98 DISPOSITIONS, FRACTURES AND 



earth. It consists in that difference of position be- 

 tween approximate strata which has been called un- 

 conformable. In this modification, the strata not 

 only depart from a general parallel order, but the 

 change is so abruptly made, that one series, or stratum, 

 is found placed at angles to another, and in contact; 

 so as that the planes of the superior repose on the 

 edges of the inferior. The angle at which this species 

 of junction may take place, is variable; and its va- 

 riations lead to no conclusions that are not very 

 obvious. 



It is easy to understand the nature of this appear- 

 ance. If we examine the deserted bed of a lake, in a 

 stratified country of the usual elevated structure, we 

 shall find horizontal strata of matters more or less in- 

 durated, deposited on the rocks beneath, in reverse 

 order. Here then is a combination of events, of 

 which we see one half, and infer, with tolerable cer- 

 tainty, the other. The conclusion is natural; that a 

 period has elapsed between the production of these 

 two orders of strata, and that, during this interval, 

 the inferior series had been displaced from their original 

 position. Extending this reasoning to the uppermost 

 class of rocky strata with which we are acquainted, 

 namely, the fresh water or tertiary deposits, we assign 

 an interval of time, in the same manner, to the dif- 

 ference of position which separates them from the 

 secondary or marine strata beneath. And thus again, 

 in the same way, we discover another revolution and 

 another interval, at the boundary which separates the 

 secondary from the primary strata ; indicated equally 

 by reverse order of position. It is not the place here 

 to pursue the train of reasoning to which these ap- 

 pearances lead, nor to specify all the instances in 

 which these reversals of position occur. A more 



