THE GEOLOGY OP MINNESOTA. 



[Mica schist. Conglomerate. 



Mic. The rock is thickly set, as it were porphyritically, with idiomorphic small 

 crystals of cordierite, which are uniformly twinned so as to show 



* 



six individuals whenever they are cut perpendicular to the bisectrix, 

 as shown by the sketch here given. These extinguish in pairs, 

 composed of opposite sectors, when well exhibited, but there are 

 numerous instances in which the grouping seems to be irregular 

 or defective. Such sections have approximately a hexagonal 

 outline, although the crystal is orthorhombic.* Most of the sections 



FIG. 52. CORDIBRITE 



of this mineral are elongated parallel with n f , and contain many 

 fine inclusions, such as biotite, but the most of these inclusions are undeterminable. 

 The rock at large containing these cordierites is crowded with biotite. One section. 

 Age. Animikie. N. H. w. 



No. 372H. CONGLOMERATE. 



S. W. M N - W. % sec. 32, T. 60-13, south of Birch lake. 



Ref. Annual Report, xvii, pages 86, 136. (Compare No. 366H.) 



Meg. Conglomerate. 



Mic. The slide shows principally quartz, of interlocking structure, varying in 

 fineness, some of it having the original detrital form of pebbles, and some of it having 

 originated since the pebbles were deposited in this position: It is also evident that 

 the rock contains pebbles of quartz of detrital origin, which were not of interlocking 

 structure, but which since have been built into the general rock by the secondary 

 deposition of interlocking quartz. Throughout the interlocking finer portion of the 

 slide are calcite and mica, hornblende, epidote and small triclinic feldspars. One 

 section. 



Age. Upper Keewatin(?) 



Remark. This occurrence of fragmental grains or pebbles of interlocked 

 quartzyte in this conglomerate indicates a break of structure between the base of the 

 conglomerate and that on which it lies. If the conglomerate be the bottom of the 

 Animikie, then these curious composite pebbles of quartzyte were before the 

 Animikie. Still, they resemble very closely the fine interlocking quartzyte associated 

 with and forming a large constituent in the iron ore rocks of the Mesabi range. 

 They are probably to be referred to an underlying older quartzyte, or jaspilyte, 

 belonging in the Keewatin. 



Since such pebbles are also common in the basal conglomerate of the Upper 

 Keewatin, they do not here demonstrate whether this conglomerate is of the base 

 of the Animikie or older. The features described by the field notes rather indicate 

 the Keewatin age of the most of this conglomerate, at least at the point where the 

 most of the specimens bearing this number were collected. N. H. w. 



* Mintralogie de France, vol. i, p. 616. 



