408 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



feet near Felcli Mountain to a maximum of nearly 1,000 feet in the western 

 part of the district. While the discrepancies may be partly due to lack of 

 precision in the data, it is probable that the thickness of the formation is not 

 uniform, but really increases from east to west. On the Fence River, 18 

 miles northwest of Randville, the thickness is probably about 1,500 feet. 

 Accordingly, accepting each of these determinations as approximately 

 correct, 700 feet may be taken as a fair estimate of the average thickness 

 of the Randville dolomite within the Felch Mountain range. 



PETROGRAPHICAL CHARACTERS. 



The outcrops of the Randville formation consist exclusively of dolo- 

 mite, moi-e or less pure, and always thoroughly crystalline. A few 

 comparatively thin layers of schists, probably both micaceous and 

 amphibohtic, and also of quartzite, are mentioned in certain drill records 

 to which I have had access as occm-ring interbedded with the dolomite; 

 and while the lithological determinations are perhaps not entitled to much 

 weight, they at least prove the existence of rocks which are not dolomite 

 within the formation. In the field, however, such interbedded layers do 

 not outcrop, and they must constitute an extremely small part of the total 

 thickness. From the results of my work the Randville formation appears 

 as a lithological unit. 



Maeroscopically the dolomites are rather coarse-grained marbles, of 

 various shades of color, of Avhich pinkish or bluish white are the most 

 common. They always inclose, more or less abundantly, large flakes and 

 aggregates of tremolite, which are particularly noticeable from their projec- 

 tion above the weathered surface. Occasionally tremolite and other silicates 

 are the most abundant, and sometimes, for small. thicknesses, are essentially 

 the only constituents. Quartz and chlorite are also often present, but in 

 much smaller amounts. The weathered surface is usually dulled to a light 

 brown or creamy yellow in a thin superficial skin, but is not deeply iron- 

 stained, except when the silicates containing ferrous iron are present. 



The following partial analyses of three specimens from different parts 

 of the range show that the carbonate is normal dolomite. The insoluble 

 portion consists chiefly of tremolite. These analyses were made for me 

 by Mr. G. B. Richardson, a graduate student in geology in Harvard 

 University. 



