Pi-of. Dr. T. O. Bonney — Troldolite, etc., in Aberdeenshire. 447 



only faint indications of it, or indicate no more than a tendency in a 

 parallelism of included microliths, while others show no sign at all. 

 Among the first, some are certainly the white mica, the larger grains 

 of which are visible to the unaided eye. These give bright 

 colours with crossed Nicols, extinguish parallel with the cleavage 

 planes, and have a rather spangled or satiny aspect, even at the 

 moment of extinction. In a word this mineral reminds me in its 

 general aspect of a hydrous mica, and is not unlike paragonite. But 

 there is another well-cleaved mineral, which, though showing bright 

 colours and well cleaved, gives more uniform tints, and I think more 

 frequently contains inclosures of fibrolite than the mica. I suspect 

 this to be kyanite. Of the remaining clear grains, including some 

 without visible cleavage, the extinction, where cleavage planes are 

 visible, is parallel with them. The colours, though they have 

 a general resemblance to those of quartz, are rather more intense 

 than is usual with it. The mineral also is not quite so clear and 

 glass-like, and not seldom gives indications of incipient decompo- 

 sition. This I consider to be very probably iolite. Other grains, 

 however, are certainly quartz. The black granules are an iron 

 oxide, and are probably in great part hfematite. As pyrrhotite is 

 present at Bodenmais, I have looked carefully for this mineral, but 

 cannot detect it. The yellow -brown mineral occurs in irregular 

 filmy patches and exhibits no definite structure. With crossed Nicols, 

 it either remains dark, or gives very faint indications of an aggregate 

 structure. Some, I feel sure, is a staining of hematite or limonite. 

 This also occurs in some of the Bodenmais rock. The brown mica 

 calls for no special remark. 



A few other microlithic minerals are present which I am not able 

 to identify with precision. In one slide, however, I find a single 

 instance of a peculiar mineral. It is a patch of irregular outline, 

 (including several of the black grains, with little if any fibrolite), in 

 diameter about "07 inch, with one or two small outliers — cleavage 

 very dubious, possibly indications of two meeting at angles not very 

 far from 90° — pale puce-grey or dove-coloured by ordinary trans- 

 mitted light — strongly dichroic — extinction seemingly parallel with 

 the best indicated cleavage, which corresponds with a slightly fibrous 

 appearance, — dull olive-green for vibrations perpendicular to this ; 

 light dove-coloured or puce-brown for parallel vibrations. The 

 general look is like that of a mica in which the usual basal cleavage 

 is not distinctly shown, as is the case with some of the brown 

 mica in the slide. 



As the fibrolite appears to be present not only in the altered iolite, 

 but in the quartz and in these streaky masses, it is probably of 

 independent origin. 



We may then, I think, confidently affirm that this Black Dog rock 

 principally consists of fibrolite (sillimanite), iolite more or less 

 altered, quartz, and some micas. 



The above described slides present considerable resemblances 

 to some from the cordierite-gneiss of Bavaria. Some of the latter 

 contain very conspicuous cordierite, but others almost exactly re- 



