310 THE VERMILION IKON-BEARING DISTRICT. 



and the peripheries are black, producing a very striking appearance. This 

 zonal structure, moreover, is parallel to the irregular contours of the 

 pebbles. The alteration is evidently due to the reduction of the iron oxide 

 (FeaOg^hematite) to magnetite (FeaOg FeO). The zonal structure in these 

 £i-agments is very good, and must have been formed after the jasper 

 had acquired its present fragmentary character, as the zones run parallel 

 with the irregular margins of these fragments. Some of the fragments are a 

 foot long, although the majority of them are only a few inches in diameter. 

 Where there is brilliant red jasper lying near the conglomerates it is 

 natural to look for some iron-bearing formation as the local source of the 

 pebbles. However, a great deal of the conglomerate occurring upon and 

 in the vicinity of Ogishke Muncie Lake has as its most striking constituent 

 brilliant-red jasper pebbles, and yet there is no known typical iron-bearing 

 formation nearer than that which occurs on Otter Track Lake, 5 miles 

 away to the northwest. The intervening area is underlain by finer sedi- 

 ments — graj^wackes and slates for -the most part. The brilliant jasper from 

 this vicinity is in its general character similar to that of Lower Huronian 

 age which occurs at Soudan, Ely, and other places in the district. It seems 

 scarcely reasonable to derive these pebbles from a source so far distant as 

 the exposure on Otter Track Lake, especially as the jasper occurs in such 

 large quantity in the sediments exposed in the area extending approxi- 

 mately from West Gull Lake southwestward to Cacaquabic Lake. Win- 

 chell" reports the occurrence of a small quantity of jasper upon Townline 

 Lake, but search failed to reveal it. Moreover, the Archean area to the 

 south and east of these sediments has been hunted over for the Soudan 

 formation, which it was supposed might be lying in troughs within it, but 

 it was not found. The Soudan formation, if it ever existed in this area, 

 and it is highly probable that it did exist, has been deeply buried under 

 the sediments or completely removed by erosion. The probability of 

 such a removal will be seen to be great when we consider the enormous 

 thickness of sediments which lie west and northwest of the Archean and 

 which conseqiiently indicate a long period of erosion and deposition. 



The matrix of the Ogishke conglomerate is the finely triturated 

 material derived from the various rocks which have been mentioned as 

 occurring in pebbles in the conglomerate. 



»Geol. and Nat. Hist. Survey of Minnesota, Sixteenth Ann. Rept., 1885, p. 315. 



