THE MINERALOGY OF SCOTLAND. 345 



2. Andalusite from Marnoch, Banffshire. 



The precise locality is the banks of the stream near the Mill of Achintoul, Kinnordy 

 Castle, 



The nature of the ground at Clashnaree — for it is covered with sward and peat — 

 and the consequent impossibility of tracing the specimens into connection with the rock, 

 as well as the confused crystalline arrangement of the constituents of the veins, prevents 

 our arriving at any information which can have a geologic bearing on its formation, 

 simple though it be in composition. This should not be the case, however, as regards 

 its occurrence at Marnoch and elsewhere in Banffshire ; though the light thrown there- 

 from is still obscure. And yet it is not so much that the amount of evidence supplied 

 by the mode of occurrence and internal structure of such substances as the andalusite of 

 Marnoch, the staurolite of Aldernie, the chiastolite of Portsoy, the apophyllite of Kil- 

 syth, and the stilbite of the Long Craig is in itself small, as that we have collected so 

 few observations on the paragenetic formations, and know so little of the physical laws 

 which govern the formation of such crystals as are built up, not according to ordinary 

 polar molecular concretion, but apparently by the sequential interlocking of tesselated 

 structures, each one of which structures seems to have been constructed in defiance of all 

 the recognised laws of crystalline accretion. 



As regards internal structural arrangement — the mode of fitting of the molecular or 

 crystallo-molecular bricks of the fabric — the imbedded andalusite crystals of Marnoch 

 yield almost no insight. Because in the formation of the crystals— however that was 

 effected — so much of the matrix has been caught up by the concreting andalusite sub- 

 stance as must be regarded as capable of interfering with the free formation of any 

 definite structure, seeing that it has chemically interfered with the purity of the material 

 attempting to crystallise apart. Possibly its potency to interfere may be all the greater 

 that the intruding substance is present not in the condition of a magma mica, but as a 

 perfected mineral formation — biotite mica. 



The crystals of andalusite at Marnoch lie all imbedded in a fine-grained schist, 

 which has, when fresh, a pale yellow-brown colour, due to a crypto-crystalline magma 

 )i silicate of alumina. This magma is sprinkled throughout with minute crystals of 

 ich brown biotite, granules of quartz, specks of magnetite, and twin crystallisations of 

 itaurolite, of less than pin-head bulk. The crystals of biotite lie in all directions, per- 

 vading the whole mass ; those of staurolite have some disposition to be arranged in 

 pecial layers ; and this is very much more marked as regards the crystals of andalusite, 

 hough there are other localities in which it is hardly observable. 



