340 EEPORT OF NATIONAL MUSEUM, 1899. 



hexagonal plates or scales piled up in long bundles or faces and masses 

 of unattached scales nearl}^ perfect. Such c\a.\s are always but little 

 plastic, but may become so on mechanical treatment such as grinding 

 and kneading; on re-examination the clay then shows the same ele- 

 ments of structure, but broken and confused, no bundles left intact, 

 scales broken and a homogenous matrix of the crushed material derived 

 from the still crystalline part. Clays are found in all states of this 

 breaking up, from the highly crystalline mass to the homogenous 

 matrix showing no plates at all; and on the degree in which the crys- 

 talline structure is retained, its plasticity depends. This theory is cer- 

 tainly plausible, and is supported by the fact that we always subject 

 our clays to secure increased plasticity to mechanical disturbance 

 which has the effect that the microscope reveals. This view harmon- 

 izes with more points than any other advanced as yet, and offers a fair 

 solution of the different degrees of plasticity which plastic clays exhibit, 

 but it does not explain, nor attempt to explain, the differences which 

 exist between flint clays and plastic clays, as Professor Cook's exami- 

 nations were entirely' confined to the latter.^ 



According to Russian authorities quoted by Ries,' the plasticity is 

 not only due to the interlocking of the clay particles, but varies with 

 the fineness of the grain, the extremely coarse and fine varieties having 

 less plasticity than those of intermediate texture. This view is also 

 held by Drs. Ries and Wheeler. 



So far as the compiler's own observations go, plasticity is not de- 

 pendent wholly upon hydration nor size nor shape of the constituent 

 particles. The glacial (Leda) clays are made up of fresh, sharply an- 

 gular particles of various minerals and contain less than 5 per cent com- 

 bined water; yet in their natural condition they are extremely plastic, 

 and scarcely less so when mixed with two-fifths their bulk of ordinary 

 siliceous sand, as is done in the process of brickmaking. The Albany 

 County, Wyoming, clay (Specimen No. 53229, U.S.N.M.), on the other 

 hand, equally or even more plastic and exceedingly pasty, is made up of 

 extremely minute particles of fairly uniform size, scarcely angular, and 

 apparently all of the same mineral nature throughout. This yields 

 some 1() per cent of water, on ignition, as shown in analysis, p. 348. On 

 the Avhole, the evidence seems to show that the plasticitv is due to the 

 manner in which the particles conduct themselves toward moisture, and 

 tliis is apparently dependent upon the size and shape and the propor- 

 tional admixture of varying sizes of the constituents rather than upon 

 their chemical composition. The work now being done by Dr. AVhit- 

 ney, of the Agricultural Department, on the relationship of soils to 

 moisture bids fair to throw important light upon this branch of the 

 subject. 



1 Geological Survey of Ohio, Economic Geology, V, pp. 6ol-652. 

 •■'Clay Deposits and Clay Industry in North Carolina, Bulletin No. 13, North Caro- 

 lina Geological Survey, 1897. 



