no 01 ! I II LD-GEOLo PARTI 



amon- tin- Jurassic beds, all of \vlii. li lie un- 

 conformably on the Palaeozoic rocks. 



Unconformability. In an overlap the strata 

 of one continuous unbroken series, the formation of 

 which does not appear to have been interrupted by any 

 great physical disturbance, nor even, in many cases, by 

 marked change of any kind in the general conditions 

 of deposit. The strata are throughout conformable 

 among themselves; at least no sensible unconformability 

 ran be detected between any portions of them. 1 5m 

 where the accumulation of a group of rocks has been 

 succeeded by elevation, exposure, and denudation, the 

 next set of strata laid down on this disturbed 

 denuded group will rest upon it unconformably. Thus 

 in Fig. 30 the Secondary formations (d i) lie uncon- 

 formably upon the Palaeozoic rocks (a e). It is not, 

 however, necessary, though it is usual, for the older rocks 

 to have been disturbed from their original horizontally. 

 They may obviously have been equably upraised, and, 

 after being exposed to denudation, may have been gently 

 depressed again without sensibly losing their original 

 horizontally. But such equable elevation and depres- 

 sion are not common. Some tilt has generally been 

 given to the strata, and consequently the overlying rocks 

 rest transgressively upon their upturned and worn 

 edges. 



An unconformable junction or unconformability, as it 

 is termed, is of the highest importance in the geological 

 structure of a district. It marks one of the great gaps 

 or intervals in geological history. The observer ought 

 to spare no pains to collect all the available data in 



