May, 1845.] 239 



The Committee to whom the above paper of Mr. Harris, 

 on certain points in the geology of the Upper Missouri, was 

 referred, having availed themselves of additional information 

 from that gentleman, and examined the specimens brought 

 home by him, submitted to the Academy the following facts 

 and inferences in the form of an appendix to his valuable 

 communication. 



Of the Red Pumice of the Missouri. 



Upon the question of the origin of this singular substance, the Committee 

 desire to express their general concurrence in the views presented in the paper 

 of Mr. Harris. From the evidence afforded by the series of specimens it is 

 clear that the so-called Pumice is not a true volcanic product, but is originally 

 an argillaceous sandstone, probably a lacustrine deposition of a tertiary age. 

 In its altered, cellular form, it is said to occur loose on the shore, throughout 

 nearly the whole length of the River, as if strewed by the freshets, its specific 

 gravity being frequently less than that of water. 



The locality, where it was seen in place, is between the Beaver and Grand 

 Rivers, above the Mandans, on the East side of the Missouri, and thereforeabout 

 200 miles below the mouth of the Yellowstone. Here the material offers the 

 most unequivocal marks of being derived from the spontaneous combustion of a 

 sedimentary argillo-arenaceous stratum. The rock is seen in almost every state 

 of gradation from a stratified mass containing beautifully distinct impressions 

 of leaves of terrestrial trees, to a light vesicular pumice. Certain half-fused 

 specimens show indeed the well presumed traces of these leaves, the partially 

 melted condition and the nearly perfect vesicular structure, all upon the same 

 mass. The presence of sulphuret of iron, and of lignite, in the neighboring clays 

 of the same group of strata, suggests a ready source for this spontaneous 

 combustion. 



This explanation of the origin of the pumice, sustained as it is by the draw- 

 ings and descriptions of the scenery given by the Prince Maximilian, and borne 

 out by the fact that no other material, of even an imputed volcanic source, has 

 ever been discovered in the plains of the Missouri, renders it highly probable 

 that the stratified pumice, alleged by Mr. Catlin to exist there, is derived from 

 the same cause. We therefore come to the general conclusion, which best 

 accords with all that is known of the topographical features of the region, that 

 no trace of volcanic action is visible East of the Rocky Mountains. 



The leaves imbedded in the stratum which produces the pumice-like material, 

 bear a near resemblance to those of the common beach tree, (Fagus ferruginea,) 

 they are at all events the leaves of an exogenous tree of a recent type. 



Proofs of a Freshwater formation, near the Mouth of the Yellowstone River. 



In the immediate vicinity of Fort Union, near the junction of the Yellow, 

 stone and Missouri Rivers, in the group of strata designated as No. 5 in Mr. 

 Harris's paper, occur incontestible proofs of a freshwater formation. At a dis- 



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