652 



NA TURE 



igeneral altitude of the Cordillera belt was at this time 

 materially changed. The greater part of the accumulated 

 pressure appears to have been relieved by folding along 

 the lines of its two bordering ranges, and it seems to be 

 not improbable, as a general proposition, that changes in eleva- 

 tion affecting wide areas are due to other causes than those pro- 

 ducing mountain ranges.^ We are warranted in assuming, how- 

 ever, that a certain movement in elevation was coincident, or 

 nearly so, with that of the great disturbances above outlined, as 

 no strata representative of the Eocene period proper have yet 

 been found anywhere in the western part of Canada. The 

 entire area of the Great Plains was thus sufficiently elevated to 

 become dry land, as occurred at the same time in the Western 

 States to the south of the international boundary." 



The Eocene period thus witnessed the formation of the great 

 interior table-land, which accordingly has present no aqueous 

 formations of this age. In Miocene times, however, there were 

 large interior lakes, with deposits rich in remains of plants and 

 insects, and on the plains fluviatile gravels with mammalian 

 bones. 



The Pliocene period inaugurated another great continental 

 elevation, which continued for a long period, and in which the 

 fiords and caiions of the Cordillera were cut down by fluviatile 

 action to the sea-level of the period. Many local illustrations 

 are given in this memoir of the curious results in regard to 

 denudation which this period of rest and elevation produced. 



This leads to the glacial history of the region, the key to 

 which is believed to be found in the unequal elevation whereby, 

 while the great plains to the east remained under water, the 

 Cordilleran Ranges became covered with a great glacier dis- 

 charging north toward the Yukon Valley and the Arctic Sea, 

 and south to Puget Sound, while glacial streams ran westward 

 to the Pacific. At this time the Rocky Mountains produced 

 but few and small glaciers on their eastern sides ; but across the 

 wide sea which covered the plains the Laurentian Mountains 

 supported another nevi discharging ice in all directions. 



This was followed by what is usually called the inter-glacial 

 period, when, as is believed, the plains were slightly elevated 

 and the mountains depressed ; and this was succeeded by the 

 second glacial period, in which the mountain glaciers were com- 

 paratively small, and the depression of the plains was so great 

 that water-borne boulders were deposited at elevations of 5000 

 feet or more on the foot-hills of the Rocky Mountains. It is to 

 be noted here that the present eastward slope of the western 

 plains had not yet been impressed on them. The series of events 

 thus indicated is illustrated by the following table, which, how- 

 ever, the author regards as somewhat provisional : — 



Scheme of Correlation of the Phenomena of the 

 Glacial period in the Cordilleran region and 

 the region of the Great Plains. 



Cordilleran 

 Cordilleran zone at a high 

 elevation. Period of most 

 severe glaciation and maxi- 

 mum development of the great 

 Cordilleran glacier. 



Gradual subsidence of the 

 Cordilleran region and decay 

 of the great glacier, with de- 

 position of the boulder-clay of 

 the Interior Plateau and the 

 Yukon Basin, of the Lower 

 boulderclay of the littoral, 

 and also at a later stage (and 

 with greater submergence) of 

 the inter-glacial silts of the 

 same region. 



\ Region of the Great Plains. 



Correlative subsidence and 

 submergence of the Great 

 Plains, with possible contem- 

 poraneous increased elevation 

 of the Laurentian axis and 

 maximum development of ice 

 upon it. Deposition of the 

 lower boulder-clay of the 

 plains. 



Correlative elevation of the 

 western part of the Great 

 Plains, which was probably 

 more or less irregular, and 

 led to the production of ex- 

 tensive lakes in which inter- 

 glacial deposits, including peat, 

 were formed. 



Cf. Le Conte, American Journal of Science, III. vol. xxxii. p. 178. 



NO. 1096, VOL. 42] 



Cordilleran Region. 

 Re-elevation of the Cor- 



[OCTOBER 30' 1890 



Region of the Great Plains. 

 Correlative subsidence of the 



dilleran region to a level plains, which (at least in the 



probably _as high as or some- western part of the region) ex- 



what higher than the present. 1 ceeded the first subsidence, and 



Maximum of second period of | extended submergence to the 



glaciation. j base of the Rocky Mountains 



I near the forty-ninth parallel. 



Formation of second boulder- 



i clay, and (at a later stage) dis- 



' persion of large erratics. 



Partial subsidence of the 

 Cordillera region to a level 

 about 2500 feet lower than the 

 present. Long stage of sta- 

 bility. Glaciers of the second 

 period considerably reduced. 

 Upper boulder-clay of the 

 coast probably formed at this 

 time, though perhaps in part 

 during the last. 



Renewed elevation of the 

 Cordillera region with one 

 well-marked pause, during 

 which the littoral stood about 

 200 feet lower than at present. 

 Glaciers much reduced and 

 diminishing, in consequence of 

 general amelioration of climate 

 toward the close of the Glacial 

 period. 



Correlative elevation of the 

 plains, or at least of their 

 western portion, resulting in 

 a condition of equilibrium as 

 between the plains and the 

 Cordillera, their relative levels 

 becoming nearly as at present. 

 Probable formation of the 

 Missouri Coteau along a shore- 

 line during this period of rest. 



Simultaneous elevation of 

 the Great Plains to about 

 their present level, with final 

 exclusion of waters in con- 

 nection with the sea. Lake 

 Agassiz formed and eventually 

 drained toward the close of 

 this period. This simultaneous 

 movement in elevation of both 

 great areas may probably be 

 connected with the more general 

 northern elevation of land at 

 the close of the Glacial period. 



Among the evidences given of the partial submergences and 

 differential elevations stated in this table, reference is made to 

 the " White Silt formation " so extensively distributed in many 

 parts of British Columbia, and indicating water action up to 

 levels of about 2700 feet , to the high-level terraces ; the peculiar 

 distribution of boulders from the Laurentian higiilands on the 

 eastern slopes of the Rocky Mountains ; the absence of glacial 

 abrasion on the plains ; the chemical character of the boulder- 

 clay, leading to the inference that it was formed underwater ; the 

 wide distribution and characters of the inter-glacial beds, the 

 character and position of the Missouri Coteau, and a variety of 

 other local facts. The objection that marine shells are not found 

 in the Pleistocene strata is treated thus : — 



" From what has already been said with respect to the 

 Cordillera region, and more particularly in connection with the 

 meaning which the White Silt formation appears to have in that 

 region, it seems probable that the water by which the northern 

 part of the Great Plains is supposed to have been flooded was 

 in connection with that of the sea.i In discussing the results of 

 my earlier investigations of the superficial deposits of this part 

 of the plains, in reference to a theory of their submergence, I 

 have stated that after a certain stage the waters entering from 

 the north and south must have formed an open strait between the 

 Arctic Ocean and the ocean to the south.- This was written, 

 however, under an assumed limitation implying an equal sub- 

 sidence of the continent ; and at the time no satisfactory informa- 

 tion was available respecting the position of the margin of the 

 glacial deposits in the corresponding western part of the United 

 States, such as has since been supplied by the work of Chamberlin, 

 Salisbury, Todd, Wright, McGee, Upham, and others. The re- 

 sult of these new facts appears to show that, instead of opening 

 broadly southward as well as to the north, any body of water 

 covering the northern part of the Great Plains could have had 

 only a tortuous and comparatively narrow communication with 

 the sea to the eastward, round the front of the great confluent 

 Laurentide glacier, and that even this communication was 

 probably formed only at the time during which the plains 

 stood at the lowest level indicated by the spread of the drift 

 deposits. If such conditions may be assumed as probably 



' It must still, however, be^admitted as possible, that a great lake was in 

 some manner produced, in the region of the plains, with a height somewhat 

 exceeding that of the sea. 



^ " Geology and Resources of the Forty-ninth Parallel," p. 255. 



