192 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [Mar. 5, 



the Alps must, in all details, have been quite different from those of 

 Miocene age, in consequence of the great disturbance that the Alpine 

 rocks underwent after the close of the Miocene epoch, and the sub- 

 sequent formation of numerous new valleys of denudation. Traces 

 of the long lapse of time between the Miocene and the later Glacial 

 epoch are in other countries but imperfectly preserved in the sub- 

 divisions of the Crag, and of other minor formations of still later 

 date. Of the finer gradations that unite these subdi\isions few traces 

 have been described. Eor long before, and during all these Crag 

 epochs and the ages between them, of which we have little trace, 

 and during all the time that elapsed from the close of the Crag until 

 the period of extreme cold came into action, the Alps stood above the 

 sea, and, suffering subaerial denudation, valleys were being formed 

 and deepened. It is possible that, while the mild climates of the 

 Lower Crag epochs endured, there may still have been glaciers in 

 the higher Alps; but at whatever period the later glaciers com- 

 menced, those who allow the extreme slowness of geological change 

 will admit that the period was immense that elapsed during the gra- 

 dual increase of the glaciers, untU, in an epoch of intensest cold, 

 the ice abutted on the Jura m one direction, in another spread 

 far beyond the present area of the Lake of Constance, and on the 

 south invaded the plains of Lombardy and Piedmont. During 

 all that time weather and running water were at work modifying 

 the form of the ground under review. But, as I have already ex- 

 plained, these two agents were incapable of scooping out deep hol- 

 lows surrounded on all sides by rocks, and it therefore follows that 

 the lakes first appeared after the decline of the glaciers left the 

 surface of the country exposed approximately as we now see it, — 

 unless we admit, what seems to me impossible, that fractures, formed 

 at the close of the Miocene epoch, remained filled with water until 

 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 debris 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 hypothesis, 

 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 deposit of an age 

 between the close of the Miocene and the commencement of the Glacial 

 epoch lies between the Alps and the Jura ; 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 



* See an admirable memoii' by G-. de Mortillet, "Des Anciens Glaciers du 

 Versant Itahen des Alpes." Milan, 1860. Though I had seen his map, I had 

 not seen this memoir when I read my paper ; and the passages in which it 

 is mentioned have been added as these pages passed through the press. His 

 theory leaves the difficulty of the first formation of the basins untouched, unless 

 we believe (which I do not) that the Alpine valleys are lines of fracture. 



