EANDVILLE DOLOMITE IN FENCE EIVEE AKEA. 431 



The exposures referred to consist of soft, light-weathering slates and 

 graywackes, with which are interbedded layers of coarser texture. They 

 are very evenly banded in pale shades of yellow, red, and green, and the 

 structure thus brought out dips eastward at an angle of 62°. Besides this 

 a secondary cleavage is quite j^roniinent, especially in the finer-grained 

 beds, also dipping eastward, but at a considerably higher angle. At the 

 eastern side the slates are overlain by the lowest marble beds, here 

 extremel}^ impure and highly charged with chlorite and quartz sand. The 

 thickness of slates exposed is about 100 feet, and between the Archean and 

 the most western outcrops there is room for about as much more. The 

 total thickness, then, can not exceed 200 feet. 



A thin section of a specimen from one of the coarser layers shows it 

 to be a graywacke, the most prominent constituent of which is quartz in 

 small roundish and oval grains. These are embedded in a groundmass 

 composed of chlorite in minute irregular plates, ferric oxide, and kaolin. 

 The quartz grains while having generally clastic shapes are bounded by 

 minutely rough edges which interlock with the fibrous minerals of the 

 groundmass. Evidently much new quartz has been deposited round the 

 original grains. 



SECTION III. THE RANDVILLB DOLOMITE. 



DISTRIBUTION AND EXPOSURES. 



In the Fence River area the dolomite, as already stated, lies on the 

 east side of the Archean, and occupies a belt over half a mile in width, 

 which extends from the mouth of the Fence River on the south for about 

 10 miles to the north and west, to our western boundary near the north- 

 west corner of T. 46 N., R. 31 W. In this distance it is twice crossed by 

 the river, and on these natural sections and in their neighborhood the only 

 known outcrops of the dolomite have been found. The northern river sec- 

 tion passes through sees. 22 and 28, T. 46 N., R. 31 W., and discloses an 

 excellent series of closely connected exposures for a distance of about 

 2,900 feet, measured at right angles to the strike. The southern section is 

 5 miles farther south, and is much less continuous, laying bare the extreme 

 upper and lower portions only of the formation. Elsewhere through the 

 dolomite belt the rock surface is concealed by swamps or glacial drift, to 

 which last it contributes but few scattered bowlders of noticeable size. 



