The lower bouldery terraces, two of which may be traced above Wheeler, are by far the most distinctly marked and extensively developed. The upper one shades, on the one hand, into the adjacent upland and, on the other, gradually subsides into the lower terrace of this group. This is especially seen above the Bijou Hills at Le Beau and about Chamberlain. The upper seems to mark about the surface of the former stream, while the lower sometimes corresponds to the bottom of ancient channels, one of the more remarkable of these, for example, extending southwest from American Crow Creek past Red Lake to the Missouri. From the vicinity of the mouth of Pratt Creek, both terraces rapidly decline down stream to a lower level which is reached near Yankton Agency, where they seem to coalesce and from that point they are indistinguishable. This is attended with a widening of the terrace between Yankton Agency and Choteau Creek, to a breadth of three or four miles. The following table sufficiently shows these facts. Heights on the lower bouldery terraces above the river. Traces of this terrace, more or less conspicuous, are found on most, if not all tributaries of the Missouri outside of the Second or Gary Moraine. It has been specially noted, by the writer, along White, Niobrara, James, Big Sioux, Elkhorn, Little Sioux, Boyer and Platte rivers. One-half to three-fourths the height of this terrace is usually composed of older rocks, and the upper portion of drift, sometimes stratified, but often very closely resembling till, with 10-15 feet of silt usually capping the whole. Boulders are often abundant, especially in the vicinity of the moraine, and where the terrace slopes rapidly, as near the mouth of Pratt Creek. The higher bouldery terraces are least perfectly developed. Their age, height above the river, and the probable lacustrine character of much of the river at the time of their formation accounts for this. There seems to have been but one, except near the Bijou Hills. Its height above the river does not vary greatly from 400 feet; and at the mouth of the Niobrara seems to become continuous with the upper or western limit of the drift in Nebraska. A comprehensive view of the data given leads to the following conclusions: 1. The southward sloping of the terraces does not demand, though it would favor the idea that there had been recent northward elevation. 2. The abrupt descent of the lower bouldery terraces below the Bijou Hills indicates that the Missouri has recently cut through the divide between the White and Niobrara rivers, as was suggested by the writer in 1884 (See Proc. A A. A. S., 1884). 3. Because the lower bouldery terrace extends up the James river, Pratt Creek, Okobojo, Little Cheyenne, etc., through the first moraine to the gaps of the second moraine, it is correlated in time with the occupation of the second or Gary moraine, while less confidently the higher bouldery terrace is similarly correlated with the first or Altamont moraine. SYSTEMATIC RESULTS OF A FIELD STUDY OF THE ARCHEAN ROCKS OF THE NORTHWEST. By Prof. ALEXANDER WINCHELL, Ann Arbor, Mich. [ABSTRACT.] Two entire seasons spent by the writer in the study of the Archæan rocks of Minnesota, Wisconsin, Michigan and Canada, in addition to more casual studies during many years previously, have resulted in a view which seems clear, simple and conclusive, respecting the grand divisions existing, and their equivalences at remote points. In the typical Huronian region of Canada, we find two massive quartzite terranes underlain by blackish, siliceous argillites—the “slate conglomerate" of Logan and, at bottom, another and more vitreous quartzite. Intersecting these are vast dikes of diabases, often slaty and in some places of such extent as to appear interbedded sheets. The "Animike" formation of Thunder Bay and of northeastern Minnesota is continuous with the siliceous argillites, and lithologically identical, but with accessions of flinty and jaspery beds, and especially in Minnesota of vast deposits of magnetitic schist. Underneath this system, and in marked discordance of stratification with it, is a system of argillitic sericitic and chloritic slates, which embrace the hæmatites of the Marquette, Gogebic and Vermilion regions. Unquestionably this system is not Huronian, though generally so reputed. Still lower, but stratigraphically comformable, occur crystalline schists, followed by vast beds of gneisses. The crystalline schists ally themselves with the gneisses and graduate into them. All these present the characters ascribed to the Laurentian. But it must be said that true unbedded granites are seldom if ever seen within the range of my observations. We have then the three following systems of Archæan rocks : III. A system equivalent to Huronian. THE USE OF FOSSILS IN DETERMINING THE AGE OF GEOLOGIC TERRANES. By Prof. H. S. WILLIAMS, Ithaca, N. Y. [ABSTRACT.] THE study of the data and the methods employed in comparing the Russian Devonian (as described in a paper by P. N. Veunkoff) with the Devonian of the Eifel and Ardenne regions of northern Europe (as discussed by M. Achile Six in a paper "on the Russian Devonian" in Annales de la Société geologique du Nord, xiv, p. 67-126, 1887) and these with the Devonian system in America, gives me the occasion to formulate and discuss the following proposition: Whenever the attempt is made to determine the relative geologic age of separate terranes the following rules are believed to be generally applicable. 1. It is not the more conspicuous and fixed characters of fossil species which furnish the most valuable data for determining age, but the slight and generally more or less plastic characters. 2. It is the degree of prevalence and abundance, and not the mere presence or absence of a particular temporal modification of a species that constitutes the most reliable indication of the age of a terrane. 3. Faunas taken as wholes are more valuable than individual speciesand the fauna thus considered includes the elements of relative abundance of the component species, and takes account of the relation which the epochal facies of each species bears to its typical form, and the temporal place the species occupies in the history of its genus. THE FOSSIL WOOD AND LIGNITES OF THE Potomac FORMATION. By F. H. KNOWLTON, U. S. National Museum, Washington, D. C. [ABSTRACT.] PERHAPS no American geological formation, which has been made the subject of recent investigation, has given rise to more extensive discussion or has furnished more valuable scientific results, than has the Potomac Formation. First clearly differentiated by Prof. Wm. B. Rogers, as long ago as 1840, it has during the past decade, and more particularly during the last three years, been made a special study by Messrs. McGee, Fontaine, Ward and Marsh, and at the present time the history of its deposition and abundant animal and plant life, is better known than is the history of many of the European forinations with which it has usually been correlated. Its exact stratigraphic position, however, is still unsettled, although strong presumptive evidence is at hand. It was called by Rogers, the Jurassocretaceous or upper secondary sandstone. In 1885, Mr. W. J. McGee, arguing from the then available paleo-botanical evidence, considered it to be "Lower Cretaceous in age-the American equivalent of the European Neocomian." Prof. Wm. M. Fontaine, who has so thoroughly worked up the plant impressions, regards it as Wealden, while Prof. O). C. Marsh, who has studied the numerous vertebrate remains, claims for it a Jurassic age. The Potomac Formation is remarkable for containing the oldest dicotyledonous flora yet discovered. Of the three hundred and sixty-five species of plants described by Professor Fontaine, no less than seventy-five species are dicotyledons. They do not consist of the highly differentiated genera and species which characterize the other dicotyledonous floras, such as the Dakota group, but are new and strangely archaic in appearance. My own studies of the Potomac Flora have been extensively confined to an investigation of the internal structure of the fossil wood, which is very abundant in this formation. It occurs under two widely different conditions, viz. as lignite and as silicified wood. Both these forms, as indeed all the plant remains, occur principally in lenticular pockets of hard, bluish clay, which pockets bear evidence of having been transported en masse from the original beds in which they were laid down. There is almost no transition between the two forms, although there is evidence that some of the silicified forms are also represented in a lignitized state; that is owing to different conditions of fossilization, some specimens of a species were silicified, while others were turned to lignite. In color the lignite is almost uniformly jet black. It has a specific gravity of about 1.333, and breaks with a true conchoidal fracture like ordinary anthracite. When thus broken, it does not exhibit superficially the slightest trace of organic structure. It may, however, be split along certain lines, notably in the direction of the medullary rays, when very plain structure shows superficially. The method of examining this lignite, was that recommended by Griffith and Henfrey in their Micrographic Dictionary (2d edition, p. 178), for the examination of coal. When prepared this way, the sections are tolerably transparent, and the structure is easily made out. The most casual examination shows that this material has been subjected to great pressure, which has so entirely crushed and distorted the cellular elements, that it is difficult to recognize the original form. The examination of a large series of sections serves to give a pretty correct general idea of it. In transverse section, the lumen of the cells is seen to be almost entirely closed up, the result of lateral pressure. In regard to the identification of this lignite, it is manifestly impossible to attempt more than an indication of its general character and position. That it is coniferous, is beyond question. The absence of wood elements other than tracheïds, which were provided in some cases at least, with bor. dered pits, and the number and arrangement of the medullary rays, make the coniferous nature clear. From the abundance of the genus Cupressinoxylon, in the Potomac Formation, as shown by the silicified examples, it is probable that most of the lignite may be also of this genus, particularly as there is in many cases, a marked resemblance, so far as I am able to interpret the distorted structure, between it and some of the species described from silicified specimens. Also several species undoubtedly entered into the composition of this lignite. The silicified material was examined by the methods usually employed in the study of petrographic material, viz.: by cutting thin sections and mounting in Canada balsam. This material is all coniferous. It belongs to two well-known genera, Cupressinoxylon and Araucarioxylon. Cupressinoxylon, as now accepted, is a somewhat comprehensive genus and is usually regarded as representing in a fossil state, the wood of Sequoia, or a nearly allied form. This view is strikingly confirmed in the present instance, as Professor Fontaine has described, from cone and leaf impressions, no less than twelve species of Sequoia, and typical cones of Sequoia have been found at Beltsville, Md., associated with the lignite and silicified wood. Four species of Cupressinoxylon have been detected from the Potomac Formation, and although possessing affinities with the other described forms, they nevertheless differ from them in important features, and have all been regarded as new to science. They have been named as follows: C. pulchellum, C. McGeei, C. Wardi, and C. Columbianum. The genus Araucarioxylon represents the wood of the Araucarian pines in a fossil state. The single species is described as new under the name of Araucarioxylon Virginianum. THR GLACIAL BOUNDARY IN SOUTHEASTERN DAKOTA. By Prof. G. FREDERICK WRIGHT, Oberlin, Ohio. [ABSTRACT.] THE object of this paper was to present the result of some observations made during June and July of the present season in the vicinity of the Missouri river between Yankton and Fort Yates in Dakota. The country traversed is in part that reported upon by Professor J. E. Todd at the Philadelphia meeting of this association in 1884 (see Proc., pp. 381-393). After giving a summary of the facts relating to the character of the glacial deposits near the boundary of ice action in the eastern and central states, the speaker proceeded to say: The surprising thing to a glacialist |