1G9 



STRUCTURAL GEOLOGY. 



Specimens illustrating structural geology are contained in 

 table cases 112, 113, and in pyramid cases 109, 111, 138. 



Wind operates by means of driving sand : the sand itself Atmosplieric 

 becomes rounded much more perfectly than on a beach, while -^^^^^^i^S' 

 at the same time it polishes and otherwise wears away the 

 surfaces of rocks and pebbles exposed to it. In the western 

 districts of this State wind-polished pebbles are common 

 (case 113). 



EuKNisG WATER rolls along with it boulders, gravel, or sand, 

 -according to its force. These become rounded by rubbing 

 against one another. Notice that the smoothing produced by 

 water is not nearly so perfect as the polishing effect of wind- 

 -driven sand (case 113). Bounded boulders (case 109). 



"Weathering : — By weathering is meant the changes effected 

 by rain-water soaking into the earth's crust. These changes are 

 produced partly by means of the solvent action of the water, but 

 principally through the action of the gases taken up by the water 

 during its passage through the soil. Thus the numerous iron- 

 stone and other gossans shown in the ore-cases have resulted 

 from the decompoi-ition of sulphides, such as pyrites, under the 

 influence of surface waters. 



9015, Sunny Corner Silver Mine. A honeycomb-like mass of 

 silica. This was originally a mass of pyrites traversed by thin 

 plates of quartz. The pyrites has been removed by weathering, 

 .leaving the siliceous portions intact. 



In case 138 is a spheroidal mass of diorite from Tumbarumba, 

 the shape having been produced by weathering. Certain igneous 

 rocks show a great tendency to weather in this way. 



Deposition by Water: — Stalactites and Stalagmites. In 

 percolating through the lime-stone rocks, water, by the aid of 

 the carbonic acid it has absorbed from the soil— dissolves a 

 certain quantity of carbonate of lime. After oozing through the 

 roof of a caA'ern, drops of the water remain suspended for a 



