474 BULLETIN OF THE BUREAU OF FISHERIES 



show as much as nine parts silica (as Si0 2 ) per million. Androscoggin water is, there- 

 fore, almost as rich in silica as the spring water average just quoted (Clark's exact 

 figures are Si0 2 , 18.63 per cent of total solids, salinity 48.3 per million). According 

 to one analysis the upper waters of the Merrimac are even richer in silica than this 

 (Si0 2 , about 31 parts per million), but since the Merrimac flows for many miles 

 through an alluvial valley in its lower course, and at the same time receives several 

 important affluents from swampy areas, it probably reaches the sea with a much 

 smaller percentage of silica in its water. 



If we can take the Androscoggin as fairly typical of the rivers of northern New 

 England (including the St. John River, of which no analyses are available), which 

 is justified by the nature of its watershed, it appears that, on the whole, the river 

 water emptying into the Gulf of Maine is 7 to 8 times as rich in dissolved silica (Si0 2 ) 

 as our analyses off Gloucester suggest as a fair average for the latter. A discrepancy 

 of this sort obtains between the silica contents of river and sea water in temperate 

 zones generally, and its effects are probably accentuated in the Gulf of Maine, 

 just as the effect of land drainage is in reducing surface salinity by the concentration 

 of the run-off from a large watershed into a comparatively small and topographic- 

 ally circumscribed area of sea. It would therefore be reasonable to expect the waters 

 of the Gulf of Maine to average high in silica when sufficient analyses are made 

 to plot the distribution of silica in boreal seas generally. 



In addition to the silica brought down by the rivers in the dissolved state, 

 probably much larger amounts are carried to the sea, suspended in the form of the 

 finely divided clay which is derived from the disintegration of felspars, etc. Though 

 most of this clay is precipitated to the bottom on mixture with the salt water, part 

 of it is carried to great distances. Murray and Irvine (1892, p. 240) suspected from 

 their cultural experiments "that the pelagic silicious organisms might, in part at 

 least, obtain the silica for their frustules and skeletons" from this clayey matter. 

 So far as I know these experiments have been neither confirmed nor refuted, 

 nor is it clear whether they were sufficiently precise to eliminate other possible causes 

 for the abundant growth of diatoms which ensued on the introduction of clay into 

 the artificial culture solution. But we must reckon with the possibility that diatoms 

 not only make use of the dissolved silica but also of the insoluble silicates, given 

 vertical circulation strong enough to keep the latter in suspension in the water. 



A third possible source of silica is the slow solution of the rocks that form part 

 of the coast line of the Gulf, and of its submarine boulders, sands, and clays. Silicious 

 deposits of this sort have commonly been regarded as so nearly insoluble in sea 

 water as to be negligible biologically; but as Clark (1916, p. 132) points out (geol- 

 ogists generally recognize this), sea water does attack and in the end dissolve the 

 most refractory silicates, even if very slowly. In fact, Joly (1901) found that sea 

 water dissolves more silica (Si0 2 ) from felspar than does distilled water. 84 But 

 there are two reasons for hesitancy in applying Joly's generalizations to conditions 

 as they occur in nature. First, I am unable to judge from his brief account whether 

 his analyses took due account of the small amount of dissolved silica which we must 



" Earlier tests by Thoulet (1889) gave the opposite result. 



