THE ORIGIN OF CERTAIN PALEOZOIC SEDIMENTS 249 
wind. These sands are almost always pure white in color, differing 
from typical desert sands, which are generally some shade of red 
or brown, so that in all probability desert conditions did not prevail 
here. It was more likely a stretch of sand-dune country that 
lay along the low, flat Upper Cambrian shore. At that early day 
those types of plant life which check the development of sand 
dunes along our shores today did not exist and perhaps there was 
no other type of plant to take their place. As a result the wind 
had unhindered opportunity to get at and work over this sand. 
The climate need not have been very dry, because, as Shaler has 
shown, rain does not have an important retarding effect upon 
wind-blown sand.’’ Even heavy 
and long-continued rains falling 
upon dune sand rarely wet it for 
more than an inch below the 
surface. In a few hours after 
the rain is over, this thin film of 
water is evaporated from the 
surface and the wind is free 
again to move about the sand. 
The cross-bedding shown in the 
few localities where this sand- 
stone outcrops is altogether 
consistent with this sand-dune 
theory of origin. When again 
the region became submerged, a little of the finer sand was carried 
out and dropped in the water. These grains then frequently served 
as the minute nuclei around which calcareous odlites formed, which 
were in many cases later replaced by silica and gave rise to the 
extensively developed siliceous odlites of the overlying beds. 
This sand-dune theory of the origin of the sandstone beds does 
not require great changes in the conditions of sedimentation from 
the Upper Cambrian to the Lower Ordovician of this region, nor 
does it require long periods of time to produce the well-rounded 
character of the sand. Mackie has shown that sand carried by the 
wind for a very few miles will be more thoroughly rounded than 
tN.S. Shaler, Bull. Geol. Soc. Am., V, 207-12. 
Fic. 7.—Photomicrograph of the Bar- 
rens sandstone. X22. Nicols crossed. 
