PBTKOGRAPHICAL CHARACTERS OF RANDVILLE DOLOMITE. 51 



granite. Ou the whole, however, the Randville dolomite has had no 

 marked efiPect on the topography or the drainage. 



PETROGRAPHICAJL CHARACTERS. 



In general the Randville dolomite consists petrographically of a fine- 

 grained dolomite, with some quartz. This grades down through a calca- 

 reous quartzite by increase of quartz into a true quartzite. The nearer the 

 granite, the more quartzitic is the formation. At the southeast corner of 

 sec. 2, T. 45 N., R. 32 W., on the west bank of the west branch of the 

 Fence River, is a very good exposure of the quartzite. Its derivation 

 from the underlying granite is here shown. The rock is a very fine-grained, 

 almost novaculitic, quartzite. It shows cui'rent bedding in some places, 

 though no true bedding was observed. Immediately below this quartzite 

 is a very schistose rock, in which one can readily distinguish macroscopic- 

 ally rounded to lenticular quartz areas, with masses of sericite flakes 

 between them. The contact between the quartzite and the schistose rock 

 seems very sharp when viewed from- a short distance, but is found to be 

 indefinite when closely examined. A close search was made along a con- 

 tact for pebbles from the granite, but such were not found. However, 

 small rounded pieces of vein quartz, most probably derived from the granite, 

 were observed. The schistose rock in its turn grades down into a grajdsh 

 granite, which is also more or less schistose. We have here evidently a 

 transition from the granite, through the intermediate schistose recomposed 

 granite, to the true sedimentary rock above. The meaning of this transi- 

 tion is considered below. 



Under the microscope the cause of the schistosity of the rock inter- 

 mediate between the granite and the quartzite is plain. Quartz and sericite, 

 with some feldspar, are alone present in it. The quartz is grayish and 

 granulated, and mashed out into oval areas representing original quartz 

 grains. Various fragments constituting the areas are, however, angular 

 and more or less equidimensional, and when not so never have a definite 

 orientation of their longer axes. Between these large areas, but not between 

 the individual small fragments constituting the areas, sericite is abundant. 

 When the sericite is not predominant, the flakes lie in a fine mass of quartz 

 grains, each of which agrees in long direction with the mica plates and 

 large oval quartz areas. The sericite flakes are both included in this quartz, 



