406 



EDWARD STEIDTMANN 



TABLE XVI 



The ratio of lime to magnesia in the average sediment is about 

 the same as in the average igneous rock, 1.4:1. The ratio of 

 lime to magnesia in the Paleozoic sediments of Missouri would be 

 about 4:1 if their composition is like that of Clarke's average 

 sediments. The numerical values are not positive, but they 

 point to the probability that the concentration of limestones on 

 the continental interiors may have had a surprising effect on the 

 lime and magnesia content of river waters, and ultimately on the 

 chemical deposits of the sea. 



The record of continental margins. — Chamberlin 1 has pointed 

 out that the sediments which fringe the margins of the continents 

 are characterized by a greater number of unconformities and 

 more intense metamorphism than the sediments of the continental 

 interiors. The imperfections of the marginal record therefore 

 make it impossible to make a fair comparison between the lime- 

 stone content of the marginal sedimentary column and that of 

 the continental interiors. It is perhaps significant that the mar- 

 ginal sediments of late Tertiary and more recent times are pre- 

 dominantly clastic, which suggests a synchronous relation between 

 continental expansion and the deposition of elastics. 



Deposition within the 100-fathom line during continental expan- 

 sion. — It is significant that in the present geologic epoch of con- 

 tinental expansion, the area of the epicontinental sea is limited 

 to about 10,000,000 square miles, perhaps less than a third of 

 what it has been during periods of great marine expansion. It is 

 also significant that the present period of continental expansion 

 1 T. C. Chamberlin, Geology, III, 526. 



