4 PREFACE 



cation. It was desired to avoid, for example, the implication 

 that mid-Ordovician was a time when limestone was being 

 deposited in all seas, the sort of impression which the beginner 

 is quite likely to get from the emphasis of so-called standard 

 sections. Space has permitted only occasional reference to 

 foreign geology. A synopsis of the groups of animals and 

 plants is given because few beginning students of geology are 

 sufficiently acquainted with them to be able to read intel- 

 ligently an account of geological history. English names 

 have been used for fossils wherever practicable, and the dis- 

 cussion of the life of the several periods has been shaped with 

 the desire to afford the student a brief, simple picture of how 

 life differentiated and developed gradually toward its present 

 condition. Forms having little relation to this general theme 

 are omitted, for in a brief course it is hardly possible to be- 

 come acquainted with many specific kinds of fossils. But the 

 larger facts of evolution can be understood. The historical 

 maps are inserted, not with any intimation that they are cor- 

 rect in details, or that they will not need radical revision as 

 time goes on, but because it is believed they will help the 

 student to picture in its larger outlines the geography of the 

 continent in past periods. 



It is perhaps needless to say that work with the text should 

 be supplemented by laboratory work with the common rocks 

 and minerals, with typical fossils, with topographic and 

 geologic maps, and by field excursions. No course in geology 

 can accomplish what it should without these things. 



We are greatly indebted to Professors T. C. Chamberlin 

 and R. D. Salisbury of the University of Chicago, both of 

 whom have read the manuscript, and have made many helpful 

 criticisms and suggestions. Thanks are due also to Professor 

 C. K. Leith of the University of Wisconsin, who has read cer- 

 tain of the Chapters, and to various friends who have kindly 

 furnished photographs and helped in many other ways. 



