CHERT FORMATIONS OF NOTRE DAME BAY 595 



and other shallow water and subaerial sediments, so common in 

 the younger formations of the district, are completely lacking. 

 May not, then, the centers from which these tuffs were erupted 

 have been submarine, at least in the early stages of their develop- 

 ment ? If this were the case, there cannot but have been a great 

 contribution of magmatic waters to the basin in which the cherts 

 were deposited. 



Flows of such extent as the pillow lavas of Notre Dame Bay 

 are generally conceded to be of fissure type, and from this source 

 also siliceous solutions would emanate. 



To a small degree siliceous solutions may have been formed by 

 the reaction of molten rock with sea water. This would be par- 

 ticularly effective in the case of the pillow flows and may account 

 for some of the silica of the chert which occurs between the pillows 

 and which forms but a very small proportion of the whole aggregate. 



Nature of Solutions and Manner of PRECffiTATiON 



Some of the processes held to be effective in the precipitation 

 of siliceous sediments may profitably be reviewed. 



Van Hise and Leith have suggested the direct chemical precipi- 

 tation of some of the jasper of the Lake Superior region. In their 

 experimental work on the origin of greenalite and ferrugenous 

 chert, they showed^ that greenalite (FeSi03) may be formed by the 

 reaction of a ferrous iron salt with sodium siHcate as follows: 



FeS04+Na20.3Si02- Fe0.3SiO.+Na.S04 



and they showed that the precipitate Fe0.3Si02 consists of a 

 ferrous silicate and silica, and that the proportions of these will 

 depend on the relative amounts of the iron salt and water glass 

 used. 



Although most of the chert of the Lake Superior region is 

 secondary after greenalite, which may be observed in all stages of 

 alteration to chert, they say: 



Certain facts have been described for the Keewatin iron formations indicat- 

 ing that the present hematitic and magnetic jaspers may not be the result of 

 alteration of earher ferrous compounds, but are original precipitates in the 



' Op. cit., pp. 521-22. 



