Mesozoic and Ccenozoic Geology and Palmontology. 305 



The greatest development is iu the couuties on the Missouri, from the 

 Iowa line to Boonville. In some places it is 200 feet thick. At Boon- 

 ville it is 100 feet thick, and at St. Louis only 50 feet. 



The Bluff Group is older than the bottom prairie, and newer 

 than the Drift. It gives character and beauty to nearly all the best 

 landscapes of the Lower Missouri. 



He found the drift abounding nortii 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, a 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 1 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- 

 foi'nia, 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 awaj and polished; garnets and tourmaline were 

 also cut and left with polished surfaces. Masses of limestone looked 

 as if the}^ 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 the 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 feldspar, and favorabl}' presented to the action of 

 the sand, the feldspar was cut awaj' around the hard mineral, which 

 was thus left standing iu relief above the general surlace. 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 



* Am. Jour. Sei. and Arts, 2cl ser., vol. xx. 



