THE IROJST-B EARING MEMBER. 289 



silica from the apices of the troughs formed by the quartzite and dikes, and 

 tlms to form ore liodies. At the same time the other parts of the formation 

 would be steadily impoverished in iron content. Much of that whicli 

 remained disseniinated tla-ough the formation would have been changed 

 from carbonate to oxide. The silica which was taken into solution in the 

 upper part of the water's course would be precipitated in its lower part. 



The processes thus outlined would penetrate to deeper })arts of the 

 formation as erosion steadily advanced until the present surface of the 

 country is reached, where, as shown upon the diagram, the ore bodies thus 

 formed at depth are now found at surface. It follows that a large amount 

 of the iron found in the ore bodies was originally in the rock which has been 

 removed by erosion. So far as the deposits are at the surface, all of the 

 iron oxide except that which came from the oxidation of iron carbonate in 

 place must have been from such a source, while it is probable that a lai-ge 

 part of the deposits located at considerable depth have been stored from 

 rock which has been broken down and scattered far and wide. It is a 

 further consequence that it is of no moment whether the amount of iron, in 

 the original 800 feet of thickness of the formation was sufficient, if concen- 

 trated at its base, to make up the large deposits. These are not so much 

 concentrations of iron oxide which were originally dejjosited as a carbonate 

 above them, as from the layers which stretched to the southward, but which 

 were subsequently by upturning placed over the ore bodies. Also, the 

 large proportion of silica now found near the surface, and particularly in 

 the southern half of the belt, is probably much greater than was here origi- 



(lioxido is suQicieut to precipitate silica from such solutions; and that a curbun dioxide solution 

 strouj; riioii^li to ])i('<iiiitatc silica by dilution may bo made ,so weak in carlion dioxidi' that it would 

 ^»' <a]iabli' of" takini; siliia into solution. All of these facts .and jjiincipks of cheiiiistry ar<> so well 

 known that no discussion or reference to authorities is needed. However, the followinj^ statement of 

 KoBcoi' and SehoilcninKr, vol. I, p. .56!), is of interest as bearing upon tlio solubility of silica and the 

 teni|iiratnre at which it crystallizes: '"In all tlirci' conditions silica i.s insoluble in water. . . 

 Siliia, howi'ver, is easily soluble in all alkalies, evi'U in annnonia, and ( he more easily, the finer its 

 state of division. The amorphous variety, especially if it contains water, also dissolves in alkaline 

 carbonates. . . . When an alkalini' solution of silica is heated in a .siNiIed tubl^ thv glass is 

 attacked and an acid silicate is formed from which silica separates out on cooUng. If the tempera- 

 ture at which the deposition occurs be above ISO*^, the silica separates out as ijuartz; if below this 

 ]ioint, it crystallizes <iut as tridymite, whilst at the ordinary temperature of the air it separates in 

 (he form of a hydratcd amoriihous mass." 

 MON XIX 19 



