PRELIMINARY HANDBOOK OF THE DEPARTMENT OP GEOLOGY. 19 



fusion, although occasional holes of slight depth are formed, as shown 

 in specimens from Oregon and Armenia.* A more complete descrip- 

 tion of these may be fouud in the bound volume of pamphlets on the 

 tablet 



Effects of temperature variations. — As is well known, the usual effects 

 of heat upon any material substance is that of expansion, and of cold 

 contraction. In countries like the arid regions of the West the alterna- 

 tions of heat and cold are often so great as to produce very marked 

 disintegration and exfoliation even in the toughest and most solid of 

 rocks. The results of such disintegration are not of such a nature as 

 to lend themselves readily to exhibition purposes. In the specimen 

 shown (a compact andesite from Madison County, Montana), the entire 

 mass of rock has on the surface become by these temperature varia- 

 tions broken into blocks of but a few pounds' weight each, and the sur- 

 face of the ground on the neighboring slopes is everywhere covered 

 with small chips thus flaked off with beautiful concave and convex sur- 

 faces, as may be observed in the specimen. The high plateaus and 

 mountain tops in this region are invariably covered by loose debris in 

 the form of thin sheets of but a few inches or many feet in diameter 

 which have been thus flaked off. 



The effects of wind. — The geological action of the wind is due mainly 

 to (1) its efficacy as a transporting agent, and (2) to the abrasive effects 

 of the transported substances. Its efficacy as a transporting agent 

 can be shown only by an exhibition of the material transported aud by 

 photographs. 



Of chief interest in this exhibit are the fine volcauic dusts, such as, 

 after being ejected from the volcanic throat, are drifted by winds, it 

 may be for many miles. Specimen 35800 was gathered at Tryssil, on 

 the coast of Norway, in November, 1875, having been blown by the 

 wind from Iceland. No. 36974 is a product of the eruption of Krakatoa 

 in 1883. This dust fell at the rate of an inch an hour on board the 

 ship Beaconsjield while at a distance of 800 miles from its source. No. 

 38588 is from a bed not less than 6 feet in thickness, and which occu- 

 pies the bed of a now extinct lake in Gallatin County, Monanta. The 

 photograph shows the thickness and position of this bed. Other 

 samples shown are from similar beds in Nebraska (37023, 37024, and 

 38545) and Nevada (77200). Of the coarser materials thus drifted atten- 

 tion ma} 7 be called to the gypseous sand from near Fillmore, Utah. 

 This, as described by Geologist Gilbert, is first formed as minute crys- 

 tals on evaporation of the water of playa lakes. Subsequently the winds 

 sweeping across the dry lake beds gather the sand into drifts. 



The siliceous and calcareous sands in arid regions or along sea and 

 lake shores are often thus blown into huge drifts or dunes, which them- 



* The gift of Mr. J. S. Diller, of the U. S. Geological Survey. 



tOn Fulgurites, Proe. U. S. National Museum, lrttffi, p. 83, and Fulgurites or Light- 

 niug Holes, Pop. Sci. Monthly, Fell., 18-i7, p. 527. See also Geikie, p. 3015. 



