120 INVERTEBRATE PALAEONTOLOGY 



distillation as to imagine that they are the products of 

 life. The extreme scarcity of ooze-building animals and 

 peat-making plants in the older fossiliferous strata 

 lessens the probability of their occurrence in yet earlier 

 periods. 



Separated from the fundamental Archaean complex 

 by a complete and intense unconformity, the pre- 

 Cambrian rocks preserve, in the main, those lithological 

 characters with which they were originally endowed. 

 Greywackes, cleaved but hardly recrystallized, constitute 

 the bulk of the non-volcanic parts ; although the upper 

 portions seem to consist of coarse arkose and con- 

 glomerate in most regions. Among British repre- 

 sentatives of this system, the Uriconian (and comparable 

 series) and Torridonian are unsuited for inclusion of 

 palaeontological evidence, the former being mainly 

 volcanic, and the latter probably terrestrial, in origin. 

 But the slates of the Longmyndian series, interbedded 

 with arenaceous and ashy beds, are not very different 

 in lithology from much of the Lower Palaeozoic grey- 

 wacke, and seem to have been deposited under com- 

 parable conditions. Worm-tracks and burrows, which 

 occur in them, prove that the designation " Azoic " is 

 inappropriate; but they have yielded no more satis- 

 factory fossils. In America, a certain number of some- 

 what obscure forms have been discovered in the 

 Algonkian (a formation not unlike the Longmyndian), 

 but the interest of this discovery is a little discounted 

 by the apparently Lower Cambrian facies of the fauna. 



In spite of the almost complete absence of direct 

 evidence, it may be taken as certain that organisms 

 inhabited the pre-Cambrian seas. The wealth and 

 variety of the Cambrian fauna demand preceding 

 episodes of evolution ; and yet, within itself, the fauna 

 seems to indicate that its precursors would not have 



