382 University of California Publications in Geology [Vol.11 



chloride as a flocculating agent, there is another method of proving the 

 objection. If one examines the analyses of waters of desert lakes, he 

 will find that many of these lakes contain large quantities of salts in 

 solution, and that their salinities may be several times as great as that 

 of sea water. In spite of these high salinities, it will be found that 

 they frequently contain appreciable quantities of silica, amounting to 

 many times that contained in sea water. It would appear from this 

 fact that the coagulating power of strong electrolytes was insufficient 

 to precipitate silica from ordinary ocean water. 



Moreover, silica may be precipitated in combination with some 

 base in the form of an insoluble silicate. Sea water contains notable 

 amounts of magnesia and a smaller proportion of calcium. Mag- 

 nesium silicate is extremely insoluble and one would expect that on 

 coming into the ocean, the silica of the rivers would be precipitated 

 in the form of magnesium silicate. Calcium may possibly aid in the 

 precipitation of silica, but its silicate is relatively much more soluble 

 and since its proportion is smaller, its effect is undoubtedly unimpor- 

 tant. Murray and Irvine 146 found that if one part of silicic acid was 

 added to 100,000 parts of sea water the silica would be precipitated 

 within 144 hours and that the precipitate was principally magnesium 

 silicate with some silicic acid and traces of calcium silicate. 



Also, on considering the analyses of enclosed lakes, it is found 

 that as a general rule when the magnesia is high, silica is absent or 

 present in only small amounts. Exceptions are found in those in- 

 stances where the salinity is so low that the absolute quantities of 

 magnesia and silica are very small. 



While the question is rather complex, involving the physical 

 chemistry of solutions of several salts, and is further complicated by 

 the colloidal nature of the silicic acid, still the general survey of the 

 available evidence indicates quite clearly that, in the presence of large 

 concentrations of magnesia, such as exist in the ocean water, silicic 

 acid comes down in the form of magnesium silicate. 



The possibility of precipitation in other ways is suggested by the 

 experiments of Murray and Irvine. They showed that in sea water, 

 magnesium silicate was soluble at the rate of ten parts per million, 

 while it is known that the actual content of silicic acid is only about 

 one or two parts per million. Murray and Hjort 147 suggest that silicic 

 acid might enter into combination with alumina : 



i*e Proc. Roy. Soe. Edin., vol. 18, p. 229, 1891. 

 i« Op. cit., pp. 183, 184. 



