THE NATURAL HISTORY SURVEY. 13 



the feldspars, pyroxenes, amphiboles, tourmaline, muscovite, 

 talc, kaolinite. Taking into consideration the solubility of 

 many minerals in water (for example, the haloids and some 

 of the sulphides and sulphates) and noting the ease with which 

 others are destroyed by the mechanical action of water, we can 

 form an idea of the relative amounts of these minerals consti- 

 tuting the sand. The large percentage of quartz may be mis- 

 leading, if its insolubility and toughness and hardness be for- 

 gotten. Such an error is often made. It is my opinion that 

 Roscbe and Schorlemmer's estimate of the percentages of the 

 common elements forming the earth's crust gives too large a 

 figure for silicon, because the amount of quartz in the earth's 

 crust has been overestimated. It is noticeable that when a 

 mountain region has been peneplaned the quartz from strata 

 many thousand feet in thickness, being the least readily solu- 

 ble and least easily ground to fragments, collects in thick lay- 

 ers, while the potassium, sodium, calcium, iron, magnesium, 

 aluminum silicates are pulverized and dissolved and carried 

 away. Thus, as one rides over an arid region, as in our west- 

 ern states, or in eastern Europe, where for hundreds of miles 

 the surface is not concealed with vegetation, the thickly 

 strewn quartz boulders and pebbles and sand give a false im- 

 pression of their relative abundance. But if a section of a 

 freshly exposed cliff be considered, a true conception of the 

 relative amounts is obtained, because the silicious beds, as a 

 rule, form but a fraction of the cliff. They are buried and un- 

 derlain by beds of hydrated aluminum silicates or of calcium 

 carbonates. A view of the geological scale conveys the same 

 impression. A geological chart shows that limestone forma- 

 tions are as oft recurring as sandstone, and shale formations 

 nearly as common. As sedimentary rocks are formed from 

 igneous rocks, the same proportion between the constituents 

 that exists in the sedimentaries will obtain in the igneous, and 

 the sedimentaries may well illustrate the constitution of the 

 earth's crust. A geological, rather than a chemical, view of the 

 subject would increase the calcium and aluminum, at the ex- 

 pense of the silicon. 



The same principle applied to the investigation of our 

 lake shore makes possible a reliable estimate of the nature of 

 the original material supplying the present minerals. An ex- 

 amination by chemical analysis and by general measurements 

 shows that the local Silurian limestone furnishes us with the 



