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JOHN W. GRUNER 



matrix of the organisms is a very fine-grained chert containing many 

 minute needles of brownish amphibole, very small crystals of 

 carbonate of the same kind as that of the fossils, some magnetite, 

 and minute specks of pyrite. This is exactly the composition 

 of the chert of the Soudan formation in place. The latter, however, 

 is slightly more recrystallized. 



For the identification of the fossils the writer is indebted to 

 Professor Josephine Tilden, algologist at the University of Min- 

 nesota. Professor Tilden classes 

 them as blue-green algae corre- 

 sponding to such types as Inactis 

 or Microcoleus. Some of the 

 structures which represent the 

 sheaths of the plants show the 

 "canals" in which the algae 

 proper lived. Figure 2 is a tra- 

 cing (taken from a photograph) 

 of such a sheath, showing the 

 "canal." 



Since the publication of the 

 paper on the origin of theBiwabik 

 iron-bearing formation,'^ more 

 algae of Huronian age have been 

 discovered in chert of the same kind and from the same locahty 

 as that which contains the iron bacteria. The algae consist of 

 chert, or are replaced by some brownish or greenish iron-containing 

 mineral. Figure 3 is a highly magnified (X 2,200) view of one of 

 these organisms which, according to Professor Tilden, also belong 

 to the blue-green algae of the type Inactis or Microcoleus. They 

 are so small and inconspicuous at a magnification of less than 100 

 diameters that they escape attention. It is not improbable, there- 

 fore, that examination of rocks with this purpose in view will reveal 

 fossils previously overlooked on account of their extremely small size. 

 In conclusion, the author expresses the belief that the blue- 

 green algae in the chert pebbles of the Ogishke conglomerate are 

 of Archean age, and that algae and bacteria were partly responsible 

 for the deposition of the Soudan formation. 



' J. W. Gruner, op. cit. 



Fig. 3. — Blue-green 

 M 931 B.) X 2,200. 



algae. (Slide 



