328 GEOLOGIC TIME. • 



at present in solntion in the ocean, to have accnmulated on the floor of 

 the ocean.""* According to tliis calcnlation the mean rate ofaccnmn- 

 lation over existing oceanic areas is ogoSuo or 0-000032 feet per annum. 



^Vas deposition of chemical sediments more rapid duriiuf Paleozoic 

 timef — It has been claimed that the qnantity of lime poured into the 

 ocean in earlier times was greater than during tlie later epochs of geo 

 logical 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 

 solution that produced salinity, but we have no evidence from chemi- 

 cal precii)itation that more calcium was i)oured into it than could be 

 retained in solution. The Laurentian limestones are crystalline, but, 

 as has been shown, this texture is consistent with either chemical or 

 organic origin. The unaltered liuiestones of the Algonkian rocks of 

 the Colorado canyon section show traces of life iu tliin sections, and 

 they may, to a great extent, be of organic origin. Tliere is no evidence 

 in the texture, bedding, or composition of ancient limestones to indi- 

 cate that they were deposited under conditions of salinity or of sui)ply 

 diflering materially from those of the present, and 1 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 snpi)ly 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 calcareous 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 today in the great oceans, the accumulation of calca 

 reous sediment has gone on in the past as steadily and uninterruittedly 

 as at present, subject to varying conditions of temperature, life, depth 

 of water, etc. 



Area of deposition in Paleozoic time. — We have no proof that the 

 salinity 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 cal- 

 cium poured into the ocean during 2,000,000 years would have about 

 equaled the amount now contained in the limestones of the Cordilleran 

 area. We have, however, to a(;couut for the calcium deposited in the 

 interior Mississippian sea and the seas over other portions of this con 

 tinent and other continental areas and on i)ortions of the tloor of the 

 ocean that are not now accessible for observation. It is also to be con- 

 sidered that the land areas subject to denudation in Paleoziuc time 

 were, in all probability, of no larger extent than at the present time. 



The area of dry land to-day is estimated to be 55,000,000 squaic 

 miles, and of oceans 137,200,000 square miles.t 



* Vrov. noyfd Soc. Edlvhurgli, 1890, vol. xvii, p. 101. 



t Dr. Johu Murray, Scottish Geog. Mag., 1888, vol. iv, p. 40. 



