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several instances known to have been evolved after the deformation of the Pliocene, and we are thus forced to place a very late geological date upon the tilting of the Sierra Nevada orographic block.

A Feldspar-Corundum Rock from Plumas County, California. By ANDREW C. CLAWSON, Berkeley, Cal.

Mr. Turner, of the United States geological survey, has called attention to the prevalence of feldspathic "albitic" dykes cutting serpentine in various parts of the Sierra Nevada. The rock of which the present paper treats apparently belongs to this series of dykes. It occurs as a white coarse-grained dyke cutting the serpentine of the eastern flank of Spanish Peak, Plumas county. The rock is composed of 84 per cent. of oligoclase and 16 per cent. of corundum in crystals up to over two inches in length, and rather irregularly distributed through the feldspathic groundmass.

The following is an analysis of the feldspar :

SiO2, 61.36; Al2O3, 22.97; Na2O, 8.08; CaO, 5.38; H2O, 1.72. Total, 99.51; Sp. g, 2.63.

The occurrence is of special interest as one of the rare cases of a rock supersaturated with alumina, and its occurrence as a dyke in a rock devoid of alumina, soda, and lime is of especial interest as supporting a case of extreme differentiation of rock magma.

The foregoing synopses were prepared by the authors of the papers. JOHN C. MERRIAM.

REVIEWS

SUMMARIES OF CURRENT NORTH AMERICAN PRE-CAMBRIAN LITERATURE.

WINCHELL' discusses the general structural geology of northeastern Minnesota. The ancient rocks of this area he places in two main systems, the Archean and the Taconic. The former is further subdivided into the Upper and Lower Keewatin, separated from each other by an unconformity. The Pewabic quartzite also is placed with the Keewatin, but is not assigned to either of the main divisions. Overlying the Archean with strong unconformity is the Taconic, represented by Animikie and Keweenawan rocks, these divisions being supposed to represent respectively the Lower and Middle Cambrian of other parts of the country. The Coutchiching and Laurentian rocks before mapped as separate formations are now included within the Keewatin.

The Lower Keewatin comprises greenstone, with associated surface volcanics which are both subaerial and subaqueous, argyllitic slates, siliceous schists, quartzites, arkoses, "greenwackes," iron ores, and marble.

'The Geology of Minnesota, by N. H. WINCHELL, U. S. GRANT, JAMES E. TODD, WARREN UPHAM, and H. V. WINCHELL: Final Rept. of the Geol. and Nat. Hist. Surv. of Minnesota, Vol. IV, 1899, pp. 630. With thirty-one geological plates. Structural geology of Minnesota, by N. H. WINCHELL: Final Rept. Geol. and Nat. Hist. Surv. of Minnesota, Vol. V, 1900, pp. 1-80, 972-1000.

The first of these volumes contains an account of detailed field work 1 northeastern Minnesota, with incidental discussion of general problems. The area is treated by counties and smaller arbitrary geographical divisions, in the description of which several men have taken part. This manner of treatment leads to repetition in the discussion of the general geological features, and in many cases it is extremely difficult to correlate the facts recorded in the different sections.

Volume V contains an account of the general structural geology of the state by Professor Winchell based on the detailed work described in Vol. IV. This general discussion of Vol. V is reviewed, with such reference to the facts recorded in Vol. IV as is necessary to make the summary intelligible.

Dr. Grant's views, as indicated in the detailed descriptions of special areas, in some cases differ somewhat widely from those of Professor Winchell.

The greenstone, designated the Kawishiwin, is the oldest known rock in the state, and is supposed to represent a portion of the original crust of the earth. With its associated volcanic rocks it occurs in two main belts. The southern belt begins in the vicinity of Gunflint Lake and extends westward by way of Gobbemichigamma Lake, the Kawishiwi River, and White Iron Lake, to Tower, and indefinitely westward. The northern belt of greenstone enters the state from Hunters' Island, appearing conspicuously at the south side of Basswood Lake. At Pipestone Rapids and Fall Lake it widens southward and apparently unites at the surface with the southern belt, the overlying Upper Keewatin being absent for a distance of a few miles. But further west it is again divided by the Stuntz conglomerate, the northern arm running to the north of Vermilion Lake, west of which its extension is unknown, and the southern one running south of the lake.

The fragmental stratified rocks of the Lower Keewatin are most important toward the western part of the area of exposure of crystalline rocks. They occupy a wide area, south, west, and north of Tower. The iron ores of Tower and Ely on the Vermilion iron range occur in the upper part of the Lower Keewatin. It is probable that the immediately enclosing rock is a sedimentary one, although composed of the elements of a basic eruptive. The sediments extend south to the Giants Range of granite, where they are metamorphosed to mica schists by the granite. Toward the west they extend as far as the Mississippi River and its northern tributaries and across the Bowstring, although the drift prevents the delimitation of the belt. To the northwest they extend toward Rainy Lake, in this direction being converted into mica-schists and gneisses by the intrusion of granite; in unmodified form they are found at one point only on Rainy Lake. These fragmental rocks of the Lower Keewatin doubtless also underlie most of the central and southwestern part of the state as far as the Minnesota River. Here they dip beneath the later formations in the southwestern portion of the state, and probably occupy a wide patch in South Dakota. South of the Giants Range they occur also, but as they are covered by the gabbro and Animikie toward the east and the drift deposits of the St. Louis valley toward the west their geographic boundaries are mostly unknown. They appear in the central and western portions of Carlton county, where their line of separation from the Upper Keewatin is quite obscure, and in the central and western portions of Morrison county. The Lower Keewatin

marble is seen at Lake Ogishke-Muncie and at Pike Rapids on the Mississippi.

The Lower Keewatin was terminated by a period of extensive folding and intrusions of granite and basic rocks.

The Pewabic quartzite belongs with the Keewatin, but whether to the Lower or Upper Keewatin is not known. This formation includes altered quartzites and iron-ores between the granite and gabbro in the immediate vicinity of Birch Lake and small patches of similar rocks in Sec. 30-62-10; on the south shore of Disappointment Lake; on the north shore of Fraser Lake; on the south shore of Gabbemichigamma; at Akley Lake, forming the so-called Akley Lake series extending from the west side of Sec. 34-65-5 to the eastern part of Sec. 27-65-4.

The Upper Keewatin occurs in troughs in the Lower Keewatin, and particularly in one main trough the axis of which is traceable from Vermilion Lake to Saganaga Lake. The northern arm of this syncline, consisting of granites, gneisses, associated mica-schists, and in some places earlier greenstones, extends from the northern part of Vermilion Lake through Basswood Lake to the northern side of Hunter's Island. The southern arm, consisting of Lower Keewatin green-schists and other schists, penetrated by the granite of the Giants Range, extends from Pokegama Falls on the southwest toward the northeast, until cut out by the encroachment of the gabbro from the south. The Upper Keewatin consists very largely of conglomerates, but also includes graywackes, argyllites, quartzites, and jaspilites, in general coarser than those of the Lower Keewatin. Volcanic rocks are less important than in the Lower Keewatin, although still present. There is no general order of succession in the Upper Keewatin excepting that it can be said that it is in general conglomeratic at the bottom.

After Upper Keewatin time both the Lower and Upper Keewatin were subjected to another folding, the axis of which had a general parallelism with the earlier folding, with the result that the Upper Keewatin lies in narrow synclines in the Lower Keewatin and in places is nearly or quite vertical.

Associated with the Keewatin rocks are granites of at least two periods of intrusion, one later than the Lower Keewatin and one later than the Upper Keewatin. The later granite is believed to be represented by the higher parts of the Giants Range and the Snowbank Lake granite. The earlier granite is represented by the granites at Kekequabic Lake, Saganaga Lake, Basswood Lake, Burntside Lake,

Vermilion Lake, Lac la Croix, and Kabetogoma Lake. The origin of the granite is discussed and the same conclusions reached as in a previous article.'

The Taconic. This is unconformably above the Keewatin rocks. It comprises the Animikie and Keweenawan divisions.

The Animikie rocks enter the state at Pigeon Point, run westward along the international boundary to the eastern part of Secs. 22 and 27 T. 65 N., R. 4 W. They reappear again southwestward from Birch Lake on the northwest side of the gabbro mass, and thence continue along the south side of the Giants Range, constituting the Mesabi iron series, to Pokegama Falls. The higher parts of the Animikie are best. developed toward the east, while the lower parts are best developed toward the west.

The Animikie rocks comprise the Pokegama quartzite, Mesabi ironbearing formation, some limestone and slate, all strictly conformable with one another. The thickness is several hundred feet, sometimes reaching nearly 1000 feet. The dip of the series is uniformly to the south, 8° to 12°.

The iron-bearing formation and the Pokegama quartzite constitute the base of the formation. The quartzite in places is beneath the iron formation; in other places it is in the same horizon; and in still others is above the iron formation. Commonly the base of the Animikie is marked by a conglomerate, containing débris from the underlying Keewatin rocks. This is a narrow horizon which soon graduates upward into a quartzite, known as the Pokegama quartzite, from its typical development near Pokegama Falls on the Mississippi River. The thickness of the quartzite is not known to exceed fifty feet, and is sometimes less than twenty-five feet.

Above the quartzite, or in alternating beds with it, or below it, appears the iron-bearing or taconyte member of the Animikie, which contains the iron ore deposits of the Mesabi iron range. The ore is usually hematite in the western part of the range and magnetite in the eastern part. It was previously supposed to have been derived from the alteration of a greenish glauconitic sand-rock; but later work has seemed to show that the green-sand is a volcanic sand, and that the so-called taconitic rock itself has resulted from igneous forces. This

The origin of the Archean Igneous Rocks, by N. H. WINCHELL: Proc. Am. Assoc. Adv. Sci., Vol. XLVII, 1898, pp. 303, 304 (Abstract). Also Am. Geol., Vol. XXII, 1898, pp. 299-310. Summarized JOUR. GEOL., Vol. VII, 1899, p. 194.

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