fresh set of materials to be mixed with the clays. Further than this, the action of the weather, of rivers and of the sea never ceases, so that a process of re-mixing and re-sorting of materials is continuously taking place, and has been doing so for countless ages. It is, therefore, a legitimate cause for wonder that such enormous deposits of clays of so uniform a character should occur throughout the length and breadth of Europe, and practically throughout the world. For although the composition of many of these beds is of a most highly complex nature, the general properties such as plasticity, behaviour on heating, etc., remain remarkably constant over large areas of country, and the clays of each geological formation are so much alike in different parts of the world as to be readily recognized by anyone familiar with the material of the same formations in this country. Considerable differences undoubtedly exist, but these are insignificant in comparison with the vastly different circumstances under which the deposits were accumulated. Leaving the consideration of the modes of formation of the various clay deposits to later chapters (III and IV), it is convenient here to enumerate some of the chief characteristics of the different clay deposits and their associated rocks. In this connection it is not proposed to enter into minute details, but rather to indicate in broad outline the chief characteristics of the clays from the different deposits. This general view is the more necessary as clay occurs in each main geological division of the sedimentary rocks and in almost every sub-division in various parts of the world. The =Precambrian, Cambrian, Silurian and Devonian= 'clays' are chiefly in the form of shales or slates, the latter being clays which have undergone a metamorphic change; the latter resulted in the production of a hard and partially crystalline material with but little potential plasticity and therefore of small importance for the ordinary purposes of clay working. _Slates_ are distinguished from shales by their splitting into thin leaves which are not in the plane of original deposition, but are due to the deposited material being subjected to great lateral pressure. The re-arrangement of the particles thus produced has imparted to the material a cleavage quite independent of the original lamination. The shales in these formations are occasionally soft and friable and are then termed _marls_, but this name is misleading as they contain no appreciable proportion of finely divided calcium carbonate as do the true marls[8]. [Footnote 8: Readers desiring more detailed information on the occurrence of the clays mentioned in this chapter should refer to the author's _British Clays_ (No. 2 in Bibliography).]