4 INTRODUCTION 



still in progress. In 1799 William Smith, an English engineer, 

 announced the discovery that the order of succession of the rocks 

 might be determined by means of the fossils, or remains of ani- 

 mals and plants, which they contain. This discovery was made 

 possible by the advances in zoology, by means of which the differ- 

 ent species could be accurately discriminated. Since the earliest 

 recorded times, the animals and plants of the earth have been 

 subject to continual change, and the degrees of change give us a 

 standard of chronology, in accordance with which the various 

 groups of rocks may be arranged in their order of succession. 

 The archaeologist makes a similar use of the coins, inscriptions, 

 and objects of art, which he finds among the ruins of buried cities. 

 These enable him to determine the races of men who built those 

 cities and the dates at which they flourished, and to fix their place 

 in the general history of civilization. 



The lines along which geology has developed were nearly all 

 laid down late in the last century, or early in the present one, but 

 the progress of the science has led to many changes in men's 

 conceptions of the subject, some of which changes have been 

 revolutionary. Geology began with the study of western Europe, 

 and on account of this narrowly restricted range of view, erroneous 

 notions naturally crept into the new science. As the study has 

 been extended to other continents, new and larger views have 

 been gained, and doubtless, when the whole earth has been accu- 

 rately examined, many of our present opinions will need revision, 

 though we cannot hope ever to reach final certainty upon all 

 points. 



To many intelligent people this continual modification of scien- 

 tific opinion, which is a necessary consequence of advancing know- 

 ledge, is a source of annoyance. This attitude of mind comes from 

 a failure to discriminate between fact, and inference or hypothesis. 

 Accurately observed facts may be added to, but they remain trust- 

 worthy: the changeable element is the inference which is drawn 

 from the facts. These inferences are of very different degrees of 

 certainty. Some such deductions which were made centuries ago 

 remain unshaken to-day, while others of far more recent date have 



