PART II. HISTORICAL BIOLOGY 



CHAPTER I 

 PRE-CAMBRIAN FAUNAS 



THE rocks that underlie the lowest beds of the 

 Cambrian system cover a considerable part of the 

 Earth's surface and attain an enormous thickness. 

 They can be divided into two sections : a lower series 

 of crystalline gneiss and schist, and an upper of less 

 altered sedimentary and igneous material. Meta- 

 morphism has destroyed all original characters in the 

 former series, so that there is little possibility of any 

 palaeontological evidence remaining, especially since a 

 large proportion seems to have been plutonic in character. 

 Eozoon, once regarded as a reef-building Foraminiferan 

 of Archaean times, has been definitely reclaimed from 

 the Biological sphere, and proved to be a peculiar inter- 

 growth of minerals in the crystalline complex of 

 Ophicalcite. But while no determinable fossils can be 

 expected in the " Laurentian " series of fundamental 

 rocks, mineral matter suggestive of organic activity is 

 not wanting. Masses of marble (crystalline calcite) and 

 graphitic schists suggest the existence of limestones and 

 carbonaceous shales among primeval deposits. How- 

 ever, knowledge of the conditions prevalent in Archaean 

 times is so imperfect that it is at least as reasonable to 

 ascribe these substances to inorganic precipitation or 



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