31 [ 237 3 



cusable evidences of the occurrence of a silurian group in the west. Hav- 

 ing attached too much importance to the term " old red sandstone f seeking, 

 in vain, over the country that I was exploring, an equivalent for it, either 

 mineralogical or pala3ontological, which would enable me to separate the 

 carboniferous from tlie silurian system, unless I chose to find it in the sand- 

 stone on the St. Louis of lake Superior, or that of the environs of Little 

 Rock, in the Arkansas ; and not fieeling 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 1S40 before the Geological Society of France, by Messrs. 

 Murchison and De Verneuil — one " on the Devonian rocks of the Boulon- 

 nais ;" the other '• on the importance of determining the limits between 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 iden- 

 tifying the relative ages of rocks, and especially those separated from each 

 other by long intervals of country, to attend exclusively to their fossil con- 

 tents. 



Starting, then, from this principle, I think I can confidently offer indu- 

 bitable proofs of the occurrence of the Devonian rocks on the Missouri 

 river. 



In latitude 4U° 50', and longitude 95*^ 42' from Greenwich, eighteen 

 miles below Platte river, there is a locality known by the name of " Five 

 Barrels island." Opposite to that group, and on the right side of the river, 

 a bluff, at the termination of a series of rocky banks, is separated by a small 

 creek from another series called Cotes de la Platte. At the base of the bluff 

 there is — 



1st. A compact argillaceous limestone of a bluish color, from one to two 

 feet thick; soft under water, but hardening when exposed to the air; it 

 weathers into thin plates, presenting an uneven- surface ; on which there 

 are impressions of euomphali, but too indistinct to be specified. 



2d. A compact argillaceous limestone of a yellowish gray color, from 

 six to seven feet thick, containing an abundance of crlnoidal remains, as- 

 sociated with beautiful and large specimens of the ajathophyllum ver77ii- 

 cwZa/c of Goldfuss. This limestone also contains a /jrot/uc^a and an un- 

 known bivalve, together with crystallized bi sulphuret of iron agglomerated 

 into bullets. 



The upmost portion of the bluffs, attaining a height of ISO feet above the 

 river, I remarked to be shadowed by trees over a beautiful green sward ; 

 but I had no opportunity of examining it particularly. 



It may be well to state here, that all such rocky banks as the one just 

 alluded to, noticed by Lewis and Clark, and subsequently by Major Long, 

 arc constantly wearing away ; so that they offer landmarks to the travel- 

 ler only for a limited period of time. But we are not to judge of their 

 oryctognostical character from the detritus found below them ; because 

 this is composed not only of the materials derived from the bluffs, but of 

 others carried down the Missouri during its season of high waters. Among- 

 these materials is the oft-mentioned pu/nice stone, which is brought down 

 from the upper parts of the river. I have ascertained, by a more careful 

 examination than had probably been given to it previously, that it is not 

 a true pumice, but a semivitreous substance, produced by pseudo-volca- 



