INTRODUCTION. 9 



sented by the missing groups. And again, if a whole 

 system of rocks be absent, we may reasonably suppose 

 that the area in question was land throughout the whole 

 of that particular period. 



If, however, the gap is very great, and several systems 

 of rocks are missing, we must not conclude that the area 

 has been continually above water during the whole of the 

 periods of time which are unrepresented. The area may 

 have been submerged more than once in the interval, and 

 may have received deposits belonging to more than one of 

 the absent systems, but all remnants of these deposits 

 may have been swept away during the erosion which 

 accompanied and followed the last elevation. Thus the 

 Trias rests in many places on Silurian or Devonian rocks, 

 but it is probable that in most of these cases the older 

 rocks had sunk beneath the Carboniferous sea, and had 

 originally been covered with some portion at least of the 

 Carboniferous system; the Dyassic or Permian period, 

 which preceded the Carboniferous, was one of great dis- 

 turbance and denudation, and large areas of Carboniferous 

 strata were then broken up and removed, so that the 

 absence of these strata at the localities in question was 

 probably due to this erosion. Such an unconformity may 

 then be evidence of land in Dyassic times, but affords no 

 clue to the position of the land tracts in the Carboniferous 

 period. 



In cases of unconformity the rocks which rest directly 

 upon the old land surface are generally conglomerates 

 or pebbly sandstones, the materials of which have been 

 derived from the rocks of which that land was composed. 

 Such conglomerates often cover very large surfaces, but it 

 is well to remember that such beds are not the invariable 

 accompaniments of unconformity. Thus, where a tract of 

 land has sunk slowly beneath the sea for a long period of 

 time, so that the newer deposits have overlapped one 



