310 POPULAR SCIENCE MONTHLY. 



meriting upon its occurrence, correctly assumes that it represents 

 one continuous volcanic eruption, the date of which might fall well 

 within a period of a few hundred years, and he speculates as to its 

 being possibly associated with an outbreak from Mount Wrangel 

 or some active cone which is represented by the Indians to exist in 

 the region of the upper White River. Beyond this, from the nor- 

 mality of its position, and the assumed fact that no fluviatile or 

 aqueous deposits have been found overlying it, the same observer 

 argues that the outbreak must have taken place subsequent to the 

 formation of the present river courses and their valleys, a conclu- 

 sion in which I do not see my way to concur. The only satisfactory 

 interpretation of this vast uniformly placed and uniformly layered 

 deposit of ash is to me that which assumes a deposition in a widely 

 extended lake basin, or in shallow lagoon waters which already in 

 part occupied the present valley surfaces. In such waters precipi- 

 tation from long-continued suspension would proceed gradually and 

 evenly, to the end of shaping a deposit of nearly uniform develop- 

 ment and of vast extent. Such depositions we find in the valleys 

 lying north of the City of Mexico (Zumpango, Tequixquiac) and 

 in the lacustrine area of Anahuac, also in the famous fossiliferous 

 basin of Florissant, in Colorado. With the subsequent formation 

 or reformation of the river's course we should have this deposit cut 

 through, with the result of presenting the even layer which is so 

 persistent in its following. This method would also account for 

 the anomalous position in which we find the ash deposits; while still 

 holding the same relation to the top surface, it occasionally rises far 

 above what might be assumed to be its normal height or level above 

 the water's surface from four to ten feet a condition that would 

 hardly be in consonance with the assumption that the ash was de- 

 posited after the actual river channels had been cut. But other 

 and more direct proof of aqueous occupation after the laying of the 

 ash is had in the fact that in one place at least, and doubtless many 

 more such will be found on closer investigation, lacustrine or flu- 

 viatile shells (subfossils) occur in the layer overlying the ash. A 

 locality of this kind is found on the right bank not many miles above 

 the Five Finger Hapids. Here, at a height of not more than four 

 feet above the river, I had the pleasure of determining species of 

 Limnea and Physa, associated singularly enough with Helix, in 

 the layers immediately above and below the ash bed, and in both 

 horizons the species were identical. This isolated fact speaks vol- 

 umes for itself. Had this been the region of Helena, Ark., I should 

 have been prompted to class the bed with a portion of the Missis- 

 sippi loess. What interested me further in this connection was the 

 fact that up to this time I had failed to bring to light one solitary 



