48 The American Naturalist. [January, 



deposits are not confined to the eastern slope, as Bailey had supposed. 

 Mr. Edwards further states that the geological age of a fresh-water 

 Diatomaceous strata cannot be determined by means of the microscope 

 unless they are proved by other evidence to be of greater age than 

 the present period. Enough is known of the habitat of certain species 

 to make it easy to tell whether the deposit has been made in pond, 

 lake, river, marsh, bay or ocean. 



On the Relationship of the Plistocene to the Pre-plisto- 

 cene Formations of the Mississippi Basin South of the 

 Limit of Glaciation.— In the American Journal of Science, May, 

 1891, is published a paper, the joint production of Mr. T. C. Cham- 

 berlin and Mr. R. D. Salisbury, on the relationship of the Loess and 

 the Orange Sand south of the limit of glaciation. The deposits inves- 

 tigated by the writers are included between the parallels of 35° and 

 that of the northern limit of the Mississippi Basin. Throughout much 

 of this territory the loess lies upon the glacial drift. It may be traced 

 across the limit of the drift from north to south. The continuity is 

 complete, and the character of the formation is the same on both sides 

 of the line which marks the limit of ice advance. North of the limit, 

 the evidence, in the judgment of the writers, is conclusive that the 

 loess belongs to the closing stages of the first glacial epoch. If, there- 

 fore, the age of the loess which covers the drift be first glacial, the age 

 of that which lies south of the drift, in the area under discussion, is 

 likewise first glacial. 



Between the relationship of the till north of the limit of glaciation 

 and the relationship of the loess to the residuary earths of the Paleo- 

 zoic rocks immediately outside the drift there is an important differ- 

 ence. The presence of a weathered and highly-oxidized zone, subja- 

 cent to the loess, south of the drift-limit, is as conspicuous as its absence 

 to the north. This oxidized zone is the upper surface of the residuary 

 earths, and clearly indicates the existence of a long interval between 

 the loess and the residuary earths beneath. 



Beneath the loess, south of the limit of glaciation, lie the series of 

 gravels and sands known as the Orange sands. It is a peculiarity of 

 the distribution of loess, that elevations within the area of its occur- 

 rence seem to be no obstacle to its presence. The same may be said of 

 the gravel. From their relative position it U evident that the latter 

 is the older of the two. That it is much older is shown (1 ) by a zone 

 of oxidation between the loess and the < Grange sand ; (2) by :i marked 

 unconformity when the loess covers a hill, indicatinir a long period ol 



