698 



SCIENGE. 



[N. S. Vol. XIV. No. 357. 



CURRENT NOTES ON PHYSIOGRAPHY. 

 DIKES AS TOPOGRAPHIC FEATURES. 



Dikes are so generally the exclusive prop- 

 erty of geological study that a good illustra- 

 tion of their topographic value is a welcome 

 novelty. Rocky walls, shown by the removal 

 of the weaker country rock from a vertical 

 dike, have been occasionally mentioned in the 

 reports of western surveys, but the finest 

 example of the kind yet published is to be 

 found in the Spanish Peaks (Colorado) folio of 

 the U. S. Geological Survey, by Hills. Here a 

 great number of dikes, arranged in a roughly 

 radial pattern with respect to the denuded 

 stocks of the Spanish peaks as a center, form 

 numerous walls from 50 to 100 feet in height, 

 stretching more or less continuously for five or 

 ten miles or more. The weak horizontal Ter- 

 tiaries have been worn away ; but the dike 

 wall retains some mark of their bedding, just 

 as a casting shows the form of its mould. 

 Several excellent photographic illustrations are 

 appended ; they are destined to frequent repro- 

 duction as type examples of this relatively rare 

 class of topographic forms. 



THE PLAIN OF ST. LAWRENCE VALLEY. 



The lower St. Lawrence valley is a broad 

 and nearly level plain of post-glacial marine 

 clays and sands, concerning which E.. Chalmars 

 gives some interesting information (' Notes on 

 the Pleistocene Marine Shore-lines and Land- 

 slips of the North Side of the St. Lawrence 

 Valley,' Geol. Surv. Canada, Ann. Rep., XL 

 (1898), 1901, 63J-70J). The height of the 

 plain seldom exceeds 15 or 20 feet along the 

 river bank, but it increases towards the valley 

 sides, and reaches 400 or 500 at the base of the 

 Laurentide hills. The junction of the plain 

 with these hills forms a very irregular line, 

 often running up the river valleys in loops for 

 considerable distances. Generally speaking, 

 this line can be traced approximately on a good 

 map by the absence of lakes on the marine 

 area, whereas on the Laurentide area lakes are 

 quite numerous. Occasionally the surface of 

 the plain is seen to ascend by steps, each of 

 which has apparently been a shore-line during 

 the emergence of the plain from beneath the sea. 

 Terraces and beaches occur on the hill slopes 



above the plain ; the highest reach altitudes 

 of more than 800 feet. 



Certain parts of the plain are subject to 

 extensive landslips, apparently due to the slid- 

 ing of water-logged silts into valleys that have 

 been cut into the plain since its elevation. 

 One in 1840 left a depression with a maximum 

 depth of 30 feet below the adjoining plain over 

 an area of 84 acres in the valley of Maskinong6 

 river ; this was described by Logan {Proc. Geol. 

 Soc, London, IH., 1842, 767-769). The St. 

 Albans landslip occurred in 1894 ; here the 

 clays and sands slid bodily into the valley of 

 the St. Anne de la Perade for the space of 3J 

 miles, leaving a depression a mile wide and 

 averaging 100 feet deep. (See Laflamme, Trans. 

 Boy. Soc. Canada, XIL, 1894, 63; and Archi- 

 bald and Mackenzie, Railroad Gazette, N. Y., 

 June 29, 1894.) The most recent large slip 

 occurred in the valley of Riviere Blanche in 

 1898, leaving a depression over 86 acres in area 

 with a maximum depth of 28 feet. The softer 

 material flowed out from underneath, while the 

 upper and more coherent clay split into blocks 

 and columns which were borne away by the 

 sliding, surging mass. The movement con- 

 tinued for three hours ; clay masses being then 

 left stranded on the floor of the depression, 

 while the mud flow spread over the valley to a 

 depth of twenty-five feet or more for nearly two 

 miles. Accounts of this slip have been given by 

 Dawson (Bull. Geol. Soc. Amer., X., 1899, 484- 

 490) and Laflamme (Rep. Comm'r Col. and 

 Mines, 1898, 131). 



THE QUESTION OF PENEPLAINS. 



De Lapparent considers the origin of several 

 peneplains in France as determined by geolog- 

 ical evidence ( ' La question de peneplaines en- 

 visagee a la lumiere des faits geologiques.' 

 Verh. 7ten Internal. Geogr. Kongr. (1899), Berlin, 

 1901, XL, 213-220). It is here not a question 

 as to the occurrence of peneplains, now more 

 or less uplifted and dissected ; the French ex- 

 amples of this class of forms are so striking 

 that the author does not regard their verity as 

 a matter open to discussion. It is only their 

 origin that he studies. The peneplains more 

 fully described are found in the Ardennes, 

 Brittany, and the Central Plateau. Strati- 



