RELATIVE AGES OF DISLOCATIONS. 



Unconformity of Stra.tijlca.tion. 



Fig. 3*. 



There is every reason to suppose that many faults are produced 

 slowly, when land is being upheaved, since the rocks which were left 

 elevated on one side of the displacement are invariably cut level by 

 marine denudation. Faults often modify the outcrop of strata owing to 

 this circumstance. Thus, in South Lancashire, a coalfield is divided by 

 a fault ; and since it was in the form of a basin, the part which was 

 thrown down shows on the surface as a large curve, while, the other 

 part owing to denudation remains as only the bottom of the basin, and 

 the curve is proportionately small. Some faults affect the rocks very 

 slightly and over a small area ; others, like the Craven fault, have a 

 downthrow of a thousand yards, and may be traced for seventy miles. 

 Relative Age of the Dislocation. The irregular operations by 

 which these disturbances and dislocations were occasioned seem to 



have happened at various 

 periods during the formation 

 of the strata. We know, for 

 instance, examples of hori- 

 zontal strata, as in figure 32, 

 resting upon other highly in- 

 clined strata, which must 

 have been forced into their 

 unnatural position before the 

 deposit of the level strata 

 upon them. 



Such a case occurs in Somersetshire, where the Coal Measures lie 

 at a steep slope beneath horizontal beds of red marl. These Coal 

 Measures are also greatly broken by faults, which in some cases throw 

 or elevate the beds on one side more than seventy fathoms above those 

 on the other side. But the beds of red marl above are altogether 

 uninfluenced either by the steepness of the dip or the abruptness of 

 the dislocations. Therefore, the convulsions by which the effects 

 were occasioned which are shown in the section happened after the 

 deposit of the coal seams and before the deposit of the red marl. 



At Aberford in Yorkshire, and at many other points along the 

 line of the magnesian limestone between Nottingham and Sunderland, 

 similar examples occur. At Yallais Bottom, near Frome, the mountain 

 limestone is found highly inclined, below level beds of oolite ; and 

 the mollusca (Lithodomi) which lived in the oolitic sea have bored 

 holes into the subjacent limestone. 



In such cases the discordance of inclination between the superior 

 and inferior strata is expressed by the term unconformity, and the 

 upper rock is said to lie unconformably upon the lower. Unconfor- 

 mity always implies an unrepresented interval of time, during which 

 (i) the sea-bed was inclined at a new angle, and (2) the previously 

 formed deposits now upheaved were denuded so as to form a new 

 horizontal surface on which the succeeding or unconformable deposit 

 was accumulated. 



Overlap. Strata are sometimes conformable in one section, and 

 yet when traced to a distance are found to be unconformable to the 





