50 GEOLOGY OF OHIO. 



mass of clays consist of these more or less impure products that have 

 been already described. No chemical formula can be given that will 

 include them, but they vary from one another indefinitely and endlessly. 

 The only thing essential to their retaining the name of clay is that a con- 

 siderable percentage of the kaolin base, namely hydrated silicate of 

 alumina, must enter uito all, giving them the quality of plasticity when 

 wet, which is the commonly accepted characteristic of this group of sub- 

 stances. The name of clay is, probably, sometimes applied to compounds 

 containing as smallan amount of the hydrated silicate as ten per cent. 



From the statements already made it will be seen that the clays of 

 high grade are likely to be found near the rocks from the decomposition 

 of which they are derived. They have been transported but short dis- 

 tances from the places where they were disengaged. In point of fact they 

 are often found intermingled with undecomposed rock in the very ledges 

 which gave rise to them. The well-known Cornish rock of southwestern 

 England, widely used in porcelain manufacture, is an example . of this 

 mode of occurrences. 



Clays of this character, however, constitute an extremely small per- 

 centage of the immense group of argillaceous deposits. The great 

 majority of them have been transported by the ordinary agencies of 

 rivers and seas far from their original sources and have been variously 

 blended with other products in the course of their removal. Entering 

 into stratified rocks of various grades and names, these clay deposits pass 

 through an unending cycle of change. Formed from the waste of the 

 dry land in one geological period, they may' themselves become the dry 

 land of a succeeding period, to be again removed and built into new rock 

 formations ; and to each stage the proportions and associations of the 

 clay may be different from any that have preceded it. Much of the clay 

 that takes part in the formation of the present surface was originally dis- 

 charged from the felspathic rock to which it is to be traced in the earliest 

 stages of the earth's history. The felspathic decomposition accomplished 

 in our own period is a relatively insignificant source of the clays that are 

 available td us for any of the uses to which we apply them. The soft 

 beds from which we make our building and our paving brick to-day, if we 

 could follow them through all their history, might lead us back at least in 

 part to the original granitic crust that constituted the first dry land of the 

 globe. Certainly a considerable part of the clay that we are at present 

 using in Ohio is taken from beds of Paleozoic age. 



Classification op Clays. 



There is no scientific, or, in other words, there is no exact classifica- 

 tion of the clay deposits of the earth's surface possible at the present 

 time; but for the sake of convenience, we divide them roughly into a few 

 general divisions. One of the commonest of these popular divisions is 



