Dr. M*^Culloch's Additional Remarks on Glen Tilt. 71 



is crystallized, like these, in hexagonal scales, which also maintain a general 

 parallelism, and are about the tenth of an inch in diameter ; but is distin- 

 guished from mica by its softness and want of elasticity, and from pinite, by 

 its lustre, which is perfectly metallic. As mineralogists have recently found 

 reason to suspect that the term mica comprises some varieties or species essen- 

 tially distinct in some important points, it is not improbable that this also may 

 be a distinct substance requiring an appropriate name. 



Calcedony. — Although this mineral is of common occurrence, I have 

 never hitherto observed it in the rock in which it is here found ; namely, in 

 micaceous schist. It does not occupy a vein, but occurs in slender rifts in the 

 stone ; covering the surface with flattish bptryoidal protuberances. 



AcTiNOLiTE. — This is found in many of the rocks about Glen Tilt in con- 

 siderable abundance. It is not uncommon in the gneiss, in which it generally 

 forms a lamina more or less continuous ; thus usurping the place of horn- 

 blende. The crystals, in this case, are of considerable dimensions. It occurs 

 also in veins of pink carbonate of lime which traverse gneiss in the Banavie, 

 and not far from the termination of the valley. Here it is fibrous, commonly 

 radiated from various centres, and of a light-green colour ; forming an orna- 

 mental contrast with the red hue of the substance in which it is imbedded. 

 Lastly, it enters, in minute crystals, into the composition of a stratum of gra- 

 nular limestone of the same colour; producing also a very ornamental variety 

 of marble, but which is in too small quantity to be applicable to any useful 

 purposes. 



Calcareous Spar. — To the instances of the occurrence of this substance 

 formerly mentioned, I may add the following : 



Veins and nodules of a bright ochre-yellow and crystallized carbonate of 

 lime, occur occasionally among the blue limestone ; the concretions being of 

 large size and of a large platy fracture. Being sometimes irregularly inter- 

 mixed with the blue basis, and with veins of white carbonate, they are often 

 capable of being cut and polished into very ornamental marbles. They are 

 not, however, sufficiently abundant to possess any value in an oeconomical 

 view. 



In some of the schistose beds there occur small strata of white and yellow 

 granular carbonate, the grains of which equal the size of the seeds of a pome- 

 granate ; and as they impress each other in a similar manner, this comparison 

 conveys the best idea that can be given of them. 



