in Europe and North America. 333 
the great glaciers filled them with ice; or believe, with De Mor- 
tillet, that the valleys and lake-hollows were charged with water- 
borne alluvial or diluvial débris before the glaciers ploughed it 
out." 
Allowing the hypothesis of De Mortillet, the rock-basins must 
have been twice filled with water; but, according to my hypo- 
thesis, they did not exist as lakes till after the disappearance of 
the glaciers. 
But the glacier map of ancient Switzerland shows that the 
areas now occupied by the great lakes, both north and south of 
the Alps, have all been covered with glaciers. No Tertiary de- 
posit, of an age between the close of the Miocene and the com- 
mencement of the Glacial epoch, lies between the Alps and the 
ura; and, had the hollows of the lakes existed prior to the 
great Glacial epoch, we ought, but for some powerful wasting 
agent, probably in these hollows, still to find some traces of fresh, 
Water deposits, perhaps of the age of part of the Crag. No such 
télies exist. 
The Great Lakes. Lake of Geneva.—The Lake of Geneva is 
about 45 miles in length by about 12 in breadth, and its delta, 
yond the mouth of the river lie in the great Rhone valley, formed 
of older Tertiary and Secondary rocks. All the rest of the lake 
's Surrounded by the low country formed of the various subdi- 
Visions of the Molasse and Nagelfluh. The lake is 1230 feet 
above the level of the sea, and 984 feet deep towards the eastern 
end, according to the sounding of De la Beche.° 
Geneva itself stands on superficial débris; but the solid rock 
appears in the river-bed below Geneva, at Vernier, at the 
the lake, or 951 feet above the deepest part of its bottom. 
Any one acquainted with the remainder of the physical geo- 
gtaphy of the country will therefore see that the water of the 
€ lies in a true rock-basin. The question thus arises, How 
Was this basin formed ? 
- It does not lie in a’simple synclinal basin; for, though 
Miocene strata between the Alps and the Jura, it is evident by 
4 inspection of the country that the flexures of that formation 
are of far greater antiquity than the lake. These flexures have 
* See an admirable memoir by G. de Mortillet,“Des Anciens Glaciers du Ver- 
wut Italien des Alpes.” Milan, 1860. Though I had seen his map, I had not seen 
ro ir when I read my paper; and the passages in which it is mentioned 
ve been added as pages pass through the press. His ry leaves the 
cificulty of the first formations of the basins untouched, unless we ieve (which 
rs ot) that the Alpine valleys are lines of fracture. 
: h Philosophical Journal, 1820, ii, 107, and plate 2. 
AM. Jour. Sc1.—Szconp Surmes, Vor. XXXV, No. 105.—Mar, 1863. 
43 
