82 The American Geologist. Augrust, isoi 
similar to that of the South, but probably not so thick. This now 
forms a large part of the till or bowlder-clay ; and in its colors, 
thoroughly mixed and greatlj- diluted with gray (trituated rock), 
we have the color of the till. A simple experiment shows that a 
small proportion of the red clay of the South mixed with gray clay 
gives a decided reddish tinge to the whole. The absence of this 
reddish tinge in the till mav, however, be at least partly explained 
b}' the strong erosion attending the preglacial elevation of the land; 
on the same principal that the red cla}- is now mainly wanting on 
the mountains of the South. It seems, however, impossible that 
a thickness of red clay in the North comparable with what now ex- 
ists in the South could have been so completely swept away or ex- 
tinguished as the character of the till would indicate. Hence we 
naturall}' fall back again upon the alternative view that the red 
color was developed very scantily, if at all, in the North, in pre- 
glacial times, and that, after all, the climatic difference is an im- 
portant factor in the true explanation of the contrast in color be- 
tween the residual clays of the North and South. Certainly no 
other explanation accounts so satisfactorily for the fact that in low 
latitudes flows of basaltic lava assume in a few years a bright red 
color, which never happens in the North. The general conclusion, 
then, to which the foregoing considerations lead is that the color- 
contrast is due chiefly to the difl'erence in climate, but that the 
operation of this principle is modified in a general way by the 
essentially spontaneous tendency of the color to change from yel- 
low to red. 
THE FAUNA OF THE LOWER CAMBRIAN OR 
OLENELLUS ZONE. 
By Jos. F. James, Washington. 
This paper* occupies pages 509 to 703 of the tenth annual re- 
port of the director of the U. S. geological survey, and it is the 
third extended paper published by the author upon the Cambrian 
during the past six 3-ears. It is profusely illustrated with three 
maps, fift}- plates of fossils, and numerous sections in the text. 
The author has been an earnest student of the Cambrian rocks for 
man}- years, and students of geology and paleontology- will appre- 
ciate this presentation of the fruits of his labor. 
The paper under consideration treats solely of the lowest divis- 
*Thc fauna of the Lower Cambrian or Olenellus zone. By C. D. 
Walcott. 
