46 Ice Marks in North Wales. [Jan., 



in the ' Philosophical Magazine.' What I particularly wish to call 

 attention to is the fact, that the only theory pnt forward even by 

 the most eminent of his opponents is, that the depressions in which 

 lakes he (when they are bounded by rocky strata and not merely 

 dammed np by moraines) have been formed by unequal disturbances 

 of the crust of the earth or upheavals of valley bottoms, and that 

 the ice during the glacial period may have filled up and shghtly 

 modified these basins, and also have prevented them from being 

 silted up, but did not form them. In no one case that I am aware 

 of has it been shown that the strata are thus tilted in opposite 

 directions so as to produce a lake basin, nor is any hint given why 

 these tiltings and depressions should have occurred in the proportion 

 of a thousand times in glaciated districts to once in countries that 

 have not been ice-ground. 



The suggestion that lakes, however numerous, formed beyond 

 the limits of the ancient glaciers, may have been all silted up and 

 converted into alluvial plains while those filled by ice have alone 

 been preserved, seems at first sight to meet the case, but a little 

 consideration shows that it is quite inadequate to solve the problem. 

 First, we have no right to start with any other assumption than 

 that lakes before the commencement of the glacial period were 

 distributed with some average regularity over the different regions 

 of the globe, if causes so universal as tiltings and depressions of 

 strata were the chief causes that produced them. Secondly, if the 

 present disproportion in the distribution of lakes was caused by 

 those not preserved by ice being silted up, it would show that the 

 process of filling up lakes is almost always very rapid, and therefore 

 that no lakes can be very old. The ten thousands of existing lakes 

 must therefore all have been originally formed just before the com- 

 mencement of the glacial epoch, and in a time not so long as has 

 since elapsed ; and yet, during the whole time that has since elapsed, 

 the process of lake forming must have entirely ceased over more 

 than one half of the globe ! Another, though a minor difficulty, 

 is that it is necessary on this hypothesis to suppose that the time 

 the glacial epoch lasted was many times longer than the time which 

 has elapsed since the ice left the lake basins, for we see that the 

 existing lakes have been only to a very small extent silted up, 

 whereas the supposition is that ninety-nine hundredths of the lakes 

 of all the rest of the world were silted up during that period. I 

 have gone a little into this general argument of distribution, 

 because it is one that a man who knows very little either of geology. 

 or glaciers may put forward without presumption, and also because it 

 seems to me to have been very much lost sight of in the discussion 

 of this question. We can all see that a true account of the origin 

 of lakes must explain their present most remarkable distribution, 



