Reviews — Dr. James Geilde's Great Ice Age. 37 



Prof. Chamberlia represents the Drift deposits of the great plain 

 region of North America as a series of sheets overlapping each other 

 in intricate fashion, and in this case only that outer zone or mai'gin 

 of each sheet which was not reached by the one next succeeding, 

 retains its original form (except as modified by superficial agencies), 

 whilst the- inner buried zone was liable to much alteration by the 

 over-riding ice of the later advances. It ought to be remembered 

 that the outermost zone of deposits, which are called the earliest for 

 convenience sake, are really those of maximum glaciation — the mid- 

 winter of the Glacial period. The extent of the imbrication of the 

 glacial deposits largely depends on the oscillations of the ice-margin 

 and the intensity of the ice-action. If the ice-margin, in its advance, 

 pushed forward all the loose debris, there would remain a series of 

 concentric moraines instead of the imbricated sheet. 



The constituents of the Drift deposits in North America closely 

 resemble those of Europe. The Till or Boulder-clay is precisely 

 similar in character. Individual sheets of it have an average thick- 

 ness ranacina: from 20 to 60 feet; its maximum thickness in a few 

 places is known to exceed 500 feet. Terminal moraines are truly or 

 gigantic size : some have been traced for several hundred miles in 

 individual distinctness. The chief of the complex belt of moraines 

 has been followed from the Atlantic coast over the Appalachians, across 

 the broad Mississippi basin, and then far out upon the North-wester i 

 plains of Canada, and it may possibly have reached to the Arctic 

 seas. The other products of glacial action, such as Drumlins, Asar, 

 Kames, Glacio-fluvial Aprons, Valley Drift, and Loss are also repre- 

 sented on an enormous scale. 



Professor Chamberlin has tentatively proposed to subdivide the 

 glacial deposits of North America. The lowest sheet is named the 

 Kansan formation from the fact of its reaching as far as what are 

 now the dry hot plains of Kansas, 1500 or 1600 miles distant from 

 the centre of the radiation of the ice-sheet. This lowest deposit is 

 essentially of Till, with assorted beds of sand, clay, and gravel. 



The Kansan formation is followed by a later drift, named the East- 

 lowan, but between these two there is a well-developed soil-horizon, 

 consisting of peat, logs, twigs, stems, and other vegetable debris, 

 in which have been recognised remains of pine, oak, elm, sumach, 

 walnut, ash, and hickory trees, with bones of Equus, Lepus, and 

 Mephitis. Overlying the East-Iowan formation is a second horizon 

 of soil and plant accumulations, and this in its turn is overlapped 

 by a very marked glacial drift, the East- Wisconsin formation, the 

 outer margin of which is formed by a distinct terminal moraine, 

 usually one to five miles wide, and from 50 to 300 feet thick. The 

 retreat and melting back of the ice resulted in the formation of 

 immense lakes. 



Very various opinions are held by American geologists as to the 

 chronological interpretation of these various formations ; some main- 

 tain the view of two periods of glaciation and two intervals of 

 deglaciation ; but there is a growing disposition amongst field 

 workers to recognise three distinct Glacial, and as many inter-Glacial 



