200 GLACIAL HISTORY. 



. 



drift, and in its subsequent retreat deposited erratic blocks on 

 the surface. In the same manner were effected the dispersion 

 of the stratified masses of rolled gravel in the Sihlfeld near 

 Zurich, in the lower parts of the basin of the Lake of Constance, 

 and in the basins of the Lago Maggiore and the Lake of Como 

 south of the Alps. Glaciers must have passed over the lakes 

 upon bridges formed by ice, and have been dispersed by the 

 streams flowing from the glaciers. The conglomerate forma- 

 tion of the Schienerberg near (Eningen, the Uetliberg, and the 

 Au, already mentioned (vol. i. p. 289), probably dates from this 

 epoch. As it was deposited on the Uetliberg, the glacier must 

 have extended as far as the ridge of the Albis. 



It is manifest that the lignites of Diirnten and Utznach were 

 formed before the stratified drift, as they lie below it. To 

 judge from the plants that they contain, the climate was like 

 that of the present day. At any rate it was not warmer, as is 

 shown by the occurrence of the mountain-pine, which now only 

 descends at a few places into the temperate zone of the Swiss 

 hill-region (as on the Maneck) . The climate may, however, 

 have been rather colder, as the known species still thrive in the 

 mountain region at a mean annual temperature of from 6 to 

 7 C. (42 0t 8-44 Q 6 F.), as well as in corresponding northern 

 latitudes. Most of the mountain-pines extend up to the limit 

 of trees ; but the occurrence of the ash and yew, and of the 

 thick trunks of the pines with broad annual rings, show that at 

 the time when these trees grew in the lignite district no Alpine 

 or Arctic climate can have prevailed. The known facts lead to 

 the assumption of an annual temperature of 6-9 C. (42'8- 

 48*2 F.) . This condition of things must have lasted some 

 thousands of years, in order to produce the thick deposit of 

 peat from which the lignite originated. But gradually the cli- 

 mate became colder, the glaciers advanced from the Alps and 

 changed the whole aspect of the country. Again the tempera- 

 ture became warmer ; the glaciers slowly melted, and retreated, 

 after many oscillations, to their present limits. 



The transition from the warmth of the Miocene epoch to the 

 temperate climate of the lignite formation and the cold of the 

 Glacial epoch was probably effected very slowly, and with nu- 



