68 COLL. MINERALS. 



forms the remaining part of the mass. We may either 

 consider this parallel disposition as the result of a common 

 polarity or crystalline tendency in all the minute portions 

 of felspar which are aggregated to form the vein, or may 

 suppose that the whole mass of rock is a portion of one 

 crystal of felspar, interrupted by intervening crystallizations 

 of quartz. Similar arrangements arising from a common 

 polarity among distinct crystals are not uncommon, and 

 must have occurred to all mineralogists. Cases perfectly 

 resembling this are of frequent occurrence in augit 

 rock, and are noticed in different parts of this work. 

 They occur also in the sandstone of Arran, described in 

 another place. Among minerals I have observed it most 

 frequently where mesotype and needlestone are associated 

 with stilbite and analcime, the former substances main- 

 taining their rectilinear course without regard to the inter- 

 ruptions caused by the latter. In one remarkable instance 

 in my possession, where detached tabular crystals of oxidu- 

 lous iron occupy the surface of a mass of rock, they are 

 disposed in those curves which indicate the directions of 

 the magnetic current between the two poles of the needle : 

 an interesting point of resemblance between the effects 

 of magnetic and crystalline polarity. 



A vein of lead ore has long been known in Coll. It is 

 a narrow string of steel-grained galena lying in a fissure 

 of the gneiss and terminating in the sea. This fissure 

 forms an angle of about 20 with the course of the beds. 

 The ore is not accompanied, as far as it is visible, by any 

 other substance, and, offering no prospect of profit, it has 

 not been wrought. 



I have deferred to the last place the mention of augit, 

 because, not having found it in situ, I am in doubt whe- 

 ther it may not be a transported rock. The specimen in 

 question was a single block, and being at a great distance 

 from the shore there is no reason to think that it had been 

 brought in ballast. Otherwise it might be imagined to have 

 belonged to the Island of Rum, of which that substance 



