160 



GEOLOGY. 



Ceaozoic. 



probably of limestones, implies the presence of life during the era 

 occupied in the formation of the Archean rocks. Since no fossils have 

 been found, nothing is positively known of the character of the life, 

 and little, except by inference, of its abundance. 



Duration of the Archeozoic era. — Of the duration of the Archeozoic 

 era nothing can be said beyond the general statement that it was very 

 great, and this conclusion is independent of any particular conception 

 of the earth's origin. If the planetesimal hypothesis be the true one, 

 there is no readily assignable lower limit to the Archeozoic system, and 

 the duration of the Archeozoic era may exceed that of all subsequent 

 time. 



For convenience of future reference, a table of the principal geo- 

 logic time-divisions is here added: 



General Table of Geologic Time Divisions. 



(Present. 

 Pleistocene. 

 Pliocene. 

 Miocene. 

 Oligocene. 

 Eocene. 

 Transition (Arapahoe and Denver). 



(Upper Cretaceous. 

 Lower Cretaceous (Comanche or Shastan). 

 Jurassic. 

 Triassic. 

 Permian. 



Coal Measures, or Pennsylvanian. 

 Subcarboniferous, or Mississippian. 

 Devonian. 

 Silurian. 

 Ordovician. 

 Cambrian. 



Great Unconformity. 

 Keweenawan. 

 Unconformity. 

 \ Animikean. (Upper Huronian of some authors.) 



Unconformity. 

 [ Huronian. (See p. 161.) 

 Great Unconformity. 



Great Granitoid Series. 



(Intrusive in the main, Laurent ian.) 

 Great Schist Series. 



(Mona, Kitchi, Keewatin, Quinne- 

 sec; Lower Huronian of some 

 authors. 



Mesozoic. 



Paleozoic. 



Proterozoic. 



Archeozoic. 



Archean Complex. 



