TEANSACTIONS OF SECTION C. 775 



during the post-Glacial period, up to what is now the 150 feet level, which 

 means a great portion of the green space of the map. 



Neither have the steppes, under 2,000 feet, been glaciated ; but many moun- 

 tain-ridges which lise over them (all under 6,000 feet) were covered with extensive 

 glaciers. 



The whole of the border-ridge has been glaciated. Immense glaciers in the 

 Tian-Shan, and immense caps of ice further north, covered it, the ice being dis- 

 charged into the typical longitudinal valleys which fringe the border-ridge, and 

 descending there to a very low level (about 1,000 feet). 



Ice also covered large part3 and tilled many valleys of the Alpine tracts which 

 fringe the plateau. The secondary smaller plateaux, rising amidst these Alpine 

 tracts (such as the Patom plateau, 58° north latitude, 11-5° east longitude, now 

 5,000 to 6,000 feet high), were totally buried under the ice. This suggestion of 

 mine has now been confirmed by Obruchefl:". 



As to the plateau itself, I am very much inclined to think that the whole of 

 the Vitim plateau and the north-west Mongolia plateau, east and south-west of 

 Lake Baikal, were totally covered with an ice cap. So also the highlands further 

 north. 



There is reason to believe that the Pamirs were ice-bound in the same way, 

 and the great extension of formidable glaciers in the Himalayas is fully proved in 

 my opinion. 



I also consider that the eastern border-ridge of the plateau — the Great Khin- 

 gan — has been ice-bound. Where I have crossed it (50° north latitude) it bears 

 traces of extensive glaciation. 



As to the southern portion of the High Plateau — Tibet, Ordos, and highlands of 

 the Hoang-ho, as also the highlands on the Amur in Manchuria and China — so far 

 as my information goes, we must suspend judgment, and are sorry that we have 

 no reliable information either in favour of or against glaciation. In fact, we know 

 nothing in this respect as regards these countries. 



But I must mention that the portions of North and Middle Asia which have 

 bee7i glaciated are, like the glaciated areas of Em-ope, always surrounded by a 

 girdle of Loess. In Turkestan and on the Lena, as well as in South Russia or on 

 the Ehine, a fringe of Loess marks the outer limits of glaciation; and those 

 geologists who consider Loess intimately connected with glaciation, and as having 

 been formed on the outer borders of large glaciers and ice caps, as it undoubtedly 

 has been in the case of Europe, will see in the Loess of China an indication, 

 though not yet a proof, of the probable extension of immense glaciers in the 

 southern part of the Great Khingan, north and west of Peking, as well as in the 

 Hoang-ho highlands. 



I leave it to persons better acquainted than myself with the geology of Persia, 

 Asia Minor, and Armenia to decide how far the south-western plateau of Asia has 

 been invaded by ice. 



My conclusion for Siberia and the adjoining parts of Mongolia might thus be 

 provisionally expressed as follows : — 



All regions 7iow over 3,000 feet of altitude have been covered either with ice caps 

 on the plateaux, or luith large glaciers in the Alpine tracts, the glaciers descending. 

 in the valleys to levels of about 1,000 /eei above the sea. Regions below 2,000 feet 

 have probably 'SOT been glaciated. 



3. On some Assumptions in Glacial Geology. 

 By Professor T. G. Bonnet, D.Sc, F.B.S. 



Three assumptions, often treated as axiomatic by modern glacialists, were 

 discussed : — 



(1) That boulder clays are ground moraines. The modes of transport of 

 d6bris by ice were described. It was admitted that, the more extensive the glacier, 

 the greater the amount (in proportion) of sub-glacial debris ; but it was denied 

 that there was any proof that such a deposit (ground moraine) ever attained s 

 considerable thickness. 



