EVOLUTION OF LIMESTONE AND DOLOMITE 413 



Clarke's data, is approximately 6:1. In the ocean, the ratio of 

 calcium to magnesium is 0.35: 1. The relative amount of calcium 

 abstracted from sea water is evidently many times greater than 

 that of magnesium. At the present time, a large proportion of 

 the calcium is being deposited in the deep sea, and is thus either 

 temporarily or permanently withdrawn from the land. A very 

 large proportion of the calcium delivered to the sea has, however, 

 been returned to the lands in the form of limestone deposits; in 

 fact there seems to have been a relatively greater return of calcium 

 carbonate to the lands than of the complementary elastics.- 



In view of the excess of limestone on the lands, it seems highly 

 probable that the high magnesium content of the ocean represents 

 a selective withdrawal of magnesium from the lands during geo- 

 logic time. This may be one factor which could have caused a 

 decline in the proportion of magnesium contributed to the sea, 

 in the same way as the sodium contribution has declined with 

 geologic time, because of its accumulation in the sea. 



Resume of results of sedimentation and their effect on the ratio 

 of calcium to magnesium of the lands. — 1. The marine sediments 

 which are revealed to the geologist on the continental interiors 

 were deposited during periods of widespread continental sub- 

 mergence. It is a significant fact that the sediments on the 

 continental interiors probably represent several times as much 

 limestone as could be gotten by redistributing an average igneous 

 rock, or average rhyolite or basalt. The gain in limestone seems 

 to be due mainly to a loss of the complementary shales and select- 

 ive deposition of limestones on the continental interiors. The 

 sedimentary mantle covers about three-fourths of the known 

 area of the continents. The ratio of calcium to magnesium in 

 the average igneous rock is about 1 .37 to 1 (Clarke). The lowest 

 ratio of calcium to magnesium in any group of limestones in Daly's 1 

 compilation is 2.93:1, and the maximum 56.32:1. The ratio 

 of calcium to magnesium in Clarke's average limestone is about 

 5 to 1. The ratio of calcium to magnesium in the average shale 

 (Clarke) is about 1 . 48 to 1 ; that of the average sandstone (Clarke) , 

 5.5 to 1 . The dominance of sedimentary over Archean and erup- 

 tive terranes and the high percentage of limestones over muds 



1 R. A. Daly, Bull. Geol. Soc. of America, XX, 153-70. 



