QUATERNARY AGE. 261 



casionally take its place. There are a few such points between La 

 Platte and Omaha. Not unfrequently this deposit is highly cal- 

 careous in its lower and upper portion. When it gradually shades 

 down into gravel and boulder beds, the latter are often covered by 

 incrustrations of calcareous and other alkaline matter. It is possi- 

 ble that the alkaline matter that has been leached out of the over- 

 lying beds was deposited on these underlying pebbles and boulders. 

 I am, however, by no means sure that this explanation is the cor- 

 rect one. The most remarkable, however, of the deposits at this 

 horizon, are the strata of calcareous and other alkaline matters that 

 are found in the upper portion of these silicious beds. The amount 

 of alkaline matter ranges from ten to ninety per cent and the beds 

 vary in thickness from a few inches to fifteen feet. Between Ne- 

 braska City and Brownville, along the Missouri bluffs, are some 

 fine exposures of these alkaline beds, though they are more min- 

 gled with sand and gravel than farther west. The calcareous 

 concretions found here are, however, exceedingly abundant and 

 beautiful. Samples are common which measure from one to five 

 inches in diameter. Inside they are sometimes partially hollow, and 

 portions of the mass being separated and loose, they rattle, on being 

 shaken after drying. In Saline County there is a thin, almost pure 

 ■snow white layer of this calcareous matter. Further west, in Web- 

 ster, Fillmore, Hamilton, York and some other counties north, as 

 well as south of the Platte, this alkaline material occurs at this hori- 

 zon at various localities. It differs greatly in thickness and extent 

 •of beds, and in the proportions of the alkalies present and silicious 

 materials with which it is combined. It has sometimes been used 

 for mortar and plastering, and from the people has received the 

 name of natural mortar. It does not, however, avail for outside 

 work, as the rain softens and gradually removes it. Not unfre- 

 quently layers of this alkaline matter are separated by layers of 

 sand, and even higher up in the series, where it occurs in the 

 Loess, as it sometimes does, it is separated into thin strata by the 

 same materials. The following section, taken about five miles 

 northwest from Fairmount, illustrates the mode of its occurrence: 



1. Black soil 4 feet. 



2. Loess 17 " 



3. Calcareous and other alkaline earths 1 " 



4. Loess 2 " 



5. Calcareous and other alkaline earths 7 " 



€. Brownish calcareous sand, exposed 3 " 



