328 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



sedimentary strata, such as schists and conglomerate 

 or calcareous sandstone, for a long time assumed 

 to be azoic. These are still designated by the 

 general name of pre-Cambrian or Huronian, on 

 account of their great development on the shores of 

 the great Canadian lake (Huron). In France, the 

 schistous strata known by the names of Schists of 

 JRennes, Phyllads of Saint Lo, or of Douarnenez, are 

 very clearly the representatives of these pre- 

 Cambrian strata in the old Armorican expanse. 



The absence or extreme rarity of organic remains 

 in this soil is easily explained by the fact that these 

 very early sediments have frequently experienced, 

 in the course of geological times and under the 

 influence of causes of internal origin which I shall 

 have to analyse later on, structural modifications 

 designated by the general term of metamorphism. 

 These schists, instead of being earthy and amor- 

 phous, are satiny or shiny through the development 

 of membranes of mica or sericite. The limestones 

 have become Marmorean or Saccharoid, and at 

 times even the crystallinity of these sediments has 

 attained a more accentuated degree, comparable to 

 those generally observed in soils termed primitive 

 or crystallophyllian. This partial or total meta- 

 morphism has had for a first effect that of destroy- 

 ing the fragile traces of the beings of varied form 

 who doubtless peopled the waters of the pre- 

 Cambrian seas, and this it may safely be affirmed 

 is the most general case. 



At certain privileged points, however, and owing 

 to circumstances difficult to define, metamorphism 

 has very fortunately spared a portion of the sedi- 



