G6 Dr. M*"Culloch's Additional Remarks on Glen Tilt. 



occurSj bears a proportion to the magnitude of the particles of the rock; 

 being minute in those of a small grain^ and large in those of a coarser texture. 



TiTANiTE. — 1 have mentioned this substance as occurring in Scarsough and 

 in Ben Gloe*. In both instances it is but in small quantity; but I have since 

 found it in considerably greater abundance in the hills that bound the south- 

 ern side of Glen Tilt. It is there crystallized in prismatic forms, and im- 

 bedded in nodules of chlorite, which are associated with quartz veins, traversing 

 micaceous schist; a situation exactly resembling that in which it is found at 

 Killin. In many cases the crystals are fractured, or else geniculated ; for it 

 is often by no means clear to which of the two causes the deviation in their 

 rectilinear directions is to be attributed. In one instance a specimen occurred 

 in which a crowd of capillary crystals, as fine as the finest hair, traversed a 

 vacant cavity in the quartz ; and it is not unusual for the larger prisms to 

 shoot through vacant spaces in the vein. I may add here, that I procured at 

 Killin one specimen of a hollow crystal of this substance, the prism of more 

 than an inch in length, having a perforation in the direction of its axis. 

 This is a circumstance of no very common occurrence in crystallization ; and 

 I may, for that reason, remark that I have also met with it in the tourmalin : 

 the specimen in question being a mere tube, very thin, and with a large 

 vacancy. 



Tremolite. — Besides the varieties formerly enumerated, I have since ob- 

 served the following ; which it will not be useless to mention, on account of 

 the diversity of aspect presented by this mineral, and the obscurity which 

 attends some of the more uncommon modifications. 



1. Compact and granular; or at least so httle marked by prismatic forms 

 that they cannot be distinguished except in certain favourable fractures. It 

 differs from the granular variety formerly described, in being of a much 

 coarser texture, and of an obscure purplish gray colour. 



2. White and granular, with a schistose fracture. I formerly described 

 this variety, as far as its texture is concerned ; but the stratum I had then 

 observed was of a massive structure : in the present instance the mineral 

 occurs in the same situation, but is readily divisible into thin laminae. 



3. The third variety worthy of notice is a compact aggregate of minute 

 spherules, not exceeding the tenth of an inch in diameter. It is of a watery 

 gray colour, and each spherule is found, on fracture, to consist of capillary 



* Geological Transactions, vol. iii. p. 53. 



