852 THE GEOLOGY OP MINNESOTA. 



[Kersantyte. Conglomerate. Porphyrite. 



Remark. This rock differs from the foregoing in not showing in the section 

 evident clastic characters, and in possessing some that are of the nature of eruptive 

 rock, viz.: there are no pebbles, nor any variation in the texture or composition 

 comparable with that mentioned in the adjoining strata. The composition includes 

 epidote and sphene, which, while not exclusively characteristic of eruptive rocks, are 

 still wanting in the elastics immediately adjoining. It is, however, to be noted that 

 the original feldspars (as well as the pyroxene) have suffered the same change as 

 seen in the tuffs, etc., viz.: the feldspars are largely micro-granulitized, and the 

 pyroxene is turned to hornblende. In the rock at large are more or less rounded 

 greenstone fragments, indicating a detrital source for the rock. N. H. w. 



No. 2178. KERSANTYTE. (Dike.) 



Same place as the last. 



Ref. Annual Report, xxiv, page 50. 



Meg. Acts as an intrusive in No. 2177, forming dikes and bosses, micaceous, the 

 mica appearing in porphyritic crystals. Compare No. 2261. This is a very rare rock 

 petrographically, having been seen in Minnesota only in this region. 



Mic. The pyroxene is augite, but is almost entirely altered to hornblende. 

 The mica is biotite, but is also largely changed to chlorite (clinochlore), which has n s per- 

 pendicular to the cleavage. Epidote, sphene, calcite are also present in notable amounts, 

 while evident feldspar, much corroded and in small interlocking grains (not micro- 

 granulitic) is in subordinate amount, a considerable amount having been lost in 

 alteration to apophyllite, which, by its very low double refraction gives a general 

 darkness to the field between cross nicols. One section. 



Age. Archean (Keewatin). N. H. w. 



No. 2179. CONGLOMERATE. (Tuff.) 



Same place as No. 2177, but from the conglomerate. 

 Ref. Annual Report, xxiv, page 56. 



Meg. Some of the finer parts of the conglomerate of the region. 



Mic. The slide varies from amphibolyte in micro-amphibolyte, and to a 

 porphyritic lava, whose groundmass is finely micro-granulitic and whose crystals are 

 feldspar and hornblende. The amphibolytes are pebbles of a once diabase rock. 

 One section. 



Age. Archean (Keewatin). N. H. w. 



No. 2180. PORPHYHYTE. ( ' Esterellyte. ) 



Near the section line between sees. 20 and 21, T. 64-9, near Moose lake. 

 Ref. Annual Report, xxiv, page 56. 



Meg. Pebble in a conglomeratic jaspilyte; weathers red. 



