this angle the consideration of desert landscape features is put on a basis entirely different from that usually adopted. On the basis of an origin through means of stream corrasion, as under the influences of a humid climate, it has been the custom to regard the intermont basins as areas of vast aggradation from débris brought down from the desert ranges. The steeper slopes of the piedmont are designated by the Spanish-speaking inhabitants of the northern Mexican tableland as the bajada belt. On the general assumption that all the intermont valleys are filling up by the transference of mountain waste from the high peripheries, some physiographers have lately extended the meaning of the term bajada to cover all the plains slopes, as opposed to the central level area, or playa, which sometimes characterizes the valleys. In western United States the arid regions are peculiarly constructed. As is well known there is no marked alternation of resistant and weak strata. Hard rocks are segregated to a thickness of more than 10,000 feet at the bottom of the geologic column and above the Azoic complex; the soft beds are collected in equal thickness at the top. In the profound faulting and folding which the region has undergone, in Tertiary times chiefly, the indurated rocks are brought into juxtaposition with the soft ones. Under these conditions any method of vigorous general leveling and lowering of the country would first produce strong contrasts of relief. To attain these results it is not even necessary to postulate the immediate dislocative conditions which the basin-range theory of mountain structure demands. It is worthy of note, to emphasize the fact that the mountain blocks are subject to two kinds of erosional attack. There is the minor effect of stream action, apparently producing normal dissection, which has attracted widest attention; and there is the major effect of eolic erosion which works into the sides of the orographic blocks, producing a remarkable shelf all around at the constantly lowering level of the piedmont plain and which is the counterpart of the plain of marine erosion that Ramsey, in the middle of the last century, so strenuously advocated for the British Isles. In this connection it is unnecessary to point out or explain in detail that the locations of the orogenic fault-lines is not usually along the present scarps of the piedmonts, but far out on the plains; that there is a general absence of thick permanent wash accumulations at the foot of the mountains; that the meeting of two opposing slopes in the centre of the intermont valley seldom gives rise to master, longitudinal water-ways; that there is a general absence of distinct drainage lines on the surface of the bolsons; that the remarkable smoothness is presented by the foundation of the plains surface itself; or that the high inclination of the plains surface cannot possibly be the result of any known phase of running water. On the basis of water action there has never been any satisfactory explanation offered for any of these phenomena. Eolative influences, however, appear to account adequately for them all. DR. J. E. SPURR gives the most explicit account of the sweeping of strong winds successively up and down, across and obliquely over the intermont plains of the Great Basin region, carrying before them dense clouds of sand and dust. Nowhere are the winds uniformly vigorous throughout the bolsons. Their activities are reduced greatly on the lee side of mountains. Their strength materially increases as they recede from the near range, and is greatest against the exposed side of the far range. The medial portion of the bolson receives nearly the full force of the winds oftenest because it is exposed to their unobstructed sweep from all directions. Elsewhere I have called attention to the locus of maximum lateral deflation as the line where desert mountain meets desert plain. Under ordinary conditions we also expect the locus of maximum lowering to be the middle of the broad bolsons. According to this recognition of conditions, eolic erosion necessarily operates from the lower to the higher elevation. As shown by Professor DAVIS, the winds in their actioln are not like water dependent on the gradient of the land surface for their gravitational acceleration; they may blow violently and work effectively on a perfectly level surface. Unlike water also they may erode vigorously uphill; and this is exactly what they manifestly and constantly do on the bolson plains. Although wind erosion operates both down and up the slope, there is, owing to the peculiar configuration of each basin-shaped tract, a preponderance of effect on the up-slope part of the course. There appears to be a limit to the gradient on which the wind is able to blow sands extensively up-hill and this limit seems to lie chiefly between a two and a four per cent. gradient. It is for this reason, seemingly, that the intermont plains are so smooth, so uniform in grade, so high in gradient. Eolic gradation thus mainly works from a lower to a higher level. The direction of movement is directly oppo site to that of stream gradation. SOCIETES GEOLOGIQUES, SERVICES GEOLOGIQUES ET SOCIÉTÉS MINIÈRES. La liste suivante de sociétés géologiques, services géologiques et sociétes minières a été soigneusement compilée par le Secrétaire du Comité executif, M. W. S. LECKY, de nombreuses sources et avec l'aide de personnes et publications dont la liste serait trop longue à énumérer. Il adresse ses vifs remerciements a tous ceux qui ont bien voulu prêter leur concours a cette oeuvre. Pour toute facilité cette liste est divisée en trois parties comme suit: 1. Sociétés géologiques. 2. Services géologiques. 3. Sociétés minières. On remarquera que seules les sociétés qui s'occupent exclusivement de la géologie ou d'une branche quelconque de cette science sont comprises dans la liste des sociétés géologiques. Les nombreuses sociétés scientifiques ou d'histoire naturelle qui s'interessent à la géologie n'y sont pas comprises. Dans la liste des services géologiques, on trouvera toutefois les noms et adresses de tous les services des gouvernements qui se chargent d'un travail géologique quelconque, quoique dans un certain nombre de cas, ce travail ne constitue qu'une partie des attributions de ces services. On espère que le prochain Congrès aura à sa charge la publication d'une nouvelle édition de cette liste contenant les détails du travail, de l'organisation et des membres des diverses sociétés. SOCIÉTÉS GÉOLOGIQUES Les Sociétés suivantes se livrent exclusivement à la géologie ou à quelque branche de cette science: BERLIN. BONN. COBURG. FRANKFURT-AM-MAIN. FREIBERG, SA. GREIFSWALD. HANNOVER. JENA. JENA. KARLSRUHE. ALLEMAGNE. Deutsche Geologische Gesellschaft, Invalidenstrasse 44. Niederrheinischer Geologischer Verein, Geologisches Institut, Verein für Geologie und Palaeontologie des Herzogtums Coburg Geologische Vereinigung, Viktoria Allee 7. Freiberger Geologische Gesellschaft. Palaeontologische Gesellschaft. Niedersächsischer Geologischer Verein, Sophienstrasse 1. Oberrheinischer Geologischer Verein. Mineralogisch-Geologische Gesellschaft. Verband für Wissenschaftliche Erforschung der Deutschen AUTRICHE-HONGRIE. Geologische Gesellschaft in Wien, Geologisches Institut der Wiener Mineralogische Gesellschaft, Universität. Magyarhoni Földtani Társulat, Stefánia út. 14. BELGIGUE. Société géologique de Luxembourg, 59 rue St. Jean. Société belge de Géologie, de Paléontologie et d'Hydrologie, Société paléontologique et archéologique, Boulevard de l'Ouest. DANEMARK. Dansk geologisk Forening, Östervoldgade 7. ÉTATS-UNIS D'AMÉRIQUE. Le Conte Geological Club, University of California. Geological Society of America, American Museum of Natural The Philadelphia Mineralogical Society, Wagner Free Institute of Geological Society of Washington, U.S. Geological Survey. Secretary, Dr. R. S. BASSLER, U.S. National Museum. FRANCE. Société géologique de Normandie, Musée d'Histoire naturelle. Société française de Minéralogie, Laboratoire de minéralogie de Société géologique de France, 28 rue Serpente. ILES BRITANNIQUES. Midland Geological Society. Yorkshire Geological Society, Secretary, J. H. HOWARTH, Holly Bank, Skircoat Road, Hull Geological Society, Shakespeare Hall. Leeds Geological Association, Law Institute. Liverpool Geological Society, Royal Institution. Geological Society of London, Burlington House, Piccadilly. Secretary, Dr. G. T. PRIOR, British Museum (Natural History), Palæontographical Society. Care of Dr. A. S. WOODWARD, British Museum (Natural Royal Geological Society of Cornwall. The Edinburgh Geological Society, 1 India Buildings, George IV The Geological Society of Glasgow, 207 Bath Street. |