160 Geological Society. 



The author considers that in both cases the silica was pre- 

 cipitated direct, and did not, to any considerable extent, pass 

 through an intermediate stage of secretion by organisms with 

 subsequent solution and redeposition. 



Field-evidence shows that the chert is a contemporaneous 

 deposit, though with possible rare minor exceptions, as noticed in 

 the paper. 



Field-evidence also shows that the silica was rapidly deposited, 

 and that it was consolidated before the underlying limestone. 

 Even if siliceous organisms existed on the sea-floor in sufficient 

 numbers to form the chert-beds, which field and microscopic 

 evidence shows to be extremely doubtful, there are no known 

 agencies likely to be present in the sea capable of bringing their 

 skeletons and tests into solution with the rapidity shown to have 

 taken place in the deposition of the chert. 



Cherts of magmatic origin differ in field-relations and associa- 

 tions, and often in structure, from the cherts which derived their 

 silica from the land. The latter occur mainly in the upper, 

 generally thin-bedded, dark limestone which was laid down near 

 a shore-line. It is believed that, when a sufficient concentration of 

 silica in the sea was attained, rapid flocculation and precipitation 

 would result from contact with the bivalent ions of lime in the 

 presence of carbon dioxide. 



The author adduces evidence to show that simultaneous de- 

 position of silica and calcium carbonate often took place, and it 

 is believed that, in such cases, segregation ensued, and sometimes 

 resulted in the formation of nodules and lenticular masses of 

 chert. 



It is suggested that the bedded cherts of terrestrial origin re- 

 sulted from heavier precipitation of silica, comparatively free from 

 calcium carbonate, and spread over the sea-fioor by gentle currents. 

 Meta somatic replacement of limestone and calcareous organisms 

 by silica has taken place at their contact with the chert. Impuri- 

 ties in the silica have tended to limit such replacement. The freer 

 the chert was from impurity the greater was the replacement, but 

 the chert is not a pseudomorph after limestone. Impurities have 

 also had an important effect on the crystallization of the silica. 



Organisms existing in the sea or on the sea-floor would be en- 

 tangled in the precipitated silica, and their presence in the chert 

 is thus explained. 



The blackness of some chert is shown to be due to the presence 

 of carbonaceous matter. Ferrous iron may possibly have operated 

 sometimes in the same way. 



The banding frequently seen in chert is believed to be due to a 

 segregation of impurities by diffusion and rhythmic ]:>recipitation 

 in the course of dehydration. 



