FOLDING AND THICKNESS Ot MANSFIELD FORMATION. 439 



north and south, a superficial widtli of about 1,800 feet of phy Kites is 

 exposed. The most southern exposures dip northward at a low angle. On 

 the northern rim the true beddino- is nearly vertical. Elsewhere the ver- 

 tical cleavage structure alone is distinguishable. The upper limit of the 

 formation is not found in this syncline. Making the mobt liberal estimate 

 for possible minor crumples, it is improbable that a less thickness than 300 

 to 400 feet occurs here. On the eastern side of the main axis the phyllites 

 below the Groveland formation are very much thinner than this, the thick- 

 ness at the Interrange exploration, for example, being only about 100 feet; 

 but there, as well as along the whole western edge of the Michigamme 

 Mountain syncline, the lower contact with the dolomite is probably faulted. 

 It seems entirely safe, therefore, to place the average thickness of the 

 Mansfield formation in the Michigamme Mountain area at not less than 

 400 feet. 



PETROGRAPHICAL CHARACTERS. 



The Mansfield formation consists almost entirely of -^^ery fine grained 

 mica-slates or phyllites. The prevailing colors are dark green, black, and 

 light olive-green. These are often mottled irregularly with red, due to the 

 infiltration of iron oxides along the secondary cleavages. The cleavage 

 surfaces have a dull luster, caused by the parallelism of the micaceous 

 minerals, which are too minute, however, to be distinguished by the eye or 

 lens. 



The phyUites are often finely banded in different colors and shades. 

 Near the base of the formation bands of limestone and near the top thin 

 bands of graywacke are interbedded, as has already been stated. Quartz 

 and calcite lenses are not unusual in the minutely puckered portions of the 

 formation. 



The secondary cleavage is the prominent structure, and, indeed, the 

 only structure of the outcrops where the color and texture bandings do not 

 appear. Its general direction is transverse to the main arch, or nearly east 

 and west, and its dip is almost vertical. The north-south compression thus 

 appears to have been the stronger, or to have been active somewhat later 

 in point of time than the east-west compression. 



Under the microscope the phyllites are seen to be composed principally 

 of fine leaves of muscovite and chlorite, often also with a little biotite, and 

 with a variable and usually small amount of quartz, feldspar, and sometimes 



