Geologic Time. — Walcott. 363 
ity of the ocean has remained the same as at the present during the 
whole of this period, then it has taken 680,000 years for the deposits of 
the above thickness, or containing calcium in amount equal to that at 
present in solution in the ocean, to have accumulated on the floor of 
the ocean.''* According to this calculation the mean rate of accumu- 
lation over existing oceanic areas is . %g . or .000032 feet per annum. 
Was deposition of chemical sediments more rapid during Paleozoic 
time? It has been claimed that the quantity of lime poured into the 
ocean in earlier times was greater than during the later epochs of geol- 
ogical history, — this arising from the more rapid disintegration of the 
Archean,crystalline, and volcanic rocks. It is undoubtedly a fact that the 
ocean was stocked in Archean and Algonkian times with matter in solu- 
tion that produced salinity, but we have no evidence from chemical 
precipitation that more calcium was poured into it than could be re- 
tained in solution. The Laurentian limestones are crystalline, but, as 
has been shown, this texture is consistent with either chemical or or- 
ganic origin. The unaltered limestones of the Algonkian rocks of the 
Colorado Canyon section show traces of life in thin sections, and they 
may, to a great extent, be of organic origin. There is no evidence in the 
texture, bedding, or composition of ancient limestones to indicate that 
they were deposited under conditions of salinity or of supply differing 
materially from those of the preseut,and I do not find that we have reason 
to believe that the deposition of the carbonate of lime was more rapid in 
the Paleozoic than during the Mesozoic and Cenozoic times, even though 
the supply from the land may have been greater. Where the conditions 
were favorable for the deposition of lime, as in the Cretaceous sea of 
northern Mexico, we find evidence of an immense accumulation of cal- 
careous sediments. Of the amount of calcareous deposits in the seas 
outside of the continental areas that are not open to our inspection, 
we know nothing; but judging from the deposition that is going on to- 
day in the great oceans, the accumulation of calcareous sediment has 
gone on in the past as steadily and uninterruptedly as at present, sub- 
ject to varying conditions of temperature, life, depth of water, etc. 
Area of deposition in Paleozoic time. We have no proof that the sa- 
linity of the sea or the amount of calcium contained in it has varied 
from age to age since Algonkian time. If it has not, all of the calcium 
poured into the ocean during 2,000,000 years would have aboutequalled 
the amount now contained in the limestones of the Cordilleran area. 
We have, however, to account for the calcium deposited in the interior 
Mississippiansea and the seas over other portions of this continent and 
other continental areas and on portions of the floor of the ocean that are 
not now accessible for observation. It is also to be considered that the 
land areas subject to denudation in Paleozoic time were, in all proba- 
bility, of no larger extent than at the present time. 
The area of dry land to-day is estimated to be 55,000,000 square miles, 
and of oceans 137,200,000 square miles. t 
♦Proc. Royal Soc. Edinburgh, vol. 17, 1890. p. 101. 
fDr. John Murray, Scottish Geog. Mag., vol. 4, 1888, p. 10. 
