744 JOHN JOHNSTON AND E. D. WILLIAMSON 



precipitation would take place wherever, and so long as, a current 

 of water saturated with calcite was being warmed. These views are 

 not at all mutually exclusive; but their implications differ, and 

 it ought to be possible to decide by appropriate observation and 

 deduction in how far either has been a dominant cause on a large 

 scale. 



The magnitude of the scale of this presumed process of precipi- 

 tation through purely inorganic agencies depends primarily upon the 

 rate of circulation and upon the amount of calcium carried by this 

 water rising to the surface. We shall now consider the competence 

 of these agencies as geologic factors. In doing so let us suppose that 

 a cold current of sea-water is not saturated with CaC0 3 until it has 

 reached a temperature of 15 , and that this current after traveling 

 1,000 kilometers (600 miles) has attained a temperature of 20 ; 

 further, that the water in this stretch of 1,000 km. is changed 10 

 times a year, corresponding to a current speed of 1,000X10/365 or 

 27.4 km. a day. Now this water in being warmed from 15 to 20 

 would precipitate 5 . 4 parts CaC0 3 per million by weight, or 2 parts 

 per million by volume; on these assumptions, 1 therefore, in the 

 course of a year the mean thickness of the deposit (presuming that 

 all of the precipitate finally settles to the bottom within this stretch) 

 would be 2/1,000,000 of the depth of the current. Hence if the 

 depth of the current is 100 m., the average deposit over the whole 

 area would be, on the specific assumptions just mentioned, 2 mm. 

 yearly. 2 This estimate is probably a minimum, particularly because 

 we have supposed that deposition would take place over the whole 

 area, whereas in reality deposition would be localized (e.g., if there 

 is, as is likely, a more rapid warming up at some places) so that the 

 deposits actually formed would be thicker. Moreover, the deposit 

 would be redissolved whenever the current is underlain by colder 

 unsaturated water; therefore actual deposits belonging to this cate- 

 gory should occur only in localities bathed by currents which are 



1 The numerical values adopted were chosen as being reasonable; in any case 

 these calculations will serve as an illustration, and anyone may make similar calcu- 

 lations using whatever numerical values he deems most consonant with the facts. 



2 This corresponds to about 5,000 tons CaC0 3 per square kilometer per year, 

 or to a thickness of about 8 inches in a century. 



