CLAYS, THEIR ORIGIN, COMPOSITION AND VARIETIES. 53 



but in popular language they are separated into distinct groups. The 

 name "kaolin" has generally been given to deposits derived directly from 

 the decomposition of felspar, though it is not strictly confined to such 

 products. But the term "fire clay" is restricted to clay deposits of more 

 or less purity that are found in our several series of stratified rocks and 

 notably in the underclays or slates of coal seams. Coal Measures are 

 universally Clay Measures as well. 



It must be distinctly borne in mind, however, that the designation as 

 commonly used in the state at the present time is no guarantee of the 

 quality of the deposit to which it is given. The name is applied indis- 

 criminately to all underclays of coal seams without any reference whatever 

 to their grades or composition. Hard clays that come fully up to the 

 kaolin standard are grouped promiscuously with soft or plastic deposits 

 that may be very low in the kaolin base and very high in all the elements 

 that impair the quality. The truth is, the word has lost its proper restric- 

 tion of meaning, and unless its appropriate limits are restored it would be 

 better if it could be dropped altogether. A fire clay, properly speaking, 

 is what its name indicates, a refractory clay, and therefore a clay of high 

 grade. It becomes white by calcination. The term becomes positively 

 misleading when applied to fusible clays that will melt into slag at 

 moderate temperatures. But by far the largest use of it in the state at 

 the present time is in application to this last named class of clays. In 

 this report the designation will be limited as strictly as possible to refrac- 

 tor}'' clays. 



Fire clays are divided into two well-marked and contrasted groups, 

 viz., non-plastic and plastic clays, The former constitute our chief supply 

 in Ohio. Non-plastic fire clan's are sometimes known as rock clays and 

 also as flint cla} T s. They exhibit when broken a smooth conchoidal frac- 

 ture. On exposure to the weather they crumble into small but angular 

 grains, beyond which the disintegration does not advance perceptibly. 

 Their particles, even when finely ground, do not show the ordinary plas- 

 ticity of clay. In this respect, one of the most important and character- 

 istic of all the clays departs notably from the definition of the very class 

 to which it belongs, but it is held in its place by its chemical composi- 

 tion and behavior. Moreover, by repeated and prolonged grinding, a 

 growing measure of plasticity is imparted to the mass. 



Plastic fire clays do not necessarily differ in appearance from- other 

 plastic clays, but chemical analysis shows the ground of their separation. 

 The bes: of them equal the best of the hard clays already described, 

 closely approaching kaolin in composition, and they are of equal value 

 for refactory materials. , 



To the question so often asked, as to what the peculiarity of the 

 hard fire clays depend upon, no full and satisfactory answer can be given. 

 The more probable explanation is that the clay has assumed a definite or 

 at least an incipient crystalline form. Under the microscope it is some- 



