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



along with coarsely crystalline limestone and quartzytic lime- 

 stone containing Crinoids, Corals and Brachiopocls : all together 

 making one series of rocks of later Devonian age. My own 

 observations in the region confirm the conclusions of Prof. 

 Emerson. Such facts prove, moreover, that " massive " as 

 applied to crystalline rocks does not signify igneous. The 

 granite is not eruptive granite, but part of a stratum which 

 is elsewhere quartzyte, the quartzyte graduating into granite ; 

 the latter was never in fusion. 



Again : on the borders of New England and JSTew York 

 there are schists of all gradations from massive Cambrian 

 gneiss to Cambrian and Hudson River hydromica schist and 

 argillyte, the age fixed by fossils. Becker reports similar facts 

 from the Cretaceous of California. Such observations, and 

 others on record, make it hazardous to pronounce any gneiss 

 in an Archaean area " fundamental gneiss," or any associated 

 slaty schist the younger of the two. It may be true ; but it 

 may not be. It is probable that the thin-bedded schists are 

 absent from the older Archaean, but not that the thick-bedded 

 and massive are absent from the later Archaean. 



The little chronological value of kinds of crystalline rocks 

 in the later Archaean comes ont to view still more strongly if 

 we consider with some detail the length and conditions of 

 Archaean time. 



The earth must have counted many millions of years from 

 the first existence of a solid exterior, when the temperature 

 was above 2500° F., to the time, when, at a temperature below 

 1000° F., — probably near 500° F., supposing the atmospheric 

 pressure to have then been that of 50 atmospheres — the con- 

 densation of the waters of the dense aerial envelope had 

 made such progress that an ocean, moving in tides and cur- 

 rents, had taken its place on the surface.* There were other 

 millions afterward along the decline in temperature to the 

 180° F. mark — 180° F. the mean temperature of the ocean — 

 when, according to observations on living species, the existence 

 of plants in the waters became, as regards temperature, a 

 possibility f ; and still other millions from the 180° F. mark to 

 that of 120° F., or nearly, when marine animal life may possi- 

 bly have begun its existence. And since cooling went on at a 

 decreasing rate toward the end, time was also long from the 



* R. Mallet estimated, in view of the density of the atmosphere — over 200 

 atmospheres to the square inch — that the first drops of water may have been con- 

 densed on the earth's surface when the teraperatm-e was that of molten iron. 

 Phil. Mag.. Jan., 1880. 



f They live now in waters having a temperature of 200° P., Brewer, at Pluton 

 Creek, California ; 185°, W. H. Weed, Yellowstone Park. Moreover germs of 

 Bacilli have germinated after having been boiled for an hour. 



