10 INTRODUCTION. 



another against its sloping surface, the lower portion of 

 the newer series may be margined by conglomerates, while 

 the higher portion is not ; the latter may consist of shales 

 and sandstones, with perhaps barely a foot of pebbly mate- 

 rial at their base. Instances of such overlap with an 

 absence of conglomerates are common in the case of the 

 Carboniferous system ; where the Lower Carboniferous rocks 

 lie unconf ormably on older strata, conglomerates and pebbly 

 sandstones are always present, but where the Coal-Mea- 

 sures overlap the lower group and lie on older beds, they 

 often rest directly on the surface of the latter without the 

 intervention of any conglomerate, and with only a thin 

 pebbly basement bed. So also in the east of England, 

 where an ancient land surface is buried beneath the Creta- 

 ceous rocks, the G-ault clay overlaps the sandstones and 

 rests on the older rocks with a mere basement bed, which 

 is generally only a foot in thickness. 



When such cases occur we may perhaps infer that the 

 extent and height of the land had been so reduced by con- 

 tinued submergence that the area remaining was small 

 and of low elevation above the sea, so that the conditions 

 were unfavourable for the formation of conglomeratic beds. 

 In many cases the tract of land had doubtless been re- 

 duced to the condition of an island, an isthmus, or a low 

 promontory. 



The Difficulties of Geographical Restoration. These arise 

 chiefly from two causes the imperfection of our know- 

 ledge and the imperfection of the geological record. The 

 first is being gradually removed by the industry of geolo- 

 gists, but there are still many parts of the British Islands 

 about the geological structure of which we really know 

 very little, and there are many others about which more 

 detailed information is much to be desired. Again, there 

 are very large areas where the older rocks exist, but where 

 they are buried and concealed from view by the newer for- 



