118 AQUEOUS BOOKS 



apparently, -apon texture and structure, i. e,, upon the size and 

 shape of the individual particles, and in some cases at least 

 the presence of colloidal matter. Pure quartz, chalcedony, flint, 

 feldspar, or other silicates, will, when reduced to an impalpable 

 powder, possess the qualities usually ascribed to clay, and in the 

 pages following, the term is used only with reference to degree 

 of comminution and plasticity, regardless of mineral nature or 

 chemical composition. It includes residual products of any or 

 all forms of rock degeneration, and which may or may not have 

 been reasserted through the agency of water. (See further under 

 The Eegolith, Part V.) The oft-repeated statement that kaolin 

 forms the basis of clays, or that clay is impure kaolin, is an 

 unfounded assumption, and if accepted at all it must be with the 

 reservations made by Johnson and Blake,^ who limit the term 

 kaolin to the impure material, quite distinct from true haolinite, 

 which is a definite chemical compound corresponding to the 

 formula PI^ALSi^Og. 



Throughout the glaciated region of the northeastern United 

 States the clays are largely glacial silts or water deposits from 

 the floods of the Champlain epoch. The latter are often beauti- 

 fully and evenly stratified, as shown in the illustration on PL 28. 

 The plastic clays and siliceous sands about Woodbridge, New 

 Jersey, are regarded as derived from the Azoic rocks and de- 

 posited by sea-water in enclosed basins. The exact source of 

 the material is not always apparent; the porcelain clays of Law- 

 rence County, Indiana, on the other hand, are, according to 

 State Geologist Cox, residual deposits resulting from the de- 

 composition of impure Carboniferous (Archimedes) limestones, 

 the lime carbonate being removed in solution, while the less 

 soluble clay remains. Kaolin, as already noted, is a residual 

 deposit from the decay of f eldspathic and other aluminous rocks, 

 while the ordinary brick and tile clays of the Southern states, as 

 well as the clayey soils, are residual aluminous deposits resulting 

 from the decay and leaching out of soluble constituents from a 

 variety of rocks, both sedimentary and eruptive. (See chapter 

 on rock weathering.) 



As showiiag the comparative compositions of kaolins and other 

 clays, the following table is given: 



^Am. Jour, of Science, 1867, p. 351. 



