24 B. J. CHAPMAN ON THE 



examiuatiou of a deposit of iron ore, as revealed by natural exposures, or laid bare by- 

 trenches or trial-pits, the presence of a stock-formed mass may be inferred in most cases 

 from the great width of the deposit, and especially — whether of abnormal width, or other- 

 wise — by the variations in width, and consequent irregularity of outline, which it 

 exhibits. In the case of magnetic ores, these indications are often sufficiently revealed 

 by the dipping-needle. 



A very instructive and unmistakable example of a stock-formed ore-deposit is furnished 

 by the "Wallbridge Hematite Mine, on the twelfth lot of the fifth range or concession of 

 Madoc, one of the southern townships of the County of Hastings, in Ontario. This mine, 

 now practically exhausted, was opened about sixteen years ago by the late Thomas 

 Campbell "Wallbridge of Belleville. Since that date, with occasional stoppages, it has 

 been extensively worked, and has yielded many thousand tons of red iron ore of very 

 superior quality. 



The geological features of the district in which the mine occurs were indicated 

 briefly by Mr. Thomas Macfarlane in his Eeport on the County of Hastings, published by 

 the G-eological Survey in 1866, and more elaborately by the late Mr. Vennor in his able 

 Report of 1869 ; but as the mine had not been opened at these dates, no information could 

 at that time be given as regards the nature and extent of the deposit. It was previously 

 known that the rock formations of the district are essentially of Laurentian age, overlaid, 

 here and there, unconformably, by outlying patches of Lower Silurian limestone, now 

 called " Cambro-Silurian " by the present Survey. According to Mr. Yennor, the Lauren- 

 tian rocks of this section of country forma series of roughly-parallel synclinals with gene- 

 ral N E and S W strike, and consist of a threefold stibdivision, comprising (in ascending 

 order) : (1) syeuitic and gueissoid rocks, essentially red in colour, with some crystalline, 

 graphitic limestones ; (2) dark-green, amphibolic and pyroxenic rocks with associated 

 iron ores ; and (3) various micaceous and siliceous slates, crystalline limestones, and con- 

 glomerates, mostly of a greyish colour. "Whilst the general accuracy of this distribution 

 is undeniable, I think it would be preferable to regard the scries as consisting of four, 

 in place of three, groups, — two of these being probably eruptive or intrusive, while the 

 other two are undoubtedly metamorphic strata in the ordinary sense of the term. I would 

 thus separate, from the lower stratified gneisses, the unstratified syenites or syeuitic 

 granites, the elevation of which has caused the synclinals determined by Mr. Vennor. 

 The green, amphibolic or pyroxenic rocks are of doubtful origin. Although in places 

 they graduate into schistose layers apparently conformable with the underlying gneisses, 

 in other places they shew no distinct stratification, and at certain spots, as pointed oiitby 

 Mr. Macfarlane, they present even a sub-columnar structure. I believe them to be for the 

 greater part, if not wholly, eruptive overflows or intrusive beds from which the iron 

 deposits have separated d^lring consolidation. In many places they are scarcely repre- 

 sented at all in the series, or merely form the sheaths or enclosing rock of the iron ore. 

 They differ, thiis, in a very marked manner from the extended gneissoid beds which lie 

 beneath them, and from the slates, crystalline limestones, etc., by which they are imme- 

 diately succeeded. Here and there throughout the district, these crystalline rocks are 

 overlaid unconformably by outlying patches of fossiliferous and nearly horizontal lime- 

 stones of the Lower Trenton horizon. The elevation of the red syenites, therefore, if Post- 

 Laureutian, must necessarily have preceded the Cambro-Silurian period. 



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