C. H. Warren — Alteration of Augite-llmenite Groups. 469 



Art. XLIY. — Note on the Alteration of Augite-llmenite 

 Groups in the Cumberland, R. Z, Gabbro (Ilessose) ; by 

 C. H. Warren. 



[Contributions to the Geology of Khode Island. — No. III.] 



In a recent paper* descriptive of the geology and petrog- 

 raphy of Iron Mine Hill, Cumberland, It. I., it was shown 

 that the basic, titaniferous rock rhodose (cumberlandite) was 

 closely associated with a strongly metamorphosed and altered 

 gabbro, whose areal extent and general characteristics were 

 there described. The rocks taken together formed a roughly 

 circular area of basic igneous rock entirely surrounded by 

 highly metamorphosed, ancient sedimentaries and granite 

 intrusives. The close association of the two rocks, and the 

 fact that both are characterized by a large content of ilmenite, 

 naturally led to a detailed study of the gabbro in connection 

 with that of the rhodose. As a result, it was found that, 

 while the metamorphism and alteration of the gabbro pre- 

 sented many of the common characteristics of such rocks, cer- 

 tain mineralogical changes had taken place, which if previously 

 observed have not been adequately described, and it is the 

 object of the present article to call attention to them. 



The unaltered gabbro. — Although the gabbro is nowhere 

 exposed in an unaltered condition, a study of its less highly 

 altered forms shows clearly that it was originally a rather 

 coarse (millimeter) grained gabbro containing, beside abun- 

 dant augite and accessory apatite, an unusual amount of ilmenite 

 in the form of large irregular grains occurring in close associ- 

 ation with augite. The grain of the rock seems to have been 

 generally uniform throughout, although an occasional coarse- 

 grained, almost pegmatitic, development has been noted, as 

 well as a fine (aplitic?) phase. The optical properties of the 

 feldspar indicate a plagioclase of about the composition Ab x An x 

 or a little more acid. The textural relations of the feld- 

 spar to the augite and ilmenite is diabasic. In habit the feld- 

 spar is strongly tabular on 010, the crystals averaging perhaps 

 almost l cm square and from 2 to 3 mm in thickness. The ore 

 grains, although now more or less reduced in size by alteration, 

 are still large and abundant, many of them averaging as much 

 as 4 or 5 lDm in diameter. Their distribution seems to have 

 been fairly uniform. Locally, in several places, the grains 

 have been noted larger in size and much more abundant. 



On treating a polished surface of the rock with hot hydro- 

 chloric acid the ore grains are but slightly attacked, and no such 

 interesting structure (intergrowth of magnetite and ilmenite), 



* This Journal, vol. xxv, Jan., 1908. 



Am. Jour. Sci. — Fourth Series, Vol. XXVI, No. 155. — November, 1908. 

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