New Occurrence of Rutile and Sapphirine. 275 



Per cent Molec. ratio Ilmenite Eutile Hematite Eest 



Si0 2 2-24 0-037 



TiO, 53-35 -667 0'382 0'285 



Al 2 6 3 .._ 1-65 -016 



Fe 2 3 .._ 13-61 '085 0*085 



FeO 24-49 -340 | -344 



MnO '30 -004 j 



MgO 4-04 -101 -038 



CaO. -30 -007 



99-98 58$ 22-8$ 13'6$ 5'6f 



It is of course impossible, on account of the uncertainty as 

 to the exact amounts of the accessory constituents present and 

 of their composition, also as to the exact composition of the 

 ilmenite molecule, to calculate the true mineral composition of 

 the rock. An approximation may, however, be made by assum- 

 ing that about one-tenth of the HO in the ilmenite is MgO, 

 calculating the excess of Ti0 2 as rutile and of Fe 2 3 as hema- 

 tite and neglecting the Si0 2 , etc. Such a calculation gives the 

 approximate results shown after the analysis in the table. The 

 percentages summarized are : Ilmenite-hematite, 71*6 per cent ; 

 rutile, 22'6 per cent : rest, 5*6 per cent. These results are in 

 fair agreement with the results of the Rosival estimate given 

 in column 1 above. The hematite amounts to about one-fifth 

 of the ilmenite-hematite mixture, which is a figure somewhat 

 lower than that obtained by a very crude estimate made with 

 the microscope on an etched specimen. The results are, as a 

 whole, mutually confirmatory to a quite satisfactory degree. 



While no attempt will be made at this time to classify the 

 rock in the " Quantitative System," it is easily seen that it falls 

 in a new position in the perfemane class, perhaps in close 

 company with the interesting ilmenite-rutile-apatite rocks 

 recently described* from Virginia by Dr. Watson. In view 

 of the occurrence of so much free titanic oxide as primary 

 rntile> it appears to the writer that it will be necessary to give 

 Ti0 2 a special significance in classifying this and analogous 

 rocks in the " Quantitative System." 



Origin, Crystallization, Name. — That the ilmenite masses 

 in general so commonly associated with anorthosite or gabbroid 

 rocks are the result of magmatic differentiation seems to be 

 generally accepted. In the case of the rutile-sappliirine- 

 bearing portion there appears to be no good reason to doubt 

 that it represents a portion of one of these differentiated 

 masses in which there was a notable excess of titanic oxide 

 and a somewhat smaller excess of magnesia and alumina. 



*U. S. G. S., Bull. 430, p. 206, et seq. 



