40 GEOLOGY OF OHIO. 



deemed best to furnish to all such readers as intelligible an account as 

 possible of the origin and classification of clays and shales, as well as a 

 description of the uses to which they are being applied in the state at the 

 present time. This, however, will involve a repetition of the statements 

 and discussions of the former chapter on the. subject ; and inasmuch as 

 the mere change of form in the statements for the sake of change can be 

 of no value, the materials of the previous chapter will be used in the pres- 

 ent report with all freedom. The divisions of the chapter of Volume V 

 will be for like reasons employed in the present report, with but slight 

 . modifications. The present chapter will be devoted to the origin, com- 

 position and varieties of clay. 



SECTION I. 



The Origin and Composition and the Varieties of Clays. 



Origin op Clay. — What is clay? No substance entering into 

 the composition of the earth is more commonly met or more familiarly 

 known. All persons of ordinary intelligence have a more or less definite 

 idea as to what is meant by the word. As ordinarily used, ' clay denotes 

 any earthy substance which can be worked up with water into a plastic 

 mass and then retain the shape into which it has been formed, when dried. 

 Clay and sand are two of the most common products of the decomposi- 

 tion of the older rocks that constitute what is familiarly known as the 

 crust of the earth. They enter into almost all soils and generally their 

 aggregates make nine-tenths of these soils. Desert plains and barren 

 mountain sides as well are largely covered with clay or sand. And the 

 same is true to a great extent of the floor of the sea, especially that por- 

 tion of it that constitutes the margins of the continents. 



So wide a range of distribution naturally suggests great variety of 

 composition in the substances that are called clay and the most casual 

 examination confirms this expectation. Varying and ever-changing pro- 

 portions of sand, iron oxide, lime and organic matter and fragments of 

 many kinds of rocks are found associated in masses to which we are 

 obliged to give the name of clay. Strictly speaking, however, the term 

 applies to a single mineral, viz., silicate of alumina or kaolinite. This 

 mineral in a pure state is of comparatively rare occurrence, and large and 

 accessible accumulations of it become of great economic value in many 

 parts of the world. But it is this mineral that makes the basis of all 

 clays. The percentages of it vary indefinitely in their composition and, 

 as already stated, other substances in considerable variety are united with 

 it under a common designation. The larger the percentage of kaolinite, 

 the more characteristic is the clay containing it. 



Kaolinite is not an original product of the earth's crust. It is 

 always the result of the decomposition of other and more complex min- 

 eral aggregates. The one great source of it is felspar in one or the other 

 of its leading divisions. The most abundant mineral in the crust of the 



