Mesozoic aad Cenozoic Geology and Paleontology. 201 
The greatest development is in the counties on the Missouri, from the 
Iowa line to Boonville. In some places it is 200 feet thick. At Boon- 
3 ville it is 100 feet thick, and at St. Louis only 50 feet. 
The Bluff Group is older than the bottom prairie, and newer 
E than the Drift. It gives character and beauty to nearly all the best 
landscapes of the Lower Missouri. 
He found the drift abounding north of the Missouri river, and ex- 
isting in small quantities as far south as the Osage and Meramec. Its 
thickness varies from 1 to 45 feet. The upper part, having the ap- 
pearance of having been removed and rearranged by aqueous agencies 
since its first deposit, but before the deposit of the Bluff Group, 
is described as altered drift. The heterogeneous strata of sand, gravel, 
and bowlders, is called the bowlder formation; and below this, in some 
places, « third division exists, which is called the “pipe clay.” It 
contains bowlders more or less dispersed through the upper part of it. 
It is found in Marion, Boone, Cooper, Moniteau, Howard and Monroe 
counties, varying in thickness from | to 6 feet. 
William P. Blake* described the grooving and polishing of hard 
rocks and minerals by dry sand in the Pass of San Bernardino, Cali- 
fornia, and on the projecting spurs of San Gorgonia, he said, grains of 
sand were pouring over the rocks in countless myriads, under the in- 
fluence of the powerful current of air which seems to sweep constantly 
through this Pass from the ocean to the interior. Wherever he turned 
his eyes—on the horizontal tables of rock, or on the vertical faces 
turned to the wind—the effects of the sand were visible; there was not 
a point untouched, the grains had engraved their track on every stone. 
Even quartz was cut away and polished; garnets and tourmaline were 
also cut and left with polished surfaces. Masses of limestone looked 
as if they had been partly dissolved, and resembled specimens of rock 
salt that have been allowed to deliquesce in moist air. These minerals 
; were unequally abraded, and in tbe order of their hardness; the wear 
¥ upon the feldspar of the granite being the most rapid, and the garnets 
being affected least, wherever a garnet or a lump of quartz was im- 
bedded in compact feldspa:, and favorably presented to the action of 
the sand, the feldspar was cut away around the hard mineral, which 
was thus left standing in relief above the general surface. A portion 
however, of the feldspar, on the lee side of the garnets, being protected 
from the action of the sand by the superior hardness of the gem, also 
stood out in relief, forming an elevated string, osar like, under their 
DD? 
* Am. Jour. Sci. and Arts, 2d ser., vol. xx. 
