PALEOBIOGEOGRAPHY OF THE MARINE REALM 175 



in the light of climatologic and ecologic variables, without too 

 much regard for what anyone else thinks may have happened to 

 the crust, but with a healthy awareness of its physical properties. 

 It is, of course, important not only to arrive at correct con- 

 clusions, but to do so for the right reasons — to analyze in terms of 

 processes, energy relations, reaction rates (intensity and time), 

 phases, and components, and not merely of empirical analogy. For 

 example, abundance of calcium carbonate (as coral and other 

 organic reefs, as beds of thick-shelled moUusks and Foraminifera, 

 or simply as limestones and dolomites without regard to skeletal 

 composition) is commonly interpreted as evidence of former 

 tropical conditions. Such abundance of itself does not demand any 

 specific temperature conditions or latitudinal restriction. Rather it 

 demands persistent saturation, which can happen at any tempera- 

 ture or latitude if enough calcium and carbonate ions are present in 

 proportion to other ions in the solution. Because temperature is 

 an important factor with regard to the carbonate ion, other factors 

 being equal, the prospects of calcium carbonate precipitation will 

 increase with increasing temperature. No matter how cold or warm 

 the oceans were at any particular time, however, they can hold 

 only a finite number of calcium (or magnesium) and carbonate 

 ions before precipitation begins. Thus, assuming the generally 

 constant composition of sea water, the proportion of carbonate 

 sediments and shells at any given time should vary with latitudinal 

 or local temperature, and the total amount of calcium carbonate 

 precipitated should be a general function of world climate. The 

 general interpretation, it seems, is probably justified, but not for 

 the reasons usually given; and the reasons are important because 

 their misapprehension can lead to unwarranted narrowing of 

 objective alternatives. Thus, an abundance of either organic or 

 inorganic calcium carbonate of great age at currently high latitudes 

 may mean, for the time of deposition, either a greater general 

 uniformity of climate, the deflection of warm currents poleward 

 as a result of special terrestrial configuration, a preceding climatic 

 shift from relatively cooler to relatively warmer, changes in carbon 

 dioxide tension or ionic composition of the sea, or shifting of the 

 poles or crust. Here also Brooks (1949, pp. 133, 150-151, 204, 206; 



