f 96 On the Causes which have operated 



tlie hollow immediately above a vallev, where I expressljf 

 limited their former existence. Mr. Farey also thinks the 

 Cumberland and Westmorelai>d lakes, not having given 

 wav and becon)e valleys (an absurdity I never imagined^ — 

 flats, and not Valleys, are what I have supposed former lakes 

 to have left,) afford strong presumption, that my notions on 

 the subject are erroneous; but he ought to have known 

 that there are two distinct natural processes by which lakes 

 may become exhausted. The one is where the stream, 

 leaving the lake, has a sufficient fail over yielding strata to 

 effect the excavation of a valley back into the lake. The 

 other is where the stream flowing into the lake brings 

 down alluvia sufficient in time to till it up. In general both 

 of these operations are in action at the same time, and that 

 was strictly the case with the lakes which I described as 

 having existed in the course of etreams. There arc also 

 two distinct circumstances which mav prolong the existence 

 of lakes. The one is where the fall in the stream leaving 

 the lake is too inconsiderable either to give the necessary 

 force to the descending power of the stream, or to admit 

 of its cutting a channel backwards sufficiently deep for the 

 discharge of the lake. The other is where, with a suffi- 

 ciency of fallj the stream is crossed by a stratum of indu- 

 rated rock, so durable as effecluallv to resist the abrading 

 forces of the stream. The former is the case witii several 

 of the principal Scotch lakes, which are situated near to, 

 and but little above, the level of the sea. The latter is the 

 case with almost all the numerous lakes in Canada, where 

 the many magnificent falls, on the same streams, and al- 

 ways below the lakes, incontrovertiby point out the cause 

 which has preserved the lakes. 



Mr. Farev asserts, that " it is bv no means a general 

 truth, that, if the matters removed from the strata to form 

 valleys were replaced, there would still be a sufficient fall 

 in the country for the streams to flow the sa7)ie wny." I 

 certainly never meant, nor could any one understand me 

 to mean, that the streams would flow over precisely the 

 identical channels whicli thev now do, but in nearly the 

 same way or direction ; and I conceive, that when a very 

 great majority of the instances in a case are similar, that 

 similaritv constitutes a general truth. Now out of the many 

 hundred streams which Mr. Farev has seen, he points out 

 two solitary instances onlv, as differing from mv descrip- 

 tion ; adding, mtleed, that other instances are very nume- 

 rous, and that f shall be forced to admit that excavated 



valcft 



