106 PHYSICAL GEOGRAPHY. 



The silica is in the form of clean, white microscopic grains 

 of sand, which, as will be seen, constitute the greater part of 

 the deposit. 



That it contains much lime is also apparent from the 

 presence of numerous calcareous, concretionary lumps, some- 

 what resembling pebbles in shape and size, which are 

 generally distributed throughout the mass. It is also shown 

 in the abundant accumulation of calcareous tufa, around 

 the springs which issue in considerable numbers at the base 

 of the bluffs just where this deposit rests upon the drift. 

 The water of these springs has fallen upon the surface, and 

 percolated through the whole thickness of the Bluff Deposit, 

 and escapes laterally in the form of springs upon the less 

 pervious drift. Rain water always contains at least a minute 

 quantity of carbonic acid dissolved in it, and after it has 

 penetrated the air cavities of porous earth, much more. This 

 renders some of the carbonate of lime which the deposit 

 contains slightly soluble. When the water, holding this 

 excess of carbonate of lime in solution, reaches the atmos- 

 phere it becomes precipitated there in the form of the tufa 

 before referred to. It is the lime thus precipitated which 

 cements the sand and gravel of the Altered Drift in those 

 localities, which have been mentioned on a previous page. 

 Except the limy concretions before mentioned, not a stone 

 or pebble of any kind is to be found in the whole deposit, 

 but all is uniformly fine and homogeneous. 



P7iy steal Properties. Some of the physical properties of 

 this deposit are so unusual that they merit especial mention. 

 When it is known that there is no rocky support to these 

 Missouri river bluffs, although they are frequently so steep 

 that a man cannot climb them, it is very apparent that the 

 material composing them is different from the earth ordinarily 

 met with, and which it resembles upon its ordinary surfaces. 

 Its peculiar property, however, of standing securely with a 

 precipitous front, is best shown in artificial excavations. For 

 all practical purposes of building-foundations, even of the 

 most massive structures, and for roads, etc., the ground it 



