INTRODUCTION. 11 



nations ; thus we hardly know anything of the subterranean 

 geology of the large areas which are covered by strata of 

 Carboniferous age both in England and Ireland. Other 

 areas again are concealed by the Neozoic strata, and we 

 are only just beginning to obtain some knowledge of the 

 underground limits of the older Neozoic rocks, by means 

 of the deep borings which have been made from time to 

 time in the eastern part of England. 



The imperfection of the geological record is another 

 great source of difficulty, and one which will never be alto- 

 gether overcome. The rocks which remain to us as the 

 records of any one period are but a remnant of the depo- 

 sits which were formed during that period, and yet before 

 we can attempt to restore the geography of that time we 

 must replace in imagination the rocks which are lost, so 

 as to form a conception of the space over which they origi- 

 nally extended. For some portions of the space this may 

 be easily done, as in the case of conformable beds lying on 

 each side of an anticlinal, but when the final limit of 

 a formation toward a given direction is a fault or an 

 abrupt boundary, without even an outlier beyond, we are 

 left in complete uncertainty as to the original limits of 

 those beds in that direction. Under such circumstances 

 we are obliged to fall back on general considerations, such 

 as the lithological changes seen in the exposed areas and 

 the physical characters of the country beyond the present 

 boundary of the formation ; still it must be admitted that 

 in many such cases any attempt at geographical resto- 

 ration partakes more or less of the nature of guess-work. 



I must, therefore, ask my readers to remember that 

 some of the restorations attempted in the following chapters, 

 especially those of the earlier periods of geological history, 

 are built on slight foundations, and may have to be modi- 

 fied by the results of new discoveries. In some instances 

 the facts which are known suggest different inferences to 



