PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 837 



Graywacke. Quartzyte.] 



substance (ylauconite? ) from which the iron oxide and in part the silica are by Mr. 

 Spurr supposed to be derived. In this case, however, these green particles are of 

 fine actinolite fibres. In other cases the little rounded pebbles take elongated shapes 

 almost inconsistent with the idea that they are of detrital beach-worn derivation. 

 Still, the larger pebble, of which these are the minuter units, is unquestionably a 

 pebble of such origin, constituting a part of a siliceous conglomerate. 



Two other slides are of a quartz from some vein being coarsely fibrous and of 

 positive elongation. Four sections. 



Age. Pebbles in the base of the Potsdam, i. e., Puckwunge conglomerate. N. H. w. 



No. 2071. GRAYWACKE. 



In the valley of the Puckwunge, north of the stream, perhaps 200 feet below the conglomerate above. 

 Ref. Annual Report, xxiv, page 38. 



Meg. Forms low outcrops on the level land. A gritty, gray rock. 



Mh'. (Jiutrtz, originally in rounded clastic grains, now made angular by 

 secondary enlargement, and feldspar grains of about the same size, sometimes a 

 plagioclase like olujocla.se, and sometimes microcline, but generally unidentifiable as 

 to species, constitute the most of this rock. These feldspars are frequently composed 

 of a micro-granulitic mosaic (quartz and feldspar) substituted for the original feld- 

 spar, and thus resemble the feldspars of the Ogishke conglomerate, from which they 

 may have been derived as pebbles. Besides the debris, of the nature of the grit 

 itself, the cement consists largely of calcite, but also with a little secondary quartz, 

 some sericite (or other mica) and a few spicules of actinolite. One section. 



Age. Animikie. 



Remark. There is a general resemblance between this grit and some portions 

 of the Ogishke conglomerate, in its microscopic characters, but it is supposed to be 

 a portion of the Animikie. It is occasionally in outcrop along the Grand Portage 

 trail where it is cut, as here, by the great dikes of that region, and it has been 

 named Grand Portage gray wacke. Compare vol. iv, page 510. N. H. w. 



No. 2073. GRAYWACKE. 



On the Grand Portage trail about a mile from the (north or) west end. 

 Ref. Annual Report, xxiv, page 39. 



Meg. Similar to Nos. 2070 and 2071. 



Mir. The same characters as seen in No. 2071, except that calcite plays no 

 part in the cement. One section. 



Age. Animikie. N. H. w. 



No. 2074. QUARTZYTE. 



Grand Portage island ; near contact with a trap sheet. 

 Ref. Annual Report, xxiv, page 39. 



