1918] Davis: The Badiolarian Cherts of the Franciscan Group 405 



Two possible sources of solutions of alkaline silicates may be sug- 

 gested. It is possible that the alkaline silicates are direct contribu- 

 tions from the cooling magma, escaping from extrusive masses, and 

 also rising through fissures in the crust, from larger intrusive masses 

 below, as these gradually cooled. It is possible, also, that the alkaline 

 silicates are not direct contributions from the magmas, though pro- 

 duced as a result of their intrusion. If a body of molten magma were 

 injected into the rocks beneath the sea floor, it would set up an active 

 circulation. The water, thus set into circulation, would be sea water, 

 containing abundant alkali salts. It is conceivable that such waters, 

 being heated by the intrusion, and acting on the rocks beneath the 

 ocean floor, the unconsolidated sediments, or the igneous material 

 itself, would take large amounts of silica into solution in the form of 

 alkaline silicates. 



Regardless of which hypothesis may explain the true source of the 

 soda-rich solutions associated with the basic rocks of the Franciscan, 

 it seems certain that if large bodies of this type of magma invaded the 

 region beneath the sea floor, the intrasion Avould give rise to great 

 numbers of submarine springs, the waters of which would be rich in 

 sodium silicate. 



It would appear, therefore, that the hypothesis, which attributes 

 the cherts to the solutions coming from the ellipsoidal basalts, and 

 the hypothesis of submarine siliceous springs were mutually inter- 

 dependent. 



On this combination of hypotheses it would not be necessary that 

 there be extrusive lava in immediate association with the cherts. If 

 there were magma reservoirs beneath the surface, slowly cooling, 

 these would give rise to siliceous springs. The existence of igneous 

 rocks erupted at various times during the Franciscan period is a proof 

 of the existence of subterranean reservoirs of molten magma. 



The question of the mode of precipitation of silica from such 

 siliceous springs might present some difficulty. It has been shown 

 that dilute solutions of silicic acid or sodium silicate would precipi- 

 tate little silica on mixing with ocean water, but that the silica would 

 come down in combination with magnesium or aluminum. In the 

 case of siliceous springs, however, the solutions would probably be 

 more concentrated and the precipitation of a large proportion of their 

 silicic acid might then occur through the flocculation by electrolytes 

 present in the ocean water. It is possible, also, that the magnesium 

 in a volume of sea water around such a source of supply might be 



