430 Tarr — Origin of the Chert in the Burlington Limestone, 



The climatic conditions for the regions drained by tliese 

 streams are those of the United States east of the 100th 

 Meridian, so thej vary widely. 



Yan Winkle gives much valuable data for the streams of the 

 Pacific coast. The following are averages of the silica content 

 of the California streams : 



Percentage 

 of silica 



Granite areas 14-3 



Igneous rocks of all kinds .. 17 6 



Igneous and sedimentary rocks 8-0 



All sedimentary rocks . 6*7 



Average for California streams 143 



The following figures show that climatic factors have little 

 to do with the silica content of the streams : 



Percentage 

 of silica 



Average of all streams with a rainfall over 15 inches .. 14-4 

 Average of all streams with a rainfall of less than 15 

 inches 14"2 



Oregon streams drain all three types of rocks and show an 

 average silica content of 26*82 per cent, while Washington 

 streams on similar rocks show an average of 21'1 per cent of 

 silica. These streams are high in silica. As a rule a high 

 silica content goes with a low total of dissolved solids. An 

 example of this is shown by forty-five streams of eastern 

 United States, which average more than 10 per cent silica and 

 have 19'2 parts of silica to an average of 102 parts per million. 

 These citations are sufficient to show that the amount of 

 silica being taken to the sea annually is very large, amounting 

 on the average to 10 or 11 per cent and for many large streams 

 or areas as much as 25 per cent. 



Taking Clarke's averages again (page 105) we get the follow- 

 ing amounts in ronnd nnmbers of silica which is being added 

 to the sea water every year : 



North America . . . 54,300,000 tons 



South America 23,300,000 



Europe 35,000,000 



Asia . 68,600,000 



Africa 41,000,000 



222,200,000 tons 



Such a large amount of material added to the sea annually 

 is exceeded by that of only one base, calcium. We talk freely 

 of the formation of great limestone beds from the material 

 added to the ocean by the rivers, yet if all the calcium added 

 annually entered into the formation of limestone (which is not 

 the case, for some of it remains in solution, though all of the 



