14 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



Granite (and syenite). — Thcse roclis occupy au area of about 22 townships on tlie 

 northeast part of the survey, between the fifth correction line and the south boundary 

 of township 45 N., and east of range 32 W., in a series of irregular uplifts, frequently 

 funning high clifls and sloping ledges on the most elevated portion of this district. 

 [This covers a part of the Archeau granite oval of the Crystal Palls district, as well 

 as the large Archeau areas northeast of it. J 



Argillaceous slates. — The argillaceous slates alluded to in townships 42 and 43 N. are 

 generally overlaid by deep drift; their boundaries, therefore, could not be satisfactorily 

 defined. West of the Peshakumine River these slates appeared to have undergone 

 considerable change by igneous action, and were often associated with an oxide of 

 iron ; but east of the Peshakumme no change by igneous action in the slates was 

 observed, and on this part they have generally a reddish color 



They dip variously at a high angle, and are supposed to conform to the greenstone 

 on the north and west, and to overlie or pass into the mica-slates on the south; and 

 in their middle portion they dip about 90°, with strike nearly east and west. [These 

 slates correspond to our least metamorphosed phases of the Upper Huronian.| 



Greenstone and hornblende slate. — Thcsc rocks occupy a larger area iu the district 

 surveyed than any other class of rocks. They extend from the granitic and other 

 rocks east of them westward beyond the survey. [See their outline on map, 

 fig. L] 



The greenstone and hornblende slates form a less broken surface than the 

 granitic range; and next to it is the most elevated range in this district, having an 

 estimated altitude, in many places, of from 1,000 to 1,100 feet above Lake Superior. 



These rocks are frequently seen in the beds and banks of streams and in ridges 

 and conical hills of various heights, often forming precipitous ledges upon their 

 sides 



The greenstone of this region is generally more or less granular and syenitic, 

 with a dark green color when moist; its composition is hornblende, feldspar, and 

 quartz — the former mineral greatly predominating. In some places the feldspar and 

 quartz are nearly or quite wanting, leaving a granulated hornblende rock. Another 

 variety of this rock was frequently seen which was composed of the same ingredients 

 but very fine grained and compact and having frequently a laminated or slaty' 

 structure, the cleavages of which generally dip from the granitic rocks at a very high 

 angle 



Some of these hornblende slates have in their seams and cleavages a silky luster, 

 from the presence of mica or talc in very fine grains 



All of these rocks are traversed by many quartz veins, from a line to 4 feet or 

 more iu width, and with still larger veins and dikes of more recent trap rock. This 

 range is supposed to have become blended with the trap range of Keweenaw point as 

 it passes under the red sandstone lying between them, and probably farther west the 

 two are united in one range. [These are the altered and more or less schistose basalts 

 and accompanying fragmentals which are comprised in the Hemlock formation. | 



Mica-slates. — Thcse slates stretch along the southerly side of the argillaceous slates 

 on the south part of the survey. They extend from the Brule River on a course east- 

 northeast for about 22 miles, iu townships 41 and 42 N., ranges 29, 31, and 32 W., and 

 have an average breadth of about 4 miles. . . . . 



The mica-slates are supposed to dip northerly under the argillaceous slates at a 

 high angle, varying at the surface from 45° to 80° 



