GABBROS ETC. EST SCOTLAND AND IRELAND. 65 



almost to point to the conclusion that the augite and enstatite in 

 rocks may sometimes be dimorphoas forms of the same substance*. 



More or less ferriferous biotite occurs not unfrequeDtly in these 

 rocks, and seems also to take the place of the augite, though not in 

 so striking a manner as enstatite. 



In the same Avay the place of the magnetite is, in certain varieties, 

 taken by the isomorphous chromite and picotite, and by the hetero- 

 morphous titanoferrite. 



The secondary minerals in these rocks are exceedingly numerous, 

 and their exact determination is often a very difficult problem. 



I have already shown how, in deep-seated rocks, as a consequence 

 of the schiUerizing process, the felspars become filled out with 

 secondary products, acquiring thereby avanturine and chatoyant 

 characters, while the augites pass into diallage and pseudo-hyper- 

 sthene, the enstatites into bronzite and hypersthene, and the olivines 

 also acquire similar and distinctive characters. 



By other processes of chemical change to be hereafter more 

 distinctly specified, many new minerals crystallize out from the 

 products of decomposition of these rocks ; among the principal of 

 these are quartz, uralite, hornblendes (with the actinolitic varieties 

 known as smaragdite), biotites, " saussurite " (including zoisite), 

 and other epidotes, kaolin, various zeolites, serpentine, haematite, 

 with several hydrous varieties of ferric oxide, and leucoxene. In ad- 

 dition to these we must note a number of uncrystallized substances, to 



* It is true that the large crystals which have generally been studied by 

 mineralogists show the augites to be essentially lime-magnesia-iron silicates, 

 and the enstatites essentially magnesia-iron silicates ; but the series of analyses 

 of microscopic rock-forming enstatites cited by Mr. C. Whitman Cross (U. S. 

 Geol. Surv. Bull. I. 1883, p. 29) indicates the existence of a very considerable 

 proportion of lime in some of them, while the series of analyses published by 

 Merian (Neues Jahrb. fiir Min. iii. Beil. Bd. 1884, p. 252-316) appear to show 

 that among the rock-forming augites there are not only some in wliich the pro- 

 portion of magnesia is very high indeed, but that it sometimes actually exceeds 

 that of the lime (see also Teall, Quart. Journ. Greol, Soc. vol. xl. (1884), 

 pp. 648, 649). I need scarcely refer to the fact that Oebbeke and other 

 mineralogists have asserted the existence of a monoclinic pyroxene which has 

 the marked pleochroism of a ferriferous enstatite, and strongly simulates its 

 characters. I have myself found in an example of the piperno of Pianura near 

 Naples a crystal of augite which is beautifully zoned. The hiterior of this 

 crystal has the cleavage and optical properties of a ferriferous augite, but the 

 outer zone, which is not cut off by any sharp line of demarcation from the 

 interior, exhibits the marked and peculiar pleochroism of a ferriferous enstatite. 

 Both the inner and outer portions of the crystal appear to have the same ex- 

 tinction-angle. This outer zone is unfortunately too narrow to permit of the 

 charactei-s of its interference figures being determined by convergent polarized 

 hght. In the same way augite is sometimes found acquiring the peculiar 

 pleochroism of hornblende before assuming its cleavage and other distinctive 

 properties (see Plate VII. fig. 5). 



It appears to me that all the known facts of the case point to the conclusion 

 that mixtures of the bisilicates of lime, magnesia, iron, &c. are dimorphous and 

 may crystallize in the rhombic system (enstatites) or in the monoclinic system 

 (augites) ; but that the formation of enstatite, especially in larcje crystals (and 

 of such mineralogists have until recent times alone taken cognizance), is favoured 

 by an excess of magnesia over lime, and of augite by the excess of lime over 

 magnesia. 



Q. J. G. S. No. 165. F 



