J. D. Dana — Subdivisions in Archaean History. 457 



120° F. mark to that of a mean oceanic temperature of 90° F., 

 or below it, when Paleozoic life found congenial conditions in 

 the water. The mean temperature now is about 60° F. 



The ocean, sooner or later after its inaugural, began the 

 work of making permanent sediments, that is sediments that 

 were not speedily recrystallized ; and these sediments, through 

 the millions of years that followed must have been of all kinds 

 and of great thickness. 



The conditions became still more like the present after the 

 introduction of life with the further decline of temperature. 

 Even before its introduction, iron oxides, iron carbonate, cal- 

 cium carbonate, calcium-magnesium carbonate and calcium 

 phosphate had probably commenced to form, for the atmo- 

 sphere, although it had lost the larger portion of its water- 

 vapor, still contained, as writers on the " primeval earth " have 

 stated, the chief part of its carbonic acid, amounting to all 

 that could be made from the carbon of the limestones, coal 

 and carbonaceous products now in the world. It had also a 

 great excess of oxygen — all that has since been shut up in the 

 rocks by oxidations. And these most effectual of rock-destroy- 

 ing agents worked under a warm and dripping climate. 



The amount of carbonic acid, according to published esti- 

 mates, has been made equivalent in pressure to 200 atmo- 

 spheres, or 3000 pounds to the square inch. 200 is probably 

 too high, hut 50 atmospheres, which is also large, is perhaps 

 no exaggeration. Hence, the destruction of rocks by chemical 

 methods must have been, as Dr. Hunt and other writers have 

 urged, a great feature of the time ; and long before the intro- 

 duction of living species, the temperature had so far declined 

 that the making of silicates must have given way in part to 

 the making of deposits of carbonates and oxides. 



But with the existence of life in the warm waters, through 

 the still later millions of years, there should have been, as 

 Weed's study of the Yellowstone Park has rendered probable, 

 abundant calcareous secretions from the earliest plants, and, 

 additions later, through the earliest of animal life. Great 

 limestone formations should have resulted, and large deposits 

 of iron carbonate, and perhaps iron oxides, over the bottom- 

 sediments of shallow inland or sea-border flats, besides carbon- 

 aceous shales that would afford graphite by metamorphism. 



In fact, long before the Archaean closed, the conditions as to 

 rock-making were much like those that followed in the Paleo- 

 zoic. Surely then, all attempts to mark off the passing time 

 by successions in kinds of rocks must be futile. Some varieties 

 of the various kinds of rocks are probably ArcliEean only ; but 

 not all those of its later millions of years. Even crystalline 

 and uncrystalline may not be a criterion of chronological value. 



