598 ^y. a. tarr — oolites ix shale and their origin 



the eastern United States, and the following averages were made from 

 Jiis pajDer : 



rercentage of silica 



streams drainiug sedimentary and glacial areas T.S 



.Streams draining Precambrian, sedimentary, and metamor- 



I>liie rocks 21.2 



Streams draining Precambrian and igneous areas 28.5 



Tlie climatic conditions for these stream areas are those of the United 

 States east of the one hundredth meridian, so they vary widely. 



Data for the streams of the Pacific coast are given by Van Winkle.^- 



The average silica content for California streams is as follows : 



Percentage of silica 



Granitic areas 14 . 3 



Igneous rocks of all kinds IT . 6 



Igneous and sedimentary areas 8.0 



All sedimentary rocks 6.7 



Average for California streams 14.3 



Tlie following fignres are of interest in showing the lack of climatic 

 infiueDce on the amount of silica in solution : 



Percentage of silica 



Average of all streams with rainfall over 15 inches 14.4 



Average of all streams with rainfall under 15 inches 14.2 



The majority of the Oregon streams drain areas of igneous rocks or of 

 igneous, metamorpliic, and sedimentary rocks. 



Percentage of silica 



Average of igneous rocks 30 . 5 



Average of igneous, metamorphic. and sedimentary rocks... 22.6 

 Average of all Oregon streams 26. 82 



These streams are all high in silica. 



Washington streams show the following silica content : 



Percentage of silica 



Areas of igneous rocks 30 . 2 



Areas of igneous and metajaorphic rocks 27 . 1 



Areas of igneous, metamorphic, and sedimentary rocks 14 . 8 



Average of all streams, including some from wholly sedi- 

 mentary areas 21 . 1 



The above figures show that the silica content of stream waters varies 

 considerably, but that streams which flow over igneous and metamorphic 

 rocks are usually high in silica. It is to be noted that in most cases the 

 total amount of dissolved solids is low in streams which contain the most 



1- Walton Van Winkle : Water Supply Papers. U. S. Geol. Survey, pp. 237, 339. and 363. 



