GEOLOGIC TIME. 669 



calcium was poured 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 limestones in the Algonkian rocks of 

 the Colorado Canon section show traces of life in thin sections, 

 and they may be, to a great extent, of organic origin. There is 

 no evidence in the texture, bedding or composition of these 

 ancient limestones to indicate that they were deposited under 

 conditions of salinity or of supply differing materially from 

 those of the present, 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 calcareous sedi- 

 ments. 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 fjom the deposition that is going on 

 to-day in the great oceans, the accumulation of calcareous sedi- 

 ment has gone on in the past as steadily and uninterruptedly 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 con- 

 tained 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 about equaled the 

 amount now contained in the limestones of that area. We have, 

 however, to account for the calcium deposited in the interior 

 Mississippian sea and the seas over other portions of this conti- 

 nent and other continental areas, and on portions of the floor of 

 the ocean that are now accessible for observation. It is also to 

 be considered that the land areas subject to denudation in 

 Paleozoic time were, in all probability, of no larger extent than 

 at the present time. 



