Tarr — Origin of the Chert in the Burlington Limestone. 429 



that the amount of silica present is from 1 part in 220,000 to 

 1 part in 460,000. Thej suggest that organisms may obtain 

 a portion of the silica they contain from suspended silicates in 

 the sea water. 



It is far more probable that the true source of the silica is 

 the land and that the small amount in the sea water is evidence 

 of the effectiveness of its removal by precipitation. An addi- 

 tional means of removal would be the organisms, as they 

 would use the easily obtained colloidal silica in preference to 

 breaking up silicates. 



As evidence that the silica is derived from land the average 

 percentages of silica carried to the sea by streams working 

 under various climatic conditions should be noted. Clarke in 

 Bulletin 616, IT. S. G. S., has brought together analyses of 

 streams from all over the world, and the work of the chemists 

 of the United States Geological Survey has given us analyses 

 of a great many of the streams in the United States. (See 

 especially the following Water Supply Papers of the United 

 States Geological Survey : No. 286, by R. B. Dole, and jSios. 

 339 and 363, V W. Van Winkle.) 



Clarke gives the following averages for river and lake waters 

 of the five larger continents : 



Average percentage 

 of silica 



Waters of North America . _ _ 8*60 



Waters of South America 18*88 



Waters of Europe 8*70 



Waters of Asia^ ._ 9*51 



Waters of Africa lY'SQ 



General weighted average of all 1 1 '67 



Sir John Murray's average of river waters 10*75 



These are general averages, so it will be of value to note 

 some specific cases of the effect of the rocks in the area 

 drained on the silica content of the rivers. The data available 

 are best for those of North America. The following analyses 

 of streams in Eastern United States were taken from Dole's 

 paper, and the geologic data from the large geologic map of 

 the United States Geological Survey, Professional Paper 71 : 



Percentage 

 of silica 



Streams draining sedimentary and glacial areas . . 7 8 



Streams draining pre-Cambrian sedimentary and meta- 



morphic rocks 21*8 



Streams draining pre-Cambrian and igneous rocks 28*5 



