File:Preliminary report on the geology of east central Minnesota including the Cuyuna iron-ore district (1918) (14577772847).jpg

Identifier: preliminaryrepor00hard (find matches)
Title: Preliminary report on the geology of east central Minnesota including the Cuyuna iron-ore district
Year: 1918 (1910s)
Authors: Harder, Edmund Cecil, 1882- Johnston, A. Walfred, author Geological Survey (U.S.)
Subjects: Geology Iron mines and mining
Publisher: Minneapolis, University of Minnesota
Contributing Library: Wellesley College Library
Digitizing Sponsor: Boston Library Consortium Member Libraries

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ite and quartz with probably a considerable per-centage of white mica in fine sericitic flakes. In the fine-grained slatylayers, sericite is very abundant, and distinctly visible mica flakes arealmost absent, while in the coarser layers numerous fine flakes of biotiteoccur, disseminated in a quartzose or sericitic ground-mass. Other min-erals which occur locally in the slate and phyllite are ottrelite, garnet,amphibole, and chlorite. Ottrelite occurs as small, scattered, thin, shinyblack plates lying parallel to the cleavage in some of the phyllite layers,and garnet also occurs locally in the phyllite in tiny, disseminated crystals.Both are rare. Garnet, amphibole, and chlorite, however, are foundmore characteristically in a number of elongated, lens-shaped, concretion-like masses which occur interlayered with the slate and phyllite. Someof these masses are five or six feet long and a foot or more thick. Theyconsist of a hard, dark, siliceous ground-mass in which are embedded PLATE VI
Text Appearing After Image:
B. SLATE OUTCROP AT LITTLE FALLS SHOWING THE RELATION OF CLEAVAGE TO BEDDING ROCKS OF EAST CENTRAL MINNESOTA Oi abundant fibrous, black amphibole or dark red garnet with associateddark green flakes of chlorite. The amphibole crystals range up to onehalf of an inch in length and show no regularity in their arrangement.The garnet crystals average about one eighth of an inch in diameter,being much larger than those disseminated through the phyllite. Chloriteis commonly associated with it, and appears to be an alteration productof it. Rarely, garnet and amphibole are found together in the same con-cretion-like lenses, but commonly they are in separate lenses. Ottrelitealso is often distinctly associated with these lenses. It does not occurwithin them, however, but in the phyllite or slate layers along their bor-ders. Amphibole, on the other hand, is found only in the concretion-likemasses, and not in the slate and phyllite. The origin of the concretion-like masses has not been determined.

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