at variance with any known geologic facts, viz.: all acid eruptives are the result of the hydro-thermal fusion of siliceous sediments belonging to the super crust, and all basic eruptives are derived from below the super-crust of the earth. If, secondly, we make an attempt to apply this principle to the supercrust and to ascertain what order of sequence there has been in the outflow of basic eruptive material, we shall be able to identify the great epochs of molten, basic eruption by a casual examination of the strata. They have marked characters, and no geologist can mistake them except when they have themselves been subjected, as in the Archæan, to the same hydro-thermal re-fusion and change, or when they have been so modified by atmospheric agents that they are no longer massive but schistose, or have taken silica into their interstices, so that they do not or may not give a basic aggregate percentage of silica. Guided by these evident characters, I wish to call your attention to the four great epochs of basic eruption which I think are found in the lowest rocks of Minnesota. (1) The Vermilion group, so named in the fifteenth report of the Minnesota survey, embraces what have been styled frequently the crystalline schists of that region. They lie next above the Laurentian gneisses. Their manner of contact on the gneiss is remarkable. It is characterized sometimes by several alternations of gneiss with dark crystalline schist, in distinct layers from ten to fifty feet thick, evincing a gradual, or interrupted, transition from one to the other, as if of sedimentary nature. The dark element in these schists is mica or hornblende. Ata somewhat higher level in the terrane there is great confusion, and the gneissic rock is mixed confusedly with the schist, in the form of mutually penetrating dikes and isolated patches, which seem to imply a plastic condition for both. But occasionally the dark rock exists in enormously greater quantities and has almost wholly a massive structure. At the same time it becomes almost wholly hornblendic, the micaceous ingredient being visible only on exposed knobs and weathered surfaces; also in the fissures and accompanying the jointage planes. There is unmistakable evidence, which cannot be detailed here, that the mica, which characterizes the Vermilion group, is wholly the result of change from hornblende, which seems to have been, or is now at least, the basic mineral of the greater hills and the massive portions. This horizon marks the earliest known appearance, in Minnesota, of basic material in the super-crust of the earth in such amount as to constitute a terrane worthy of special designation. The upper side of these crystalline schists reveals a quiet sedimentary transition into the next formation — that named Kewatin (as limited by the fifteenth report) by Mr. A. C. Law. son of the Canadian survey. There are many alternations of acidic sediments with basic, beautifully arranged in conformable stratification. In this part of the Vermilion group the rocks are characteristically hard mica schists at first, of dark color, but become graywackes as the micaceous element fades out. It is very easy to refer this mica to a changed condition of grains of hornblende derived abundantly from the disintegration of the basic eruptive just preceding. It is also very easy to explain its gradual disappearance, as the sediments passed into the graywackes, becoming more acidic by reference to the necessary progressive silicification of the super-crust already mentioned. (2) The next eruption of basic materials succeeds the gray wackes. It has not received any systematic name, but forms the falls of Kawasachong, in Minnesota. It is not so prominently a hill-making rock, but often appears as chloritic schists, seeking more retired portions of the topography. It is also of a lighter color, and sometimes embraces grains of free quartz which seem to be indigenous. It is prominently conglomeritic, or perhaps more correctly agglomeritic, with masses of rock like itself, though containing also, in some places, abundance of jaspilyte in fragments, as well as much chalcedonic silica in small pebbles and minute grains. This rock is prevalent at the iron mines at Tower, at Ely, and at Negaunee, and is styled, when stained by iron, "paint rock," and when not stained, soapstone and greenstone. The eruptive origin of this rock at Negaunee has been shown by Dr. M. E. Wadsworth. It stretches from Tower eastward to Ely and to Ogishke Muncie lake. It was later in date than the origin of the hematites of the formation, and seems to have terminated the Kewatin formation. It is supposed to be the equivalent of the "Serpentine group" of Dr. C. Rominger, a remarkable old eruptive of the Marquette region of Michigan. The contact of this eruptive on the gray wackes about Tower is most remarkable, but cannot be described here. The graywackes and argillytes are wonderfully broken and thrust together again, and the eruptive rock seems to embrace them in the same manner as mentioned of the basic and gueissic rocks at the bottom of the Vermilion group. This horizon of disturbance has been illustrated by numerous figures in the fifteenth report of the survey. (3) We have knowledge of no formation, in the northwest, intervening between this eruptive and the unconformable overlying Animikie or Huronian. But there may have been later sedimentary deposits that are hid by this unconformable overlap. Whether there were or not, the next great epoch of basic eruption followed immediately after the Animikie, and is well described as the gabbro or Mesabi overflow. This is by all considerations the most notable eruptive terrane in Minnesota. The rock, while usually a well characterized gabbro, such as has been described by the Minnesota reports, and by the late Prof. R. D. Irving of the U. S. Geological Survey, varies to diabase and to dioritic rock, and apparently to a red-weathering diabasic rock. The minerals of this gabbro exhibit great variations in their relative amounts. At Duluth and at Iron lake, back of Grand Marais, the titanic magnetite so prevails that the mass of the rock, in some limited portions, is regarded as iron ore. In the region of Little Saganaga lake and in Carlton's peak the labradorite element is almost the sole ingredient; and in numerous instances, but in limited areas, about Birch lake, the mineral olivine is almost the only one present. This eruption seems to have flowed from numerous vents that occurred along the region where it is now found in greatest amount. It was so copious that the basaltic covering which it forms is one of the most conspicuous parts of the visible geology within much of the region of the Animike. It lies not only on the slates and quartzytes of the nearly horizontal Animike, but also extended northward so far as to lie over the graywackes, schists, and older eruptives of the Kewatin. More than that, it also flowed on northward over the granitic acid rocks of the Laurentian and conceals them from sight in considerable areas, a circumstance which seems to have induced some Canadian geologists to classify it as "Upper Laurentian." This eruptive is considered to be identically the same in characters and age as the "hypersthene-rock" of Emmons which constitutes the main mass of Adirondacks in New York. (4) The fourth, and latest, basic eruptive rock known in Minnesota is that which is seen in the great Cupriferous formation. That this is of much later date than the gabbro, although it lies immediately on the gabbro, is shown by the singular pudding stones seen near Beaver bay where large transported boulders of the light-colored "labradorite rock" of Carlton's peak are embraced in a dark matrix of dolery te belonging to this eruption. Opinion is divided yet as to the age of the copper-bearing rocks of the northwest. If they are of the age of the Potsdam sandstone, these eruptives which are interstratified with the sediments, constitute the only known instance of a general outflow at that horizon and age. If they be of Mesozoic age they have their parallels in a general eruption which is found distributed from Connecticut and New Jersey to the western territories. It is the purpose of this paper only to call attention to them, as the latest in the Northwest, and not to inquire into their precise age. THE OCCURRENCE OF CHALK IN THE NORTH AMERICAN CRETACEous. By EVIDENCE THAT THE MOHAWK RIVER AT A VERY REMOTE PERIOD CHANGED ITS CHANNEL OF DRAINAGE. By Dr. A. S. TIFFANY, Davenport, Iowa. EXTRA-MORAINIC STRIÆ IN THe Missouri valLEY. By Prof. J. E. TODD, Tabor, Iowa. THE ARCHIMEDES LIMESTONES AND ASSOCIATED ROCKS IN NORTHWESTERN ARKANSAS. By Prof. FREDERICK W. SYMONDS, Arkansas Geol. Survey, Fayetteville, Ark. GEOLOGICAL HISTORY OF THE OZARK UP-LIFT. By Prof. G. C. BROADHEAD, Pleasant Hill, Mo. ON A NEW METHOD OF CONSTRUCTING GEOLOGICAL MAPS. BY JAMES T. B. IVES, F. G. S., Toronto, Canada. SOME PHYSIOGRAPHIC NOTES ON NORTHEASTERN MINNESOTA. By Prof. C. W. HALL, University of Minn., Minneapolis, Minn. ADDITIONAL FACTS RESPECTING THE LAW GOVERNING THE DISTRIBUTION IN SPACE OF SEISMISM. By Dr. RICHARD OWEN, New Harmony, Ind. PROBABLE DERIVATION OF THE TERRESTRIAL SPHEROID FROM THE RHOMBIC DODECAHEDRON. By Dr. RICHARD OWEN, New Harmony, Ind. ON THE TRAP DIKES OF KENNEBUNKPORT, ME. By J. F. KEMP, Cornell University, Ithaca, N. Y. NOTES ON THE PRE-GLACIAL DRAINAGE OF WESTERN PENNSYLVANIA. By P. MAX FASHAY, Beaver Falls, Pa. IVORYDALE WELL IN MILL CREEK Valley, OHIO. By Prof. Jos. F. JAMES, Agricultural College, Maryland. A NEW GAS WELL AT CLEVELAND, OHIO. By H. P. CUSHING, Cleveland, Ohio. GEOLOGY OF CLEVELAND, OHIO. By H. P. CUSHING, Cleveland, Ohio. THE CLEVELAND SHALE AND ITS FOSSIL FISHES. By Prof. J. S. NEW |