Identifier: annualreportsofd003unit (find matches)
Title: Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society
Year: 1901 (1900s)
Authors: United States Department of the Interior
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Contributing Library: Clemson University Libraries
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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U-, Oy W.HHerron GEOLOGICAL MAI Ui^ LACCOLITHIC INT EAST OF DEAD WOOD, SOUT**))Geology by T. A.JaggarJi-. assisted h^ Scale Contoiir inten-al 50 feet1900 TWENTY-FIRST ANNUAL REPORT PART III PL. XX
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JAGGAR.) INTEU8IVE PORPHYKIES. 183 rock of granitoid texture, showing a tendency, however, to automorphisui in tliecomponent minerals; it contains hornblende, plagioclase, quartz, biotite, andaccessory orthoclase. Lamprophyre.—A basic rock in small dikes cutting the diorite-porphyry of theNeedles, and again near the head of Iron Creek; it is composed of a line automorphicaggregate of augite and feldspar, with accessory hornblende and magnetite. Irvingconsiders it the final and basic representative of the soda-rich magma that con-stitutes the principal Black Hills eruptive series. It will be seen from the above that the rocks represented are notunlike many others that have been described from laccolithic moun-tains of the Rocky Mountain region/ and it is not possible at presentto state positively whether a sharp distinction, marking differenteruption periods, may be drawn between the phonolite-grorudite andthe rhyolite-andesite magmas. The evidence points rather to a gradualtransitio

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