PETEOGEAPHICAL CHARACTEES OF RAKDVILLE DOLOMITE. 53 



from the feldspar and in the crushing of the quartz grains, thus completely 

 destroying the rounded clastic grains and obliterating all the sedimentary 

 character of the rock, except the macroscopic structure of current l^edding. 



On the west side of the granite ellipse, at N. 1750, W. 1550, sec. 12, 

 T. 44 N., R. 32 W., about 100 yards from the granite, to the north, and 

 lower down on the slope of the same hill on which the granite is found, is 

 found a carbonaceous quartzite or quartzose dolomite. The strike is N. 

 25°-35° W. The surface only is seen, so that the dip could not be taken. 

 Microscopical examination shows the rock at the eastern side of the exposure 

 to be made up of quartz grains held together by a fine-grained carbonate 

 cement. This grades up to the west by increase of calcite and correspond- 

 ing diminution of quartz to a quartzose dolomite. 



At N. 500, W. 1650, sec. 1, T. 44 N., R. 32 W., one-fourth mile distant 

 from the granite, is seen another outcrop of a very dense quartzose dolo- 

 mite, appearing macroscopically almost like a vitreous quartzite, but reall)^ 

 with just enough quartz grains in it to enable the qualifying term "quartzose" 

 to be appropriately used. The brown ferruginous crust on the weathered 

 surfaces point to a percentage of iron in the magnesium-calcium carbonate. 

 The pm-e limestones are to be sought slightly farther away from the 

 Archean shore, where the conditions were more favorable for the production 

 of a pure nonclastic sediment. 



RELATIONS TO UKDERLYHSTG AND OVERLYING FORMATIONS. 



At only the one place cited above has a contact between the granite 

 and the Randville dolomite been found. It is probable that unconformable 

 relations exist, even though no basal conglomerate has been discovered as 

 evidence of wave action on the Archean coast. 



Relations between the Randville dolomite and the overlying forma- 

 tions have not been observed in the part of the district studied b}" me. 



THICKNESS. 



Reliable data for estimating the thickness of the Randville dolomite 

 have oidy been obtained in tliat area surveyed by Smyth. (See p. 433.) 

 According to his estimate, the formation possesses a maximum thickness of 

 1,500 feet. 



