74 



LYELL'S ELEMENTS OF GEOLOGY. 



Curved Strata in the Alps Unconformable Stratification 



Fig". 73. Lutschine, between Un- 



terseen and Grindelwald, 

 curves of calcareous 

 shale are seen from 1 000 

 to 1500 feet in height, 

 in which the beds some- 

 times plunge down ver- 

 tically for a depth of 

 1000 feet and more, be- 

 fore they bend round 

 again. There are many 

 flexures not inferior in dimensions in the Pyrenees, as those near 

 Gavarnie, at the base of Mont Perdu. 



Curved strata of the Iselten Alp. 



Unconformable stratification. — Strata are said to be uncon- 

 formable, when one series is so placed over another, that the 

 planes of the superior repose on the edges of the inferior. In 

 this case it is evident that a period had elapsed between the pro- 

 duction of the two sets of strata, and that, during this interval, 

 the inferior series had been tilted and disturbed. Afterwards the 

 upper series was thrown down in horizontal strata upon it. If 

 these superior beds are also inclined, it is plain that the lower 

 strata have been twice displaced ; first, when they were them- 

 selves brought into an inclined position, and a second time when 

 the superior beds were thrown out of the horizontal line. 



It often happens that in the interval between the deposition of 

 two sets of unconformable strata, the inferior rock has been de- 

 nuded, and sometimes drilled by perforating shells. Thus, for 



