ALGAE, BELIEVED TO BE ARCHEAN 



147 



« 



beneath it. In some places greenstone pebbles predominate; in 

 others, granitic ones, while in some areas chert pebbles, associated 

 with red jasper pebbles, are abundant. Most of them are sub- 

 angular though well worn, and as 

 a rule they do not exceed 2 inches 

 in diameter. Their chert and jas- 

 per cannot be distinguished from 

 that of the Soudan formation. 

 Even the finely banded structure 

 so common in the Soudan forma- 

 tion is frequently found in the 

 pebbles. There seems, however, 

 to be a slight difference in the 

 size of the grain of the Soudan 

 chert found in place and that 

 found in the pebbles. It is the 

 opinion of the writer that the 

 pressure on the chert pebbles was 

 diverted to some extent by the 

 yieldmg of the more or less hetero- 

 geneous and possibly still porous 

 matrix around them. The chert 

 pebbles, being the hardest and 

 simplest in chemical composition, emerged from metamorphism 

 without apparent change, while greenstone pebbles were elongated 



in many instances. This reason- 

 ing led to the original micro- 

 scopic examination of pebbles. 

 The pebbles examined were 

 dug with difficulty out of appar- 

 ently glaciated rock surfaces on 

 a number of islands in Ogishke 

 Muncie Lake. Some were col- 

 lected from the outcrops on the north shore of the lake. The two 

 pebbles which contain the algae (Figs, i, 2, and 3) come from an 

 island in the S.W. I, Sec. 23, T. 65 N., R. 6 W. The pebbles are 

 gray in color. The fossils consist of carbonate, the exact nature 

 of which cannot be determined without risking the slides. The 



Fig. I. — Blue-green algae. (Slide M 

 1055 D.) Inactis or Microcoleus. 

 X190. 



Fig. 2. — Tracing of alga with canal 

 The same slide as in Figure I. X400. 



