J. D. Dana — Subdivisions in Archaean History. 46 L 



grand divisions based on the progress of life, and they include 

 together its complete range. There is no room for another 

 grand division between Archaean and Paleozoic any more than 

 for one between Paleozoic and Mesozoic. In contrast, the 

 Algonkian division is not above the Cambrian in grade, it being 

 based on series of rocks. Its true biological relations are in 

 doubt, because fossils representing the supposed life of the 

 period are unknown, or imperfectly so. The discovery in any 

 rock so-called of Trilobites, Crustaceans, Mollusks, Brachiopods, 

 or Crinoids, whatever the species, would entitle such rocks to 

 a place in the Paleozoic, and either within the Cambrian group 

 or below it. Walcott has already reported such fossils from 

 the beds at the bottom of the Colorado canon referred by him 

 to the Algonkian, namely : besides a Stromatoporid, a small 

 Patella-like or Discina-like shell, a fragment of a Trilobite and 

 a small Hyolithes — forms which make the beds Paleozoic 

 beyond question. 



3. Subdivisions based on Physical and Biological conditions. 



Although the physical and biological conditions of the early 

 globe are not within the range of observation, there are gene- 

 rally admitted facts which afford a basis for a philosophical 

 division of the time ; and from it geology may derive instruc- 

 tion. The subdivisions to which we are led are the following : 



I. The Asteal seon, as it has been called, or that of 

 liquidity. 



II. The Azoic aeon, or that without life. 



1. The Lithic era, commencing with completed consolida- 

 tion : the time when lateral pressure for crust-disturbance 

 and mountain-making was initiated, and when meta- 

 morphic work began. 



2. The Oceanic era,- commencing with the ocean in its 

 place: oceanic waves and currents and embryo rivers 

 beginning their work about emerged and emerging 

 lands, and the tides, the retarding of the earth's rota- 

 tion. 



III. The Archeozoic £eon, or that of the first life. 



1. The era of the first Plants: the Algte and later the 

 aquatic Fungi (Bacteria) ; commencing possibly with 

 the mean surface temperature of the ocean about 180° F. 



2. The era of the first Animal life ; the Protozoans, and 

 forms related to the embryos of higher invertebrate 

 species ; commencing possibly with the mean surface 

 temperature of the waters about 120° F., and ending 

 with 90° F. or below. 



The subdivisions, as is evident, mark off great steps in the 

 progress of the developing earth, although the rocks bear no 

 marks of them that can be distinguished. 



