Davis.] 
342 
[January 18 , 
proved, as many of these lakes have other and sufficient causes ; 
second, that the distribution or location of true rock-basins is 
not sufficiently controlled by glacial but rather by orographic 
conditions. 
In spite of this disagreement with the more pronounced glacial- 
ists, I have no intention of siding with the extreme conservatives. 
Glaciers have undoubtedly been important in cleaning out, more 
or less completely, many partly filled old lakes, so giving them a 
Longer lease of life ; they have frequently conspired with other 
causes, constructive, obstructive, or both, and lent their aid to the 
formation of a basin ; and from their own exertions alone they 
have greatly increased the list of lakes by the addition of numer- 
ous tarns, both existing and extinct. 
It may be well to name certain lakes, frequently accounted 
rock-basins, and refer them to the species to which we consider 
them as properly belonging; but the difficulty here arises from the 
almost universal complexity of causes by which lakes have been 
produced, and the impossibility of giving a quantitative estimate 
to the share of each. We may begin the attempt with the classic 
lakes of Switzerland. 
Ramsay considers all the larger ones as essentially excavated by 
ice, on the line of preglacial valleys ; most Swiss geologists give 
ice-action a very subordinate place. Following the latter, we 
would place all the northern marginal lakes under the W arped 
Valley Basins, (p. 330), the lakes of the plain about Zurich under 
Moraine Barriers (C. 3), and the Italian lakes as belonging to 
both these species. As nearly all these lakes are drained by out- 
lets running through valleys that are deeply silted up with allu- 
vium, something of the present height of their water surface is 
pretty surely due to drift obstruction (C. 4). 
Norway and Sweden 1 undoubtedly possess many true rock- 
basins of glacial excavation, but I question whether drift barriers 
may not explain the greater number of lakes there, as they do in 
Finland : the same remark applies to Scotland 2 and Ireland, 
1 A. Helland, Die glaciate Bildung der Fjorde und Alpenseen in Norwegen. Pogg. 
Ann., cxlvi, 1872, 538-562. 
2 J. Geikie advocates a glacial origin for all the larger Scotch lakes. Great Ice 
Age, 1877, 267. 
