June i6, 1892] 



NATURE 



153 



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 tem- 

 perature. Even before its introduction, iron oxides, iron 

 carbonate, calcium carbonate, calcium-magnesium car- 

 bonate, and calcium phosphate had probably commenced 

 to form, for the atmosphere, although it had lost the 

 larger portion of its water-vapour, still contained, as writers 

 on the " primaeval 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 oxy- 

 gen — all that has since been shut up in the rocks by oxida- 

 tions. And these most effectual of rock-destroying agents 

 worked under a warm and dripping climate. 



The amount of carbonic acid, according to published 

 estimates, has been made equivalent in pressure to 200 

 atmospheres, or 3000 pounds to the square inch. 200 is 

 probably too high, but 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 introduction of living species, the 

 temperature had so far declined that the making of sili- 

 cates 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 shal- 

 low inland or sea-border flats, besides carbonaceous 

 shales that would afford graphite by metamorphism. 



In fact, long before the Archaean closed, the conditions 

 as 10 rock-making were much like those that followed in 

 the Paleozoic. Surely, then, all attempts to mark off the 

 passing time by successions in ki7ids of rocks must be 

 futile. Some varieties of the various kinds of rocks are 

 probably Archaean only ; but not all those of its later 

 milHons of years. Even crystalline and uncrystalline may 

 not be a criterion of chronological value. The beds of 

 the Upper Archaean, under the conditions existing, may 

 well, over some regions, be uncrystalline still, and may 

 include carbonaceous shales that hold to this time their 

 carbonaceous products. Such uncrystalline beds may now 

 exist over the Continental Interior ; for the great Interior 

 has generally escaped when metamorphic work was in 

 progress on the Continental borders. 



The amount of carbonic acid is most readily estimated 

 by first obtaining the probable amount for all post- 

 Archaran sources, and then adding to this that which is 

 indicated by Archaean terranes. The calculation is here 

 given in detail that others may use it for deductions from 

 other estimates. 



For the estimation there are the following data. A 

 cubic foot of pure limestone which is half calcite and 

 half dolomite and has the normal specific gravity 275, 

 weighs 171*4 pounds ; and this, allowing for ,\,th impurity, 

 becomes 157 pounds and corresponds to 72 pounds of 

 carbonic acid. A cubic foot is equal to an inch-square 

 column 144 feet in height. Since 72 is half of 144, each 

 foot of the column of such limestone contains half a pound 

 of carbonic acid. Hence a layer of the limestone one foot 

 thick would give to the atmosphere, on decomposition, 

 half a pound of carbonic acid for each square inch of 

 surface. 



A foot layer of good bituminous coal containing 80 



NO. 1 1 8 r . VOL. 46] 



per cent, of carbon, G==r5, will give to the atmosphere 

 by oxidation i 9 pounds of carbonic acid per square inch 

 of surface. 



If the mean thickness of the limestone over the whole 

 earth's surface, that of the oceans included, reckoned on 

 a basis of ^'..th impurity, is 1000 feet, the contained 

 carbonic acid amounts according to the above to 500 

 pounds per square inch, or 34 atmospheres (of 14I pounds), 

 and if the mean thickness of the coal is one foot, the 

 carbonic acid it could contribute would be i"9 pounds 

 per square inch. Adding these amounts to the carbonic 

 acid corresponding to the carbon in the mineral oil and 

 gas and other carbonaceous products of the rocks and 

 organic life, supposing it to be six times that of the coal, 

 the total is 5135 pounds, or 35 atmospheres. The mean 

 thickness of Archaean calcium, magnesium, and iron car- 

 bonates is not a fourth of that of post-Archaean. Estimat- 

 ing the carbonic acid they contain and that corresponding 

 to the graphite of the rocks at ten atmospheres, the whole 

 amount becomes 45 atmospheres. 



To bring the amount up to the estimate for early Archaean 

 time of 200 atmospheres of carbonic acid, the mean 

 thickness of the limestone for Archaean and post-Archaean 

 time should be taken at nearly 6000 feet. 



Part of the limestone of post-Archaean terranes was 

 derived from the wear and solution of Archaean lime- 

 stones, iron carbonate, &c., and hence all the 35 atmo- 

 spheres to the square inch were not in the atmosphere 

 at the commencement of the Paleozoic. But if we 

 reduce the 35 atmospheres, on this account, to 25 

 atmospheres, it is still an enormous 'amount beyond what 

 ordinary life, even aquatic life, will endure. Reducing 

 the estimated mean thickness for the limestone layer over 

 the globe from 1000 to 500 feet would make the amount 

 nearly one half less.' 



The making of carbonates early began the work ot 

 storing carbonic acid and purifying the atmosphere ; and 

 the introduction of life increased the amount thus stored, 

 and added to it through the carbonaceous materials from 

 living tissues contributed to the earthy deposits. But 

 with all the reductions that can be explained, the excess 

 is still very large. It has been proved by experiment 

 that an excess also of oxygen diminishes the deleterious 

 influence of carbonic acid on plants ; and that if the 

 amount of this gas is made equal to that of the oxygen in 

 the present atmosphere, plants will still thrive. How far 

 this principle worked in early time cannot be known. 



2. Subdivisions based on Stratification. 



The stratification in an Archaean region affords the only 

 safe and right basis for subdivisions. This method has 

 been used in the separation of the Huronian from the 

 older Archaean ; and recently, with good success, by 

 Irving and Van Hise in the study of the Penokee-Mar- 

 quette region, or the Huronian belt of Wisconsin and 

 Michigan. The intimate relation of the beds in the series 

 has been worked out and their unconformability with the 

 lower rocks thus ascertained, besides the stratification 

 and constitution of the iron-ore series within the belt. 

 This is the first step toward that complete study which 

 should be carried on throughout all Archaean areas, how- 

 ever "complex." The distribution of the rocks and their 

 apparent or real stratigraphic succession, whether 

 massive or schistose, the positions of the planes of folia- 

 tion or bedding, the unconformities in superposition, and 

 those of mere faulting, and all structural conditions, should 

 be thoroughly investigated. Correlation by likeness of 

 rocks has its value within limited areas, but only after 



I A right estimate is very desirable. If made for North America, it could 

 not be far out of the way to assume it to be a mean for like areas of the other 

 continents .is regards the limestone. But with the best possible result for 

 the continents, the oceanic area, three times that of the continents, and out 

 of the reach of investigation as to depths of bottom deposits, remains a large 

 source of doubt. 



