Correspondence — Dr. J. W. Dawson. 137 



ments of the general ice-cap were somewhat quickly brought to an 

 end by the submergence of the still ice- covered country in the sea. 

 I have made a similar suggestion as explanatory of the wonderfully 

 little moraine matter (distinct from pinnel or possible iceberg drop- 

 pings) to be met with in the Lake District (Geol. Mag., Sept. 1872). 



Nothing perhaps is more calculated to teach young geologists to 

 hesitate before pronouncing a drift-mound to be a moi'aine, than the 

 fact that the " practised eye " of a very great Glacialist, many years 

 ago, led him to regard the intensely marine-bedded sand and gravel 

 knolls around Carnforth as moraines. 



In the above remarks the term moraine is used in its customary or 

 Swiss acceptation, and not as including deposits of Boulder-clay, Till, 

 or Pinnel. D. Mackintosh. 



AMERICAN LAKE BASINS AND ARCTIC CURRENTS. 



Sir, — While I am much gratified by the not unfriendly review of 

 my Post-Pliocene Geology of Canada in your January Number, I 

 may perhaps be permitted to explain briefly one point of some 

 geological importance which I appear not to have stated with suffi- 

 cient clearness. I refer to the apparently contradictory statements 

 that the basins of the great American Lakes were cleaned out by cold 

 Arctic currents, and that these basins are parts of old valleys com- 

 municating with the sea, and which may have been excavated by 

 subaerial denudation. There is really no contradiction ; and as 

 the nature of our lake-basins is often misunderstood abroad, it may 

 be well to put the facts of the case plainly. 



It is well known, and may be seen by a mere glance at Sir William 

 Logan's beautiful Geological Map of Canada, that the basins of the 

 Great Lakes of the St. Lawrence are hollows of denudation, excavated 

 in the softer members of the Silurian and Devonian rocks. But 

 until attention was directed to the matter by Newberry and Hunt, 

 it was not so generally known that they are connected with each 

 other and with the sea by deep valleys, now filled up with Post- 

 pliocene deposits, and which have not been re-opened by the modern 

 rivers. Nor has sufficient attention been directed to the fact that the 

 old Post-pliocene filling remains on the "lee" or south-west side of 

 the Adirondacks and Laurentide ridge, while it has been cleaned out, 

 if ever deposited to the same extent, opposite the gap of the St. 

 Lawrence valley, and the depression north of the Lauren tides, leaving 

 the basins of the five great lakes in their present form. 



These facts imply that the original rock excavation is of 

 " pre-glacial " date, and in part at least referable to the epoch of 

 continental elevation in the Tertiary period ; and that the subsequent 

 partial cleaning out of the sediment or preservation of the basins in 

 an unfilled state, was due to oceanic currents flowing from the 

 north-east, and having the same powers of erosion and deposition 

 now possessed by the Arctic currents off the American coast. 



In short the original rock excavation may have been a process of 

 atmospheric denudation, finished in the Pliocene period. The subse- 

 quent filling and cleaning out belong to that action of the northern 



