XXXII OUTLINE OP THIS MONOGRAPH. 



Ill Section I there are described a number of intrusive rocks which can not be connected genet- 

 ically with one another. These comprise ordinary biotite-granites, with micropegmatitic -v-arieties, 

 muscovite-biotite-granite, motadolerite, metabasalt, and liicrite-jjorphyry. Where the granites have 

 intruded the Upper Huronian series they have contorted the strata, and include and metamorphose 

 the roclvs, producing muscovite-biotite-gneiss, and staurolitiferous and garnetiferous mica-schists. 

 The metadolcrites possess no special points of interest in themselves, but where they have intruded 

 the Mansfield slates they have caused interesting exomorphism. The slates are converted into 

 adinoles, ,spilosites,and desmosites. Chemical analyses indicate the chief change which has taken 

 place in the production of these rocks from the clay slate to have been in the increase of silica and 

 soda as the contact is approached. There seems thus to have been a direct transference of sodium 

 and silicon from the igneous rock to the sedimentary. The raetabasalt dikes are of little interest. 

 The ultrabasic piorite-porphyries are extremely altered. This alteration has produced in one case 

 chiefly tremolite and in another serpentine. One of these serpentine pierite-porphyries is polar-mag- 

 netic. The pierite-porphyries are presumed to have contained a vitreous base, and correspond to the 

 modern Tertiary limburgite. 



In Section II there is given a study of a series of rocks varying from those of intermediate 

 acidity through those of basic composition to ultrabasic kinds. The exposures of these rocks are 

 found in au area underlain by the Upper Huronian series, extending from Crystal Falls southeast to 

 and a short distance beyond the Michigamme River. The prevailing rocks are, on the one hand, 

 diorites of intermediate acidity, ranging to more acid rocks, tonalites, quartz-mica diorites, and 

 granite. On the other hand, we have hornblende-gabbrosy gabbros, norites, and, lastly, peridotites 

 of varying mineralogical character. Only those kinds of rocks of which analyses have been obtained — 

 mica-diorite, hornblende-gabbro, norite, and wehrlite — are discussed. The diorites are holocry- 

 stalline rocks of medium to coarse grain. In texture they show some variation from those which are 

 hypidiomorphic granular to those in which the texture is imperfectly ophitic. As facies of the dioritic 

 magma there are described diorite, mica-diorite, quartz-mica-diorite, tonalite, and plagioolase-bearing 

 granite. A quartz-mica-diorite-porphyry occurs in narrow dikes cutting the mica-diorite. Analysis 

 of the mica-diorite shows it to stand upon the border between the lime-soda feldspar rocks and the 

 orthoclase rocks. The gabbros and norites are holocrystalline rocks of moderately fine to coarse grain. 

 They show considerable variation in texture. The hypidiomorphic granular texture predominates, 

 but some few show a good parallel texture. Others are noticeably porphyritic, a few have poikilitic 

 textures, and less commonly there is an approach to the ophitic texture. Hornblende and labradorite 

 is the most common mineral association, giving typical hornblende-gabbro. A monoclinic pyroxene at 

 times becomes abundant, giving a transition to the normal gabbro. Bronzite at times is the promi- 

 nent bisilicate constituent of these rocks, giving bronzite-norite. A bronzite-norite-porphyry also 

 occurs. Locally the hornblende-gabbro has been crushed, and there is produced therefrom a schistose 

 rock which represents a transition to a hornblende-gneiss. Of these rocks the hornblende-gabbro was 

 first formed. It was intruded by normal gabbro, and both of these types were then cut by dikes of 

 bronzite-norite and bronzite-norite-porphyry. The rocks included under the peridotites show con- 

 siderable mineralogical variation. There is produced an amphibole-peridotite, which, when augite 

 becomes predominant, grades to wehrlite. This in its turn becomes feldspathic, and indicates a tran- 

 sition to olivine-gabbro. The amphibole-peridotite also becomes feldspathic and quartzitic, indicat- 

 ing a transition toward diorite. When the order of crystallization of the minerals composing the 

 granular rocks of the entire series described above is considered, it is seen to have been as follows: 

 Bronzite is apparently the oldest. The olivine and monoclinic pyroxene come next and are of essen- 

 tially the same age. Mica and hornblende follow, and are contemporaneous. Then comes plagio- 

 clase, orthoclase, and quartz. A consideration of the chemical analyses of the rocks above described 

 shows them to belong to a series ranging from a diorite, on the one hand, to hornblende-gabbro and 

 norite and to peridotite on the other. On the acid sideof the series variations are shown microscopic- 

 ally, but of these rocks chemical analyses have not been obtained. It is not possible to state which 

 of these rocks most nearly resembles in its composition the original magma of which the different 



