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AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES. ] 



Art. X. — A Contribution to the Study of Contact Meta- 

 morphism ; by J. Morgan Clements. 



[Published by permission of the Director of the U. S. Geological Survey.] 



The various Huronian sediments which form a great por- 

 tion of the iron-bearing districts of the Upper Peninsula of 

 Michigan have in all of these districts been found to be pene- 

 trated bj dikes of igneous rocks, which are predominately 

 basic in character. The influence which these dikes exert in 

 the formation of ore bodies where they cut the iron-bearing 

 formations has been carefully worked out and described by 

 Yan Hise in numerous papers. The effects of contact meta- 

 morphism produced by the dikes upon the intruded sediments 

 are of much less economic importance, and, to a very consider- 

 able extent, have been overlooked. In this paper I shall 

 describe the products which have resulted from the intrusion 

 of basic dikes in the Mansfield slate formation, a Lower 

 Huronian iron-bearing formation of the Crystal Falls district 

 of the Upper Peninsula of Michigan.* 



The Mansfield formation, in its typical development, occu- 

 pies a narrow valley, about three miles in length, through 

 which flows the Michigamme River. In this valley there is 

 situated the village of Mansfield and the mine of the same 

 name. The formation consists of graywackes, slates (these 

 predominate, hence the name), and phyllites, of demonstrably 

 sedimentary origin. Associated with these, either as beds or 



* For details concerning the position which the Mansfield formation holds with 

 reference to the other Huronian rocks, the reader is referred to an article entitled 

 The Crystal Palls Iron-bearing District of Michigan, soon to be published in the 

 19th Ann. Rept. U. S. Geol. Survey, and in a Monograph of the Survey. 



Am. Jour. Sci.— Fourth Series, Vol. VII, No. 38.— February, 1899. 

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