XXX OUTLINE OF THIS MOJSTOGKAPH. 



Section II. Tlio Maiistiekl slate i.s best exposed in the viciuity of the town of the same name. 

 It here occupies a valley, through which flows the Michigamme River. Petrographically this 

 formatiou iucludes graywackes, clay slate, phyllite, siderite-slate, chert, ferruginous chert, and iron 

 ores, with various metamorphic products derived from them. The slate predominates. The Mauslield 

 slates are intruded by basic igneous rocks, which underlie them. They are overlain by volcanics, 

 which contain fragments of the slates, and are hence younger than they. The Mansfield slates strike 

 north and south and dip on an average 80'^ to the west. They represent the limb of a westward- 

 dipping monocline. The maximum thickness of the belt is 1,900 feet. Followed south away from 

 the point of maximum thickness it rapidly thins out and disappears. In the slate but a single ore 

 body of commercial importance has been found. This is exploited by the one Bessemer ore-producing 

 mine of the district. The ore body is presumed to have resulted from the alteration of a siderite, 

 and the concentration in a favorable position of the iron from the portions of the ferruginous beds 

 removed by erosion. A possible continuation of the Mansfield slate is suggested by the occurrence of 

 small outcrojis of somewhat similar slates about 5 miles slightly to the west of north. 



Section III. The Hemlock formation consists almost exclusively of volcanic rocks, both basic 

 and acid, with crystalline schists derived from them. Sedimentary rocks play a very unimportant 

 role. Exposures are numerous west of the belts of previously described rocks, and where erosion 

 has removed the drift the formation has a marked influence on the topography. The thickness is 

 estimated from the dip to reach 23,000 feet, but this is probably illusory because of reduplication due 

 to folding. In the northern portion of the district the formation overlies the Kona dolomite. In the 

 southern portion it overlies conformably the Mansfield slate. It is probable that volcanic activity 

 began in the north and moved south, and that some of the volcanics to the north are contemporaneous 

 with the Mansfield slates. The volcanics are cut by a few acid dikes. Basic dikes forming enormous 

 bosses of basic rock are of frequent occurrence. The volcanic origin of the major portion of this 

 formation is perfectly clear. Some of the volcanics are submarine. The greater proportion, however, 

 were derived from volcanic vents, which could not be located, but were probably situated near the 

 Huronian shore line. The Hemlock volcanics are divided into igneous and sedimentary rocks. Under 

 the igneous rocks there are described both acid and basic lavas and pyroclastics. Under the sedimen- 

 tary rocks there are described volcanic sediments, both of eolian and water-deposited character. By 

 extreme metamorphism crystalline schists have been produced from both igneous and sedimentary 

 rocks. The acid volcanics include rhyolite-porphyries and aporhyolite-porphyry. The rhyolite- 

 porphyry shows interesting micropoikilitic textural characters. Some of the porphyries have been 

 rendered schistose by pressure. Acid pyroclastics are scarce and were derived from the aporhyolite. 

 The basic lavas correspond to the modern basalts. They are much altered. To indicate these facts 

 and at the same time show their correspondence to the Tertiary and recent basalts, they are called 

 "metabasalts." The basic lavas include uonijorphyritic, porphyritic, and variolitic types. A columnar 

 structure was not observed, but an ellipsoidal structure is very common. This structure is described 

 iu detail and the conclusion reached that basalts possessing this structure were originally very 

 viscous and correspond to the modern aa lavas. The amygdaloidal structure, which is almost univer- 

 sally present in the volcanics, is described and illustrated. The alteration of the basalts is discussed 

 and special cases described. As result of this alteration the textural as well as the mineralogical 

 characters may be completely changed, and the volcanic origin of the resulting rocks could not be 

 determined but for their association. In the zone of weathering, calcification is the controlling 

 alteration process. In rocks more deeply buried, silicification is the process which predominates. 

 The pyroclastics comprise eruptive breccia, including thereunder friction breccias and flow breccias, 

 and volcanic sedimentary rocks. The eolian deposits, which are described as tutt's, grade from fine 

 dust deposits up into tliose in which the fragments are bowlders. The water-deposited volcanic 

 fragmentals are known as volcanic conglomerates, and likewise grade from those of which the particles 

 are of minute size into those of which the fragments are of very large size. At various places 

 clastic rocks occur which are now schistose, and whose exact mode of origin — that is, whether eolian 

 or water deposited — could not be determined. Normal sediments consisting of slate with limestone 



