256 ANNUAL OF SCIENTIFIC DISCOVERY. 



the frontier line of the United States, and westward from Lake Su- 

 perior to the Pacific Ocean. 



It began by showing that the central portion of North America is 

 a great triangular plateau, bounded by the Rocky Mountains, Alle- 

 ghanies, and Laurentian axis, stretching from Canada to the Arctic 

 Ocean, and divided into two slopes by a water-shed that nearly fol- 

 lows the political boundary line, and throws the drainage to the Gulf 

 of Mexico and the Arctic Ocean. The northern part of this plateau 

 has a slope, from the Rocky Mountains to the eastern or Laurentian 

 axis, of six feet in the mile, but is broken by steppes, which exhibit 

 lines of ancient denudation at three different levels : the lowest is of 

 fresh-water origin ; the next belongs to the drift-deposits ; and the 

 highest is the great prairie-level of undenuded Cretaceous strata. 

 This plateau has once been complete to the eastern axis, but is now 

 incomplete along its eastern edge, the soft strata having been removed 

 in the region of Lake Winnipeg. 



The eastern axis sends off a spur that encircles the west shore of 

 Lake Superior, and is composed of metamorphic rocks and granite, 

 of the Laurentian series. To the west of this follows a belt where 

 the floor of the plateau is exposed, consisting of Lower Silurian and 

 Devonian rocks. On these rest cretaceous strata, which prevail all 

 the way to the Rocky Mountains, overlaid here and there by de- 

 tached tertiary basins. 



The Rocky Mountains are composed of carboniferous and Devo- 

 nian limestones, with massive quartzites and conglomerates, followed 

 to the west by a granitic tract, which occupies the bottom of the 

 great valley between the Rocky and the Cascade Mountains. The 

 Cascade chain is volcanic, but the volcanoes are now inactive ; to 

 the west of it, along the Pacific coast, cretaceous and tertiary strata 

 prevail. The description of these rocks was given with considerable 

 detail, on account of their containing a lignite which for the first time 

 has been determined to be of cretaceous age. This lignite, which 

 is of very superior quality, has been* worked for some years past by 

 the Hudson Bay Company, and is in great demand for the steam 

 navy of the Pacific station, and for the manufacture of gas. Exten- 

 sive lignite deposits in the prairie were also alluded to; and, like 

 those above mentioned, were considered to be of cretaceous age ; 

 but, besides these, there are also lignites of the Tertiary period. 



CHANGES OF CLIMATE INDUCED BY CHANGES IN THE EARTH'S 



AXIS. 



The following letter has been addressed to the London Athenceum 

 by Professor Airy, the British Astronomer Royal, in answer to the 

 speculations of Sir Henry James (see Annual of Scientific Discovery, 

 1861, p. 287), Chief of the Ordnance Survey of Great Britain: ' 



" On the possibility of a great change in the terrestrial position of 

 the earth's axis of rotation having been produced by the elevation 

 of mountain masses," Mr. Airy says, " I do not question the accuracy 

 of the principle of Sir Henry James's speculations ; but I very greatly 

 doubt the adequacy, in magnitude, of the cause to explain the sup- 

 posed effects. 



