Chapter I: Preface

This little book is intended for those who are not specialists in geology, and it may perhaps be accepted as a contribution for the general reader. To all who are interested in the earth, the study of rocks is an important branch of natural history. If detailed works on petrology are to be consulted later, F. W. Clarke's _Data of Geochemistry_ (Bulletin, U.S. Geological Survey, ed. 2, 1911) must on no account be overlooked. Its numerous references to published papers, and the attention given to rock-origins, make it a worthy companion to C. Doelter's _Petrogenesis_. Many things have perforce been omitted from the present essay. It seemed unnecessary to review the Carbonaceous rocks, since the most important of these have been admirably dealt with in E. A. N. Arber's _Natural History of Coal_, published as a volume in this series. I should like to have described occurrences of rock-salt, of massive gypsum, and other products of arid lands, where "black alkali" poisons the surface, and the casual pools are fringed with white and crumbling crusts. Rock-taluses, and all the varied alluvium carried seaward as the outwash of continental land, well deserve a chapter to themselves. But there is really no end to the subject, which embraces all the accumulative processes of the earth. A few vacation-journeys, judiciously planned out, teach us that text-books are merely signposts to set us on what is believed to be the way. When the path enters the great forest, or rises above green lakelets to the crags, we find there those who went before us, pointing to unconquered lands.

G. A. J. C.

Royal College of Science
for Ireland, Dublin.
_February 1912._

CONTENTS

CHAP. PAGE

List of Illustrations viii

I. On Rocks in General 1

List of the common Minerals that form Rocks 8

II. The Limestones 12

III. The Sandstones 56

IV. Clays, Shales, and Slates 78

V. Igneous Rocks 103

VI. Metamorphic Rocks 143

References 162

Table of Stratigraphical Systems 189

Index 170

LIST OF ILLUSTRATIONS

FIG. PAGE

1 Surface of Limestone plateau, Causse du Larzac, Aveyron 45

2 Ravine in Limestone, Cañon of the Dourbie, Aveyron 47

3 Waterworn cliff of Limestone, Millersdale 49

4 Limestone country dissected by ravines, Hercegovina 51

5 Sand developing from Sandstone, Cape of Good Hope 59

6 Siliceous Conglomerate, Co. Waterford 75

7 Quartzite Cone, Croagh Patrick 77

8 Shrinkage-cracks in Clay, Spitsbergen 81

9 Landslide of Limestone over Shale, Drôme 93

10 Weathering of Shale, Isère 95

11 Boulder-clay, Crich, Derbyshire (_Phot._ H. A. Bemrose) 97

12 Nordenskiöld Glacier, Spitsbergen 99

13 Sefström Glacier, Spitsbergen 101

14 Ash-layer of 1906 on Vesuvius 111

15 Puy de la Vache, Puy-de-Dôme 113

16 Granite invading Mica-schist, Cape Town 121

17 Weathering granite, Lundy Island 139

18 Granite weathering under tropical conditions, Matopo Hills 141

19 Composite Gneiss, Co. Donegal 153

20 Composite Gneiss, Ängnö, Sweden 155

(Figs. 11 and 17 are reproduced from the Cambridge County Geographies of _Derbyshire_ and _Devonshire_ respectively; the rest of the illustrations are from photographs by the author.)