222 



J. F. KEMP — GABBROS OF LAKE CHAMPLAIN. 



TiTANIFEROUS MAGNETITE OrE BoDIES. 



Another interesting feature of the gabbro exposures is found in the 

 contained bodies of titaniferous magnetite. The one on Split Rock 

 mountain is shown by the excavations to be large. The breast is ten 

 feet or more across, and has been taken out on two levels for 100 feet. 

 The wall rock right up to the ore on each side is a dark, basic gabbro, 

 with the reaction rims well developed. The ore itself is simply an ex- 

 tremely basic phase of the gabbro magma. The following analyses are 

 quoted by Mr A. J. Rossi.* 



Fe... 



TiO^. 

 Fe,03 



I. 



II. 



III. 



44.77 

 13.15 



32.59 

 14.70 



40.42 

 16.37 



61.8 



45.0 



55.8 



The metallic iron has been calculated as Fe203 in the third line, but it 

 only serves to show that the ore is very low grade, and that there remain 

 25 to 40 per cent unaccounted for. Beyond question this remainder con- 

 sists of AI2O3, MgO and CaO, the familiar bases of pyroxene, hornblende 

 and labradorite.. Mr Rossi remarks on the general high percentage of 

 AI2O3 in these titaniferous ores from neighboring prospects. They also 

 run, as a general rule, low in sulphur and phosphorus. The same geo- 

 logic surroundings accompany the other titaniferous ores referred to in 

 a previous page, but at times black hornblende (as on Ledge hill. West- 

 port) is very abundant next the ore, for the wall shows little else than 

 coarsely crystalline masses of this mineral, which pass into the ore itself. 

 In all of these mines there is no evidence of a fissure or of vein forma- 

 tion, no slips or salbands, nor at Split Rock, at least, does the wall rock 

 lack a purely massive habit. The origin of certain iron-ore bodies as 

 excessively basic developments of an igneous magma has been favored 

 by not a few of the late waiters. The interesting ore at Cumberland Hill, 

 Rhode Island, described years ago by Dr Wadsworth f as an intrusive 

 peridotite ; the ore bodies in Minnesota gabbros, cited by N. H. Win- 

 chell;J others in nepheline rocks in Brazil, described by 0. A. Derby, § 



* A. J. Rossi : Titaniferous Ores in the Blast Furnace. Trans. Am. Inst. Min. Eng., Februarj--, 1893.- 

 Many other analyses of titaniferous ores from other regions are given by Mr Rossi. 



t M. E. Wadsworth : Bull. Mus. Comp. Zoo!., vol. vii, 1881, p. 183. 



JN. H. Winchell: Tenth An. Rep. Minn. Geol. Survey, 1882, pp. 80-83. 



§0. A. Derby: Am. Jour. Sei., April, 1891, p. 311. Mr Derby informs the writer that this ore con- 

 tains up to 20 per cent TiOo. Even with high Ti02 percentages it may be a natural lodestone, a 

 fact not to be overlooked in discussions of magnetic separations of titaniferous ores. 



