PLATE LXXIX. 



SNOWBANK LAKE PLATE OF THE MESABI IRON RANGE, 1899. U. S. GRANT. 



It is within the area of this plate that can be seen some of the most interesting 

 relations existing between the clastic and the massive rocks. The Archean exhibits 

 here nearly all the original rocks, whether massive or fragmental. The Upper Kee- 

 watin occupies the larger part of the Keewatin area. It is usually plainly conglom- 

 eratic and often bedded. The older Keewatin is probably that which occupies a belt 

 running south westwardly across the northwest corner of the plate. 



The Upper Keewatin becomes crystalline. It makes mica schists and gneisses, 

 as can be seen plainly about the shores of the northern and western parts of Snow- 

 bank lake. It also forms a granitic rock in which the forms of boulders are more or 

 less preserved; as the transformation proceeds the crystalline product loses all out- 

 ward trace of clastic origin and its megascopic appearance and intrusive action on 

 the adjoining mica schist reveal it as an igneous rock. It is then only by micro- 

 scopic examination that the old fragmental structure can be seen. The various 

 original grains, say of feldspar, are divided between fresh growths and remnants of 

 decayed feldspar grains. The decay products are clustered together, usually at or 

 near the centres of the feldspars, and they are themselves crystallized into new sili- 

 cates, largely muscovite, chlorite and epidote. 



The southern arm of the syncline of the Lower Keewatin is involved in the 

 gabbro, and the gabbro also embraces the more basic southern part of the Upper 

 Keewatin. The relation of the gabbro to the Keewatin greenstone can be studied 

 at the iron location at the south shore of Disappointment lake, and northeastwardly 

 along the border of the gabbro. The basic rock of the Keewatin becomes crystallized 

 with the formation of numerous new minerals, such as characterize basic igneous 

 rocks, viz., hypersthene, magnetite, diallage, augite, labradorite, olivine. There is a 

 stage of the transformation from greenstone to gabbro corresponding to the mica 

 schist or gneissic stage that precedes the production of granite. It has been named 

 muscovadyte, but it varies in composition according to the nature of the original 

 basic rock. In one of its conditions it is noryte, and it is sometimes plainly allied 

 to the greenstone, and can be traced directly into it, and sometimes it is plainly 

 a phase of gabbro. There are almost infinitely varied shades of structure and 

 composition in the rock muscovadyte, but they all lie between basic Keewatin green- 

 stone, sometimes conglomeratic, and coarse gabbro that originated from the green- 

 stone in early Keweenawan time. The great hill of Upper Keewatin conglomerate 

 which is on the east side of sec. 34, T. 64-8, a very conspicuous object from nearly 



