PETER SANDSTONE 
297 
that weathering proceeded well in advance of the removal of the 
residual soil. 
The land-mass was apparently without vegetation, for, if it may 
be judged by the best available evidence the world over, land 
plants had not yet developed in Early Ordovicic time. If they had, 
they were probably of minute size and easily destroyed. Practic¬ 
ally no important remains of land plants are found before Late 
Devonic times. This absence of vegetation probably was an im¬ 
portant factor in increasing the efficiency of wind-work, since 
it must have given even to a humid region many of the char¬ 
acteristics of a desert. 
The wind circulation must, unless we assume profound shifting 
of the pole, have been much as at present. This factor, together 
with the other points already cited, the absence of vegetation, the 
comparatively flat topography and the deep weathering, seem to 
constitute vital points in the proper understanding of one of the 
apparent anomalies of the early Paleozoic sediments of the north 
central United States, namely the remarkable rarity of shales 
as compared with sandstones, from the base of the Cambric sec¬ 
tion to the top of the Peter sandstone. The average igneous rocks 
should yield several times as much clay as sand. The early Paleo¬ 
zoic sediments, derived from the pre-Cambrian shield, afford 
several times as much sandstone as shale. 
In such an area as postulated above, with its continental slope 
to the south, with deeply weathered soils, and with no protective 
cover of vegetation, westerly winds sweeping across the drainage 
lines would be given the maximum power of removing the clay 
in actual suspension, while drifting the sands into the rivers 
to be carried south into the sea. The dust, which was probably 
not carried far at any one time, except during unusual storms, 
came to rest to the eastward, each time on land instead of being 
dropped into the sea, until finally blown far eastward into the 
Atlantic Ocean and lost beyond possibility of study. This process 
would account for the fact that the Potsdam formation, though 
it has many shaly beds, contains far less than the theoretically 
normal amount of clay. The wind drift would also aid in pro¬ 
ducing the perfection of rounding seen in these sand-grains. 
With the close of Cambric time and the northwardly encroaching 
Ordovicic sea, the southern fringes of the Potsdam sandstones 
I 
