EVOLUTION OF LIMESTONE AND DOLOMITE 423 



to indicate that less and less dolomite was deposited in successive 

 periods of geologic history, thus pointing to a progressive change 

 in the conditions of deposition. Of the four factors controlling 

 the deposition of carbonates in the sea, viz., temperature, pressure, 

 life processes, and chemical composition, only the last two show 

 any probability of progressive change with time. There is no 

 evidence for a change in the nature of life processes. There is 

 evidence for a change in the chemical composition of the sea, 

 specifically for an increase in the ratio of calcium to magnesium 

 contributed to the sea from the lands, which will appear from the 

 following considerations. The present lands contain a much 

 larger proportion of limestones than could be gotten by redis- 

 tributing a granite or basalt, generally accepted as being equiva- 

 lent to the materials of the primitive lands. It is inferred from a 

 consideration of the relation between the composition of river 

 waters and the terranes which they drain that the present rivers 

 have a higher ratio of calcium to magnesium than those of the 

 primitive lands. 



The accumulation of limestones on the lands of increasing 

 calcium content, with time, seems to be related to a reworking of 

 the land over and over again along certain selective lines. A 

 higher percentage of calcium than of magnesium tends to be lost 

 from all kinds of rocks when subjected to all kinds of metamorphic 

 processes. Hence there is a continuous selective removal of cal- 

 cium from the lands of the sea, as is evidenced by the fact that 

 the ratio of calcium to magnesium of rivers tends to be higher 

 than the ratio of calcium to magnesium of the lands which they 

 drain. This involves a selective retention of magnesium in the 

 elastics. The transportation and deposition of elastics is at a 

 maximum during periods of continental expansion and at a mini- 

 mum during periods of continental submergence. The elastics 

 are therefore deposited mainly on the margins of the continents 

 and in the deep sea. Those deposited in the deep sea are per- 

 manently lost to the lands and with them goes a selective loss of 

 magnesium from the lands. The carbonates, calcium and mag- 

 nesium, are deposited mainly in shallow epicontinental seas during 

 periods of continental submergence, in consequence of organic 



