OUTLIIfE OF THIS MONOGRAPH. XXXHI 



types represent the differentiation products. Tlie horublende-gabbro is tliat type wliich apparently 

 first reached its present geological position. It was followed in the acid part of the series Ijy the 

 diorite, which in its turn was succeeded by the diorite-porphyry. Along the basic series hornbleude- 

 gabbro was succeeded by gabbro, followed by the bronzite-uorite and the jieridotite. In general the 

 forces of differentiation have been toward increasing acidity and increasing basicity. 



PAIiT II. 



Chapter I treats of geographical limits and physiograjihy. The geographical limits of the area 

 described are given and a brief statement made concerning the conditions under which the work was 

 done. In the preliminary sketch of the geology the rocks represented are stated to range in age from 

 Archean to early Paleozoic. In that part of the district north and west of the Michigamme River the 

 Archean is exposed in several regularly outlined oval areas from 10 to 12 miles long and from 2 to 6 

 miles wide. The intervals between these ovals are occupied by highly tilted metamorphosed sedimen- 

 tary and igneous rocks of Algonkiau age. In the southern and eastern portions of the district the edges 

 of the tilted rocks are covered by the gently dipping Cambrian sandstone. Nevertheless, field work 

 shows that the distribution of the Archean in ovals, which is so characteristic for the areas north, 

 also holds here. The chief surface feature is a rolling plain, which slopes gently to the southeast, 

 and upon which is superimposed the glacial drift with its characteristic topographical features, 

 multitudinous in variety and detail, but insignificant in relief. AVhile the details of the topography 

 are mainly glacial, the broader features have often clearly been determined by the presence of the 

 more resistant Archean and Algonkiau rocks. The drainage is to the southeast, mainly into Lake 

 Michigan, through the Michigamme and the Sturgeon rivers. Details of the drainage have been 

 determined by the distribution of the rocks. It is interesting to note that the Michigamme flows 

 along the eastern edge of its drainage basin, having no eastern tributaries. 



Chapter II treats of magnetic observations in geological mapping. Certain of the rocks 

 occurring in the Crystal Falls district contain magnetite in such quantity that they have a marked 

 influence on the magnetic needle. Advantage is taken of this fact in the mapping of the rocks where 

 exposures are wanting. The instruments and methods of work used in making magnetic observations 

 are described. Facts of observation are mentioned, and general principles are laid down. Applica- 

 tion of these princiiiles to special cases is then considered, and finally a description of the method used 

 in the interpretation of complex structures follows. 



Chapter III. In Section I the position, extent, and previous work done in the Felch Mountain 

 range is described. An abstract of the literature covering the area is given. 



Section II contains a general sketch of the geology of this range. The rocks range from Archean 

 to early Paleozoic. These last are not considered for the present. The Archean is distributed in areas 

 which represent the cores of large arches formed over the whole region by mountain building energy, 

 and subsequently truncated by deep Cambrian denudation. The rocks, chiefly of sedimentary origin, 

 intermediate between the Archean and Paleozoic, to which the name Algonkiau is applied, occupy a 

 narrow strip ranging from a mile and a half to less than a mile in width, and extending east and west 

 for a distance of over 13 miles. This strip constitutes the Felch Mountain range. It is bordered on 

 the north and south by the Archean. The lowest part of the Algonkiau occupies parallel zones next 

 to the Archean both ou the north and on the south, and is succeeded toward the interior of the strip 

 by the younger members. The general structure therefore is synclinal, but is not simple. The strip 

 contains two or more synclines separated by anticlines. They have likewise been aft'ected by cross 

 folds, which give a dift'erent pitch to the axes of the east and west folds. The structure is also compli- 

 cated by faulting. The Algonkiau is divided into two series separated by an unconformity. In the 

 first occur, from the base upward, the Sturgeon quartzite, the Randville dolomite, the Mansfield 

 schist, and the Groveland iron formations. Above these follows a younger series which is undivided. 



Section III treats of the Archean. This limits the Algonkiau rocks on the north and south, and 

 is very well exposed. The topography is very rough. Usually, but not always, a topographical 



MON XXXVI III 



