288 AN ENQUIRY INTO THE NATURE OF [Ch. XIII. 



" though, at first sight, it may appear extremely easy to 

 observe, whether one rock rests on the upturned edges of 

 another, or not ; and so it undoubtedly is, in many cases ; 

 but when they meet at small angles, or the one rests on the 

 contortions of the other, the enquiry becomes more difficult, 

 and it requires numerous observations to be certain of the 

 general fact. When the contortions are small, the evidence 

 is distinct; but when on the large scale, the great bends 

 being measured by miles, instead of fathoms, the subject is 

 not so easy. It may be stated, as an example, that the mass 

 of the calcareous Alps is considered to rest unconformably on 

 the mass of those composed of protogine, gneiss, &c. ; but the 

 situations where the contrary opinion may be formed are 

 very numerous, the sections there exposing perfect conforma- 

 bility. It also requires great care in tracing strata up to 

 a mountain range, for the purpose of ascertaining its relative 

 antiquity, to distinguish between those beds which have been 

 decidedly upturned subsequently to deposition, and those 

 which may have originally taken a small angle during their 

 formation, on the flanks of a chain previously elevated to a 

 certain extent." Again, the same author has observed, that 

 " a general unconformability does not always prove a move- 

 ment in the inferior rocks, prior to the deposition of the 

 superior : for supposing a given series to be so produced, that 

 the newer rocks may be formed within successively diminish- 

 ing areas, and another deposit to cover the whole ; it is evident 

 that the upper mass will so far rest unconformably on the in- 

 ferior rocks, that it will cover them all in succession. Now, 

 this is what has happened with the chalk and oolite groups in 

 England. The angles at which the cretaceous and other 

 rocks meet, in Dorset and Devon, are so small, that their 

 unconformability could scarcely be determined at any par- 

 ticular point, though in the mass it is evident." 



Thus we learn that, without entering on the more intricate 

 problem of the nature of these inclined and horizontal strata, 



