T1IK I.ITKIIATURE OF HISTORICAL GEOL< 33 



had a geography of its own, hut the special phaw- with which it 

 opened slowly changed toward the evolution of tin- .tiMiing phase 

 by the upheaval of one region and the subsidence of others, or by a 

 general n-ijional movement of elevation or subsidence. Sometime* 

 indeed great changes seem to have taken place within the duration 

 of a single period. The present arrangement of land and sea, as 

 well as their relative heights and depths, ia merely the last of these 

 phases, but it is at the same time a result of the geographical 

 evolution which has been in progress ever since the earth's crust 

 was cool enough for water to condense and settle upon it ; in other 

 words, modem geography is the outcome of all the past geographical 

 changes. 



The restoration of ancient phases of geography, the attempt to 

 indicate the limits of the seas and the positions of the land areas of 

 any age or period, is one of the most difficult problems that a 

 geologist can endeavour to solve. Shore-lines are seldom preserved, 

 and the essay involves a careful consideration of the conditions 

 under which various local deposits have been formed, and a studied 

 use of the scientific imagination in estimating the original extension 

 of deposits, of which only small portions are in many cases pre- 

 served or exposed. 



Such endeavours to restore the geographical conditions of past 

 periods must at present be tentative, and with respect to the older 

 periods they are probably more suggestive than real ; but they 

 exercise the faculty of inductive reasoning, and they help to show 

 the student that there are aims and objects to be attained by the 

 study of historical geology which are more interesting than the 

 mere enumeration of rock-groups or the recognition of character- 

 istic fossils. 



A brief account of the conditions under which each series of 

 rocks has been accumulated and of the relative positions of land 

 and sea in the European region during each period will be given 

 in the following pages ; but for more detailed consideration of the 

 geographical changes in the British area the reader is referred to 

 the author's treatise on the Building of the Hritith I$U$ (third 

 edition, 1911, Stanford), and to Professor Hull's Contribution* to 

 the Physical History of the British Isles (1882, Stanford). 11 

 also find maps that deal with a wider area in the last edition 

 (fifth) of De Lapparent's Traite de Geologic (1905). 



