446 Prof. Dr. T. G. Bonney — Troldolite, etc., in Aberdeenshire. 



mark effectually on my right hand in the form of three cuts, one 

 pretty deep. At last, after one or two more unsuccessful attempts, 

 I secured a moderate-sized fragment of a rock which I may fairly 

 describe as one of the hardest and toughest that I have ever assailed. 

 It needed but a glance to show that the rock was highly crystalline, 

 and quartzose, without any mineral that could possibly be compared 

 with talc, except the not very abundant silvery mica. I believe 

 there is only one Black Dog, or I should have supposed, so 

 obviously incorrect is Prof. Heddle's statement, that he were speak- 

 ing of some other rock. 



The rock on a smoothed surface has a dull grey or bluish-grey 

 colour, mottled or rather streaked in an irregular fibrous manner 

 with a darker, almost green-black tint, and spangled with a few 

 small flakes of a silvery mica. A broken surface exhibits a more or 

 less crystalline structure, with an irregular fracture, as if the com- 

 ponent minerals were tough and had been torn asunder rather than 

 snapped. On these surfaces the grey assumes a rather more dis- 

 tinctly violet-blue tint. The outer part and the surfaces of fissures 

 are stained brown. The slides, viewed with reflected light, in 

 certain positions also exhibit in parts a slightly bluish hue. This is 

 very faint, but I think indubitable, and much resembles the peculiar 

 violet-blue of some varieties of cordiei'ite (iolite). 



On examining the slide with a one-inch objective, by transmitted 

 light, we see a number of wavy, more or less translucent subparallel 

 streaks, varying in colour from dull light-brown to dusky grey, 

 which have a fibrous aspect and evidently consist of fine aggregated 

 hair-like minerals which stick out in the streaks in various directions, 

 though on the whole they maintain the same general orientation. 

 We may in fact compare the streaks to rather matted locks of hair. 

 Between these is a mineral generally clear, but sometimes a little 

 dirty-looking, which occasionally exhibits a granular structure ; 

 among both these a number of black grains, sometimes more or 

 less rounded, sometimes angular, are scattered rather irregularly. 

 Here and there we observe a mineral resembling a white mica in 

 detached crystals, with irregular patches of a rather gummy-looking 

 yellowish-brown mineral, and some small crystals of a brown mica. 



After a careful examination of the hair-like mineral with high 

 powers, I have come to the conclusion that in all probability the 

 greyer and the browner patches are alike aggregates of the acicular 

 microliths which project from the edge, but possibly the latter 

 may be darkened in colour by the inclusion of some exceedingly 

 minute ferrite (though even with a one-eighth objective I can make 

 out nothing definite beyond the crowded microliths) ; and that we 

 may safely regard this mineral, so abundant in the slide, as a variety 

 of fibrolite. It is almost exactly like that in the cordierite-gneiss 

 of Bodenmais, which, however, sometimes has the microliths of 

 larger size, resembling those in a typical specimen of fibrolite from 

 the same locality, which I have in my collection. 



Among the colourless minerals I notice that a fair proportion 

 have one well-defined cleavage, while of the remainder, some present 



