CLAYS, THEIR ORIGIN, COMPOSITION AND VARIETIES. 51 



that which classifies argillaceous material into two groups, namely, clays 

 and shales. 



Clays and Shales — This distinction is recognized in geology also. It 

 does not necessarily mark any difference in quality or in chemical compo- 

 sition, though many of the leaner varieties of clay will be found under 

 the head of shale; but it is based upon the presence of a thinly stratified 

 or fissile structure in the latter which is wanting in clays proper. This 

 structure is called lamination. It is quite possible for materials of ident- 

 ically the same chemical composition to be found in both divisions. The 

 clays proper lack this fissile texture, but between well-marked types of 

 the two extremes every gradation can be recognized. The line separat- 

 ing such a series into clays and shales will be an altogether arbitrary one. 



The clays may contain such large amounts of one or another of the 

 impurities already mentioned as of common occurrence in these deposits 

 that they can be named from these impurities. Thus we find calcareous 

 clays, silicious clays, ferruginous clays, carbonaceous clays. 



Similar divisions of shales are recognized, but in addition to those 

 already named as calcareous, silicious, carbonaceous, or bituminous and the 

 like, we sometimes speak of argillaceous shales implying a decided pre- 

 ponderance of the last named element. We also recognize alum shales, 

 pyritiferous shales and various others. Shales differ among themselves 

 very much as to hardness and other physical properties. Some of them 

 break up easily into clays, under the action of water, while others can 

 with difficulty be made to show the plasticity which is the first test of all 

 true maltered argillaceous deposits. Shales must be carefully distinguished 

 from slates, with which they often agree closely in appearance and in general 

 composition. Slates are shales or other argillaceous deposits that have 

 been hardened and otherwise metamorphosed by heat combined with high 

 pressure. Slates are among the most durable rocks, while almost all 

 shales are perishable upon exposure. 



As a rule, shales originate in deeper water than clays. The lamina- 

 tion which is characteristic of them may take its rise either from inter- 

 mittent deposit of the materials composing them, or from the effect of the 

 pressure of the overlying beds, or from both these factors combined. 



Several varieties of shale have been found to be better adapted to 

 important lines of clay manufacture than even the higher grades of clay, 

 and a great enhancement of value is in progress in respect to such 

 deposits. 



Varieties op Clay. 



Bearing still in mind that the divisions of argillaceous deposits which 

 we find it convenient to recognize are for the most part popular and not 

 scientific, and furthermore that the exact boundaries of these divisions 

 cannot be laid down, we can separate, for our present purpose, all of them 

 into two main groups, viz., high grade clays and low grade clays. 



