26 A. p. VINOGRADOV 



taining haematite and magnetite, which is a product of its metamorphosis; that 

 is to say, they contain oxidized iron. We may mention the iron-containing quart- 

 zites of Krivoi Rog, of the Kursk magnetic anomaly, of Canada, Sweden (Kiruna 

 and other places), Africa and other places. When studying the breccia of Suo- 

 deniemi in comparison with the primaeval formation of Finland (age 2 x lo^ 

 years), Rankama [11] did not find any excess of Fe"^ in the breccia. Geijer [4] 

 denied this and found haematite in the breccias. These ores of the Archaean and 

 Proterozoic are many times greater in amount than all the iron ores of later dates. 

 About the same age is assigned to the universal appearance of calcareous rocks 

 in the form of marbles and crystalhne dolomites. These crystalline carbonates 

 originally formed part of the composition of conglomerates and breccias. They 

 were gradually transformed, especially during the Proterozoic, into enormous 

 strata on the platforms. In parallel with this, traces of the most ancient organisms 

 have been found in the Proterozoic and lower down in the Archaean limestones 

 etc. The first such find was that of the well known CoUenia walcotta, an alga from 

 the limestone bed of the Grand Canyon, from the Grenville limestones of the 

 Great Lakes which are about 1-5 x lo^ years old. Grant and his fellow-workers 

 described an organic structure, similar to CoUenia in the Huron formations which 

 are of about the same age. In the Lower Huron sediments of Minnesota, Griiner 

 found the alga Inactis. In the Jatulian formations of Karelia (about i-6 x 10^ 

 years old) Carelozoon jatulica has been described. Tyler and Barghoorn found 

 the remains of blue-green algae and flagellates in the formations of the Canadian 

 shield (the formations are about 2 x 10^ years old). MacGregor found traces 

 of algae in the limestones of Rhodesia, the age of which is estimated at about 

 2*7 X 10^ years. Other similar finds have been made. Thus we have direct 

 evidence that at least as long as about 2 x 10^ years ago O2 was present in the 

 atmosphere. In as much as the primary atmosphere contained H2O and CO2 and 

 other oxygen-containing compounds there is absolutely no reason to suppose 

 that some part, at least, of the O2 in the Archaean atmosphere was of abiogenic 

 origin, being formed by photochemical dissociation of these compounds in the 

 upper layers of the atmosphere. This process is very slow and the intense volcanic 

 activity did not permit the accumulation of O2 in the primary atmosphere. 

 Moreover, the absence of O2 did not prevent the development of anerobic life. 

 Even if there were no ozone screen, which is supposed to have shielded the 

 organisms from the harmful effects of ultraviolet irradiation, the waters of the 

 seas, even when quite shallow, would have provided an excellent screen. 



Carbonaceous schists and graphite are associated with all these finds. Shungite 

 is also found at the time of the transition from the Archaean to the Proterozoic 

 ages (i-6 X lo^ years ago). 



Although Thode et al. [15] thought, on the basis of determination of the 

 ratio 32S/34S in sulphides of sedimentary origin, that the earUest limit for life 

 is somewhere about 700-800 miUion years ago, we have proved experimentally 

 that this is not true, by determining the isotopic composition of the S in similar 

 sulphides. From this we see that, as I have already said, there is a more or less 

 determinate limit somewhere between 2-5 x 10^ and 2-0 x lo^ years ago and 

 the correct figure is probably nearer to the longer than to the shorter period. 



