520 ■ PEOF. T. G. BO^'XEX OX IHE SO-CALLED DIOEITE 



I think, be anterior in consolidation to the brown hornblende, as their 

 crystals do not indicate definite orientation or any relation with it. 

 The brown hornblende exhibits the nsual transition to a pale green or 

 even colourless variety with a slight alteration in the extinction-angle. 

 The serpentinons mineral in which some of these white augite crystals 

 are imbedded is very faintly tinged with yellowish green and has 

 but a slight influence on polarized light, resembling a steatitic 

 mineral. In other parts of the slide the colour is a little stronger, 

 and the mineral gives brighter tints between crossed nicols, showing 

 the aggregate fibrous structure often seen in serpentines. The 

 serpentinons grains mentioned above as also interspersed in this 

 ground-mass, are rather irregular in form, though showing some 

 tendency to rectilinear boundaries ; many of them exhibit one well- 

 marked cleavage. They are rather a stronger yellowish olive-green 

 in colour, frequently crowded with dusty-looking granules containing 

 occasional specks of opacite and numerous minute belonites *. "With 

 polarized light they exhibit a minutely fibrous, aggregate structure 

 with fairly bright colours (Plate XYI. figs. 3, 4). I believe the 

 mineral to be an altered enstatite. I have also noted one or two 

 iiakes of brown mica, more or less altered. 



In conclusion I will venture a few remarks on the paragenesis of 

 the minerals in this interesting group of rocks, including therewith 

 the Schriesheim rock and a picrite from Gippsland (Australia), The 

 latter has been described by me in the ' Mineralogical Magazine,' 

 vol. vi. p. 54, and is microscopically closely allied to them except 

 that the hornblende is green instead of brown ; in this also 

 unaltered olivine stiU remains. I fear, however, that they will 

 state difficulties rather than solve them. We find olivine (or 

 serpentinons pseudomorphs of it) included in brown and green 

 hornblende and in brown mica, also in light-brown augite (if we 

 include the Inchcolm picrite and that from Hain). Olivine also is 

 included in enstatite in certain serpentines. In other serpentines 

 we have enstatite separately crystallized ; and in some of the above 

 picrites serpentinons products which suggest the former presence of 

 enstatite rather than of olivine occur in a similar manner. 



jSTow as regards the augite, there is, it appears to me, some reason 

 to believe that we can have the following succession of alterations in 

 the above group of rocks — light-brown augite, sometimes practically 

 clear in thin sections ; clove-brown hornblende f ; green hornblende ; 

 colourless, rather fibrous hornblende. In these changes the augite 

 undergoes what I may call for distinction the "uralitic change," 

 because there is apparently no alteration in the outward form of the 

 crystal. Another line of change appears to be the formation of 

 distinct crystals of green hornblende or of abundant microhths of 

 actinolite. This we may term " actinolitic change." In some rocks 



* Like those described by me in the Porthnobla specimen, Quart. Journ. 

 Geol. Soc, vol. xxxix. p. 255. 



t If not there are Terj singular cases of intergrowth of the pale augite and 

 the brown hornblende. 



