Mr. J. Ball on the Formation of Alpine Lakes. 501 



lake-basins, if filled with diluvium, could have been cleared out 

 by glaciers. Whatever may be the true solution of the in- 

 teresting problem proposed by M. de Mortillet, that furnished 

 by him will not, I feel sure, be accepted by geologists. 



I believe that, in indicating the presence of the sea in the 

 valley of the Po, I have pointed out the quarter whence a true 

 solution of the difficulty is to be derived. The solitary argu- 

 ment that seems to be relied upon by M. de Mortillet to nega- 

 tive this belief, is the fact that marine remains have not been 

 discovered in the diluvium. If negative evidence of this nature 

 were held to be conclusive as to the validity of geological theories, 

 we should have to alter many of the received doctrines of the 

 science. Deposits of enormously greater extent than the dilu- 

 vium in the valley of the Po are universally believed to have 

 been formed by marine action, although no marine fossils have 

 yet been identified in them ; and it is especially in regard to 

 strata formed of rolled fragments of hard rock that we should 

 expect the destruction of whatever scanty organic remains they 

 may once have contained. I may be allowed to cite the follow- 

 ing facts as illustrative of the small value of negative evidence 

 in regard to the very geological period now under discussion. 

 The presence of the sea at a considerable height on the moun- 

 tains in the British Islands during a portion of the glacial 

 period is now universally admitted ; but no marine remains have 

 been found on the lower western slopes of the mountains of 

 North Wales, although these must have been in succession por- 

 tions of the shore during the advance and subsequent retirement 

 of the sea. Only in two or three spots, at a height of from 

 1200 to 1350 feet, have sea shells been found. The chief de- 

 posit, first discovered by Mr. Trimmer, is near the summit of 

 Moel Tryfaen, a sharp conical peak, where no one would suspect 

 its presence. From this locality, as I have been informed by 

 Sir Charles Lyell, fifty-six existing species of shells, for the 

 most part Arctic or northern British, have been extracted, 

 while in another spot, where similar remains were detected by 

 Professor Bamsay, but few and imperfect fragments have been 

 found. When it is remembered that the district in question has 

 been over and over diligently examined by careful geologists, the 

 absence of organic remains over so wide an area affords a strik- 

 ing commentary on the value of negative evidence in geology. 



Even though the presence of salt water in the valley of the 

 Po during the glacial period were absolutely disproved, this 

 would not remove the possibility of the existence of an inland 

 freshwater sea in the same area, whose physical effects in the 

 production and diffusion of the diluvium would have been un- 

 distinguishable. 



