Jtdy 2 2, 1875] 



NATURE 



243 



lies between the forty-i.inih and fifty-fifth parallels, and extends 

 from the base of the Rocky Mountains to a ridge of Laurentian 

 rocks that runs norlh-west from Lake Superior towards the 

 Arctic Sea, and is called by the author the "Laurentian axis." 

 This plateau is crossed by two watersheds ; one, starting from 

 the base of the Kotky Mountains at about the forty- ninth parallel, 

 runs due east to the 105th meridian, when it turns to the south- 

 east, dividing the Red River from the Missouri ; the other 

 crosses from the Rocky Mountains to the Laurentian axis near 

 the fifty-fifth parallel. The whole region between these two 

 transverse watersheds slopes gradually eastward, but is divisible 

 into three prairie steppes or plateaus of different elevations. The 

 lowest includes Lake Winnipeg and the valley of the Red River; 

 its average altitude is 800 feet. The second, or the "Great 

 Plains," properly so called, has an average elevation of 1,600 

 feet. The third or highest is from 2,500 to 4,200 feet above the 

 sea, and is not so level as the other two. 



Glacial Phenonuna of the Laurentian Axis. — The neighbour- 

 hood of the Lake of the Woods is taken by the author as fur- 

 nishing an example of the glaciation visible in many parts of the 

 Laurentian axis. This lake is seventy miles long, and has a 

 coast line of three hundred or four hundred miles. The details 

 of its outline closely follow the character of the rock, spreading 

 out over the schistose and thinly cleavable varieties, and becom- 

 ing narrow and tortuous where compact dioritic rocks, green- 

 stone, conglomerate, and gneiss prevail. The rocks both on the 

 shores and the islands in the lake are rounded, grooved, and 

 itriated. The direction of the stria; is from north-east to south- 

 west. 



Drift Plateau of Northern Minnesota and Eastern Manitoba. — 

 This plateau consists of a great thickness of drift deposits resting 

 on the gently sloping foot of the Laurentian, and is composed 

 to a depth of sixty feet or more of fine sands and arenaceous 

 clays, with occasional beds of gravel and small boulders, pro- 

 bably reposing throughout on boulder-clay. The only fossil 

 found was a piece of wood apparently of the common cedar 

 ( Thuja occidentalis). The surface of the plateau is strewn with 

 large erratics, derived chiefly from the Laurentian and Huronian 

 to the north ; but there are also many of white limestone. The 

 fossils in some of the latter bein^ of Upper Silurian age, tho 

 author is inclined to believe, with Dr. Bigsby, that an outcrop 

 of Upper Silurian is concealed by the drift deposits in the Lake 

 of the Woods region. 



Lowest Prairie Levtl and Valley 0/ the Red River. — This prairie 

 presents an appearance of perfect horizontality. The soil con- 

 sists of fine silty deposits arranged in thin horizontal beds resting 

 on till or boulder-clay. Stones were exceedingly rare. The 

 western escarpment was terraced and covered with boulders. 

 It is therefore probable that this prairie is the bed of a pre-glacial 

 lake. 



The Second Prairie Plateau is thickly covered with drift de- 

 posits, which consist in great part of local debris derived from tho 

 underlying soft formations, mixed with a considerable quantity 

 of transported material, especially in the upper layers. Large 

 erratics are in places abundant ; they consist mainly of Lauren- 

 tian rocks, but Silurian limestone also abounds. The following 

 is the percentage of the boulders from the different formation* 

 present in the drift :— Laurentian, 28-49; Huronian, 971 ; 

 Limestone, 54-01 ; (^uartiite Drift, 1-14. The last is derived 

 from the Rocky Mountains, the other three from the Laurentian 

 axis. There are also on the surface of this plateau some re- 

 markable elevated regions, apparently entirely composed of 

 accumulated drift materials. 



Ed^e of the Third Pratrie Plateau, or the Missouri Coteau, is 

 a mass of glacial debris and travelled blocks averaging from 

 thirty to forty miles in breadth, and extending diagonally across 

 the country for a distance of about 800 miles. 



'Third or Highest Plateau. — There is a marked charge in the 

 drift on this plateau, the quartzite drift of the Rocky Mountains 

 preponderating, seldom showing much glaciation. Its general 

 character may be seen from the following percentage of its com- 

 position : — Laurentian, 2705 ; Huronian, ?; Limestone, 15-84; 

 Quartzite drift, 52-10. Some of the lower parts of this steppe 

 show thick deposits of true till with wcU-glaciated stones, both 

 from the mountains and the east, and debris from underlying 

 tertiary beds, all in a hard yellowish sandy matrix. On the 

 higher prairie sloping up to the Rocky Mountains the_drift is 

 entirely composed of material derived from them. 



The Rocky Mountains themselves show abundant traces of 

 glaciation. Nearly all the valleys hold remnants of moraines, 

 some of them still very perfect. 1 he harder locks show the 



usual rounded foims, but striaticn was only observed in a single 

 locahty, and there coincided with the main direction of the 

 valley. The longer valleys generally terminate in cirques, with 

 almost perpendicular rock-walls, and containing small but deep 

 lakes. 



State of the Tnterior Region of the Continent previi us to the 

 Glacial Period. — The author considers that previous to the glacial 

 epoch the country was at about its present elevation, and that 

 its main physical features and rivcr-diainage were already out- 

 lined. Subaiirial denudation had been in operation for a vast 

 period of time, and an enormous mass^of tertiary and cretaceous 

 strata removed, 



Afode of Glaciation and Formation of the Drift Deposits. — The 

 author did not find any evidence rendering the supposition of a 

 great northern ice-cap necessary, but suggests that local glaciers 

 on the Laurentian axis furnished icebergs laden w.th boulden, 

 which were floated across the then submerged prairies towards 

 the Rocky Mountains. 



On some important facts connected with the Boulders and 

 Drifts of the Eden Valley, and their bearing on the theory of a 

 Melting Ice-sheet charged throughout with rock-fragments, by 

 D. Mackintosh. In this paper the main object of the author is 

 to defend generally received opinions, especially as regards the 

 great glacial submergence, in opposition to the theory announced 

 in the Quart. Journ. Geul. Soc. for lait February (vol. xxxi. 

 p. 55). He brings forward a number of facts aird considerations, 

 founded on repeated observations, to show that the dispersion of 

 Criffell granite-boulders is so interwoven with that of boulders 

 of porphyry and syenite from the Lake-district as to be incom- 

 patible with the theory of transportation by currents of land-ice : 

 and that the limitation of Criffell boulders along the S. E. 

 border of the plain of Cumberland to about 400 feet above the 

 sea-level is inconsistent with the idea of a boulder-chargtd ice- 

 current 2,400 feet in thickness. His main argument against the 

 theory of land-ice " charged throughout with rock-fragments of 

 all sizes," is derived from the purity of the interiors of existing 

 ice-sheets ; and he quotes Prof. Wyvillc Thomson iu support of 

 his statements. 



Observations on the unequal distribution of Drift on opposite 

 sides of the Pennine chain, in the country about the source of 

 the River Calder, with suggestions as to the causes which led to 

 that result, together with some notices on the high-level drift in 

 the upper part of the valley of the River Irwell, by John Aitken. 

 The author, in calling attention to the unequal distribution of 

 the drift on the opposite sides of the Pennine chain in this dis- 

 trict, points out that on the western side of that range an exten- 

 sive series of drift-deposits is found, spreading over the great 

 plains of Lancashire and Cheshire down to the Irish Sea. It 

 also occurs on the west flanks of the chain at elevations of from 

 1, 100 to 1,200 feet, thus rising several hundred feet above the 

 watersheds of some of the valleys penetrating that elevated 

 region. On the eastern side, however, there is, with one or two 

 slight exceptions, an entire absence of such accumulations, even 

 in the most sheltered and favourable situations, lor a distance of 

 twelve or fifteen miles from the water-parting of the country. 

 This absence of drift on the eastern side might, the author con- 

 isidcrs, be satisfactorily accounted for by supposing that the 

 transverse valleys of tne chain were, during the glacial epoch, 

 completely blocked up with congeaied snow or ice, by which 

 means all communications between the opposite sides of the 

 range would be entirely cut off. The southward flow of the ice, 

 which was probably not so thick as to cover the higher portions 

 of the chain, would, on encountering such an obstacle to its pro- 

 gress, be deflected westwards, and finally debouch into the 

 plains of South Lancashire, and would there deposit on its 

 retreat the debris it contained. — {To be continued.) 



Geologists' Association, July 2. — Mr. Wm, Carruthers, 

 F.R.S., president, in the chair. — On some of the causes which 

 have contributed to shape the land on the North Wales border, 

 by D. C. Davies. In a series of diagrams the author showed 

 the probable results of an upheaving force acting upon different 

 kinds of strata ; and, in the second part of his paper, gave a 

 detailed account of several instances, along the Welsh border, 

 where important physical features now existing had been deter- 

 mined by faults and anticlmals. These were shown in a second 

 series of diagrams in which the actual relation of numerous 

 valleys, gorges, &c., to faults, <!i:c., was pointed out. The 

 various agents of erosion such as sea-water, rain-water, and ice 

 had modified, and in some cases altered, the features due to dis- 

 turbance ; but the author claimed that a proper regard should 



