20 KEPORT OF NATIONAL MUSEUM, 1889 



selves travel slowly across the country, burying everything which hap- 

 pens in their path. The shell sand shown in specimens 20255 and 20256 

 from the island of Bermuda is thus blown up from the shore, and like 

 huge drifts of snow has buried garden, woodlands, and even houses. 

 Photographs illustrative of the sand dunes of the Lake Michigan region 

 are also here included. 



The abrasive effects of material transported by the wind is often 

 manifested in a highly interesting and instructive manner. It can be 

 readily understood that sand sharply blown against any stationary ob- 

 ject would have a tendency to wear it slowly away, a fact which is taken 

 advantage of in the artificial sandblast used in glass and stone cutting. 

 In many sandy regions, and particularly those where dry winds prevail 

 a considerable portion of the year, this abrasive action becomes notice- 

 ably conspicuous. 



Under this head are here exhibited a small series of rocks thus carved 

 and polished. The most curious of these is the conglomerate (20472) 

 from Nevada, into which the natural sandblast has drilled irregular 

 worm-like holes. Others of interest are the beautifully polished spec- 

 imens from Montana, collected by Dr. A. 0. Peale and G. P. Merrill, 

 (Nos. 38575, 70602), and also the grooved basalt and peculiarly etched 

 pebbles from Arizona (Nos. 37200, 38828, and 39094), collected and de- 

 scribed by G. K. Gilbert while geologist of the surveys west of Jbeone 

 hundredth meridian. Here, too, is exhibited a large plate of glass from 

 a light-house on Cape Cod, Massachusetts. During a heavy storm this 

 became so abraded all over its exposed surface by windblown sand as 

 to be no longer serviceable, and to necessitate its removal (No. 35537). 

 This series is supplemented by photographs copied from Gilbert's re- 

 port on the geology of the region west of the one hundredth meridian. 



WATER. 



Water as a geological agent acts both chemically and mechanically. 

 The chemical processes involved are grouped by Professor Geikie (p. 

 317) under the heads of (1) oxidation, (2) deoxidatiou, (3) solution, (4) 

 formation of carbonates, and (5) hydration. In nature it frequently 

 happens that any or all of these processes are going on at the seme 

 time in a rock mass, the general result of the combined forces being 

 spoken of as weathering. In the exhibits one or more typical illustra- 

 tions are given of each of the processes working alone and a larger 

 series illustrative of the more complex process of weathering. 



Oxidation is illustrated by argillite (73267), in which the iron oxide 

 has segregated in zones of varying color, giving the rock a beautiful 

 banded structure. 



Deoxidatiou is shown by sandstone (18927) colored brown by iron 

 oxides, but from which a portion of the coloriug matter has been leached 

 out by organic acids. 



Solution by a fine block of limestone fluted by the action of rain 



