326 REPORT OF NATIONAL MUSEUM, 1899. 



fine clay particles from the coarser jmpurities and deposited them by 

 themselves, as in the case of that from Florida (Specimen No. 67256, 

 U.S.N.M.). In the majority of cases, however, natural washing- has 

 but served to still further contaminate the materials, giving rise to the 

 complex transported clays to be noted later. Many rocks, such as the 

 aluminous limestones, are so impure that on decomposing and the 

 losing of their soluble lime carbonates they leave only very inferior 

 varieties of clay, suitable for brick and tile or pottery making. Such 

 are often highly colored by iron oxides (Specimens Nos. 62564, 62673, 

 63463, and 63493, U.S.N.M., in Rockweathering series). 



The assorting and transporting power of running waters rarely 

 allow the beds of kaolin or of clay to remain in a condition of virgin 

 purity or even in the place of their origin. The minute size and the 

 shape of their constituent particles render them easily transported 

 by rains and running streams, to be deposited again in regularly 

 laminated beds (see Plate 18) when the streams lose their carrying 

 power by flowing into lakes or seas. It is through such agencies that 

 have in times passed been formed the so-called Leda clays (Specimen 

 No. 73036, U.S.N.M.) and the loess. Such may contain a very large 

 proportion of mechanically derived material and proportionately little 

 kaolin. 



Speaking of clays of this nature as they exist in Wisconsin, Cham- 

 berlain says: 



They owe their origin mainly to the mechanical grinding of glacial ice upon strata 

 of limestone, sandstone, and shale, resulting in a comminuted product that now 

 contains from 25 to 50 per cent of carbonates of lime and magnesia. This product 

 of glacial grinding was separated from the mixed stony clays produced by the same 

 action by water either immediately upon its formation or in the lacustrine epoch 

 closely following. The process of separation must have been rapid and comparatively 

 free from the agency of carbonated waters, otherwise the lime and magnesia would 

 have been leached out. 



The formation of beds of clay has been confined to no particular 

 period of the earth's history, but has evidently gone on ever since the 

 first rocks were formed and when rock decomposition began. The 

 older beds are as a rule greatly indurated and otherwise altered, and 

 in many instances no longer recognizable as clays at all. Throughout 

 the Appalachian region clay beds of Cambrian and Silurian ages have, 

 by the squeezing and sheering incident to the elevation of this mountain 

 system, become converted into argillites and roofing slates. 



Mineral and chemical composition. Formed thus in a variety of 

 ways, and consisting not infrequently of materials brought from diverse 

 sources, it is easy to comprehend that the substances ordinarily grouped 

 under the name of clays may vary widely in both mineral and chemical 

 composition. It may be said at the outset that the statements so fre- 

 quently made to the effect that kaolinite or even kaolin is the basis of 

 of all clays is not yet well substantiated. 



