The thoroughness with which the methods of physical chemistry have been applied to geological and mineralogical problems during recent years has been of very great assistance to the student of clay problems, as will be seen on studying some of the works mentioned in the short bibliography at the end of the present volume. When the principles of hydrolysis, ionization, mass reaction and reactional velocity have been applied in still further detail to the study of clays, our knowledge of their natural history will increase even more rapidly than it has done during the past few years. No industry exercises so great a fascination over those engaged in it as do the various branches of clayworking; no other substance offers so many problems of such absorbing interest to the artist, the craftsman, the geologist, the chemist and the general student of nature, whilst the differences in legal opinion as to the nature of clay could themselves occupy a volume far larger than the present one. A. B. S. The White Building, Sheffield. _November 1911._ CONTENTS CHAP. PAGE Table of clay rocks viii I Introduction. The chemical and physical properties of clays 1 II Clay and associated rocks 48 III The origins of clays 70 IV The modes of accumulation of clays 84 V Some clays of commercial importance 103 VI Clay-substance: theoretical and actual 135 Bibliography 168 Index 170 LIST OF ILLUSTRATIONS FIG. 1 Quartz crystals 9 2 Pyrite 14 3 Marcasite 14 4 Illustrating the structure of a 'clay crumb' 24 5 Chart showing rates of drying 27 6 Seger Cones indicating a temperature of 1250° C. 34 7 Ludwig's Chart 36 8 Coal Measures sequence in North Staffordshire 55 9 Lias clay being worked for the manufacture of hand-made sand-faced roofing tiles 58 10 Oxford clay near Peterborough 60 11 Cliffs of Boulder clay at Filey lying on Calcareous Crag 66 12 China clay pit belonging to the North Cornwall China Clay Co. 72 13 Orthoclase Felspar 75 14 Illustrating the successive deposition of different strata 90