3i8 The American Geologist. November. 1902 
WHAT CONSTITUTES A CLAY. 
That kaolin is the basis of all clays is the commonly ac- 
cepted opinion of most writers. The evidence upon which 
such an opinion is based is largely chemical and, it must be 
confessed, unsatisfactory — so unsatisfactory, in fact, that 
the present writer has ventured at times to doubt the accuracy 
of the statement altogether. That kaolin is a clay is unques- 
tionable, but that all the materials possessing the plasticity of 
clays and their property of becoming indurated on drying, or 
baking, can be found to contain even non-essential portions 
of kaolin, has not yet been proven. 
Even the question as to what kaolin itself may be is 
evidently problematic in the minds of many writers, a major- 
it}- of Avhom will simply refer back to the time-worn text- 
book definition to the eti'ect that kaolin is a hydrous silicate of 
aluminum corresponding to the fonnula Si^O^ALH^, which 
is that of the mineral kaolinite — a mineral micaceous in 
structure, hexagonal or rhombic in outline, but monoclinic 
in its crystallographic properties. 
As to the primary origin of kaolin, we are taught that it 
results from the breaking down of aluminous rocks, prin- 
cipally feldspars, through the ordinary atmospheric agencies 
comprised under the name of weatliering. tlie processes con- 
sisting in the removal, wholly or in part, of the more soluble 
salts, as those of potash, soda, and lime, and the accumulation 
of the practically insoluble hydrous aluminum silicate as a 
residue. With many writers the name kaolin is applied to the 
re-idual materials, whether left in place by the decomposing 
agencies or separated from mechanically admixed impurities 
through the action of water. 
Through becoming contaminated with various impurities, 
the kaolins, we are taught, pass over into the various types 
of clay, classified, according to their demonstrated fitness for 
kny particular kind of work, as brick, tile, terra cotta, pipe, 
fireclays, etc.. the last name being applied to clays which, com- 
posed mainly of aluminum silicates and relatively poor in the 
alkalies, lime, or iron, are highly refractory and suitable for 
fire brick. 
The cause of the plasticity of clays as a whole is little un- 
derstood, and it is doubtful if any of the attempted solutions 
