend exclusively to their fossil 

 contents. 



" Starting, then, from this principle, I tliink I can confi- 



