PHILOSOPHICAL SOCIETY OF WASHINGTON. 121 



found upon the old hill tops and slopes, as well as in the valley 

 bottoms, and exhibiting a general "unconformity by erosion." It 

 is composed of exceedingly fine matter without any fragments of 

 rock of notable size, such as pebbles or stones. It contains, how- 

 ever, bands of calcareous concretions in lines which are usually 

 horizontal, and these concretions are often elongated with their 

 longer dimensions vertical. It also holds those calcareous fibres 

 which Richthofen observed in the loess deposits of China, and which 

 he believed to be casts of roots of plants. Another interesting 

 occurrence is that of charcoal, which is found in several places in 

 the midst of the deposits in thin bands. The fossils of the loess are 

 the shells of geophilous mollusca. 



Mr. Todd held the view that the loess is a post-pliocene lacus- 

 trine deposit, and that the region in discussion was in post-glacial 

 time covered with a very large fresh-water lake. 



Prof. T. C. Chamberlain, of Wisconsin, being present, and in- 

 vited to take part in the discussion, remarked that while Mr. Todd 

 had presented in a very able and clear manner the reasons for 

 attributing the loess to the deposit of silt in a lake bottom, he was 

 of opinion that the objections to the acceptance of that view were 

 very great. If such a lake existed over the region in question 

 during quarternary time, it must have been of immense extent. 

 According to the observations of Dr. C. A. White, these deposits 

 extend to the borders of the region which drains immediately into 

 the Mississippi river in Iowa, and they are found nearly as far 

 west as the Rocky Mountains. Their north and south extensions 

 are not accurately known, but they are believed to be very great. 

 Independently of these deposits no evidences of such a lake are 

 now known. Its boundaries are not marked by any known bar- 

 riers on the east where the configuration of the couutry is now such 

 that no barriers could have existed, unless the region which they 

 should have occupied has undergone remarkable changes of which 

 the nature cannot be specified, and of which no traces exist. To 

 produce such a lake basin very great depressions would be necessay, 

 and there is no evidence known to him which warrants a belief in a 

 former depressed condition of that region sufficient to account for it- 

 Further research may indeed relieve us of some of these difficulties 

 or all of them, but at present they are very great. Prof. Chamber- 

 lain could not but commend, however, the earnest and scientific 

 spirit in which Mr. Todd had pursued his valuable investigations. 



