56 Transactions. 



absence of such skeletons in any of the numerous microscopic sections 

 examined removes any ground for accepting such an explanation for the 

 origin of the silica in the present case. 



Chert-beds associated with dolomites are of frequent occurrence in the 

 jH-e-Cambrian and Palaeozoic, and in these cases it is generally held that 

 the silica has replaced limestone or dolomite, whether its origin lies within 

 the mass of the carbonate rock or not. The Chalk flints are believed to 

 result from the replacement of the chalk by silica, the latter being dissolved 

 from sponge-spicules throughout the mass of the chalk and collected into 

 <'oncretionary bodies by some process not well understood. The absence 

 of the casts of sponge-spicules in the Amuri limestone militates against 

 such an explanation for the origin of the silica in the present case, although 

 it is not a fatal objection, since the limestone has practically everywhere 

 been subjected to such pressure as might be competent to close up any 

 spaces of dissolution so left. The unbroken state of the tests of Foraminifera, 

 however, suggests that little internal movement of the limestone has taken 

 place. The fact that the flint-beds are thickest where the whole series is 

 thickest is in accord with the derivation of the silica of the flints from 

 the overlying limestone series, but again the total absence of the flint- 

 beds at Amuri Bluff and farther south is, on this assumption, difficult 

 of explanation. It seems most probable, therefore, that the silica was 

 chemically deposited along with the dolomite or that it is of extraneous 

 origin. If the flint-beds were of strictly local occurrence, an extraneous 

 origin would be not improbable, though it would be difficult to explain the 

 silicification of only the lowest beds of the limestone series ; but in view 

 of the widespread occurrence of the flint-beds, apparently at a definite 

 horizon, the theory of original deposition seems most acceptable. The 

 chemical deposition of silica along with dolomite is believed to have taken 

 place in other parts of the world. Writing of the magnesian limestone of 

 Durham, England, Trechmann (1914, p. 258) states, " At certain periods, 

 it seems, a deposit of siliceous material took place. Silica, in the form of 

 compact or friable nodules of chert, occurs in several beds, but chiefly in 

 the middle bedded rocks on the eastern side of the reef. In many cases 

 the nodules merge with every gradation into the surrounding dolomite." 



The conditions that permit the precipitation of silica are intimately 

 related to those that permit also the precipitation of dolomite, calcium 

 carbonate, and sulphides (Daly, 1907). They are found best displayed 

 at the present day in the Black Sea (Andrussow, 1897), and are believed 

 to have been operative also in the closed sea of Permian times in western 

 Europe, in which were deposited the Kupferschiefer and the Zechstein 

 dolomite (Schuchert, 1915). The fundamental cause of these conditions is 

 the absence of bottom-scavengers, the lack of which allows the sea-bottom 

 to become foul with accumulated remains of the swimming animals of the 

 upper levels of the sea. The absence of bottom-dwelling life is caused by 

 the lack of oxygen, a condition which can exist only in a more or less 

 enclosed or sheltered sea not accessible to the creep of heavier and colder 

 polar waters which oxygenate the bottoms of the open oceans. In the foul 

 bottoms sulphur bacteria thrive, and decompose the organic matter, pre- 

 cipitating sulphides and liberating ammonium carbonate. The latter acts 

 on the calcium and magnesian salts in solution in the sea, precipitating 

 calcium and magnesium carbonates, and probably also on dissolved 

 colloidal silica, which is precipitated as flint or chert. 



There is no direct evidence that such conditions caused the formation 

 •of the flint-dolomite rocks at the base of the Amuri limestone, but it is 



