SEARCH FOB COAL. 



151 



sition or partial denudation of a single bed, and produced 

 the thinning out of a particular stratum, as exhibited in the 

 diagram annexed. (Fig. 86.) 



AGE OF MOUNTAIN-CHAINS. The conformable or uncon- 

 formable position of the strata affords a safe and satisfactory 

 guide to many investigations of interest and importance ; 

 from data thus furnished we learn that the mountain-chains 

 were not all of contemporaneous origin, but have been raised 

 at different epochs. Thus, if on the sides of one mountain 

 (fig. 87) we find a series of strata, a, raised and covered 

 unconformably by another group, b, it is obvious that the 

 central chain must have been thrown up after the series a 

 had been deposited, but before the formation of the beds b. 



FIG. 87. 



But if on the sides of another mountain (fig. 88) we find 

 both the series a and b tilted and covered unconformably by 

 another series, c, we have proofs that this mountain-chain is 

 of more modern date than that on the sides of which the 

 same strata, b, are undisturbed. 



SEARCH FOB COAL. The dip of strata is frequently con- 

 nected with questions of practical as well as of theoretical 

 interest. We will suppose a case one of very frequent 

 occurrence where a landowner, who is aware that coal 

 exists on an adjacent estate, is desirous to ascertain if it is 

 also to be found on his own, and if the attempt to discover 



