THE MINERALOGY OF SCOTLAND. 347 



The second theory has received the support of Delesse, but in opposition to it 

 the existence in the rocks in question of broken crystals which have been re-cemented 

 by the surrounding clay-slate substance has been pointed to. 



Striking facts, drawn from the microscopical structure of the rocks, have been 

 adduced by Zirkel in favour of the third theory. 



Later metamorphic action must not, however, be excluded in seeking to account for 

 the origin of the crystalline constituents of clay-slates. 



A review even of the theories themselves suffices to show that four distinct stages 

 at least may be considered in the series of changes by which the rocks in question may 

 have acquired their present character : — 



1st, the deposition of the mud ; 



2nd, the formation of minerals during the plastic state ; 



3rd, the separation or segregation of other materials after solidification ; and 



Ath, the action of metamorphic processes. 



If such processes have operated locally, it will have to be considered whether they 

 most favour the second or the third of these theories, for they may be local in their 

 operation either geographically or geologically. They may have operated in close 

 proximity to igneous outbursts, or to limestone formations where there has been 

 much crushing of the beds, or even when there has been disturbance alone. And, 

 geologically, the change may be apparent throughout the whole sweep of a formation, 

 but only up to a certain thickness of its deeper-seated beds. 



TOURMALINE. 



This substance, common in granitic veins as it is, does not often occur in Scotland 

 either in well-developed forms or of marked purity. The finest crystal I know of, 

 the terminal portion of which I examined, was found in the coarse granite vein of 

 Rubislaw quarry. It occurred along with microcline, muscovite, beryl, and garnet. It 

 was 8| inches in length by 1^ in width. It was curved like the figure 6, but was 

 perfectly terminated and formed throughout. Fine crystals are rarely found in 

 granite veins in andalusite schist in North Glen Clova in Aberdeenshire. 



Material sufficiently pure for analysis was prepared from several localities, but 

 our want of any satisfactory method of determinating boracic acid induced the 

 writer to postpone the analyses, except in the case of crystals which were found in the 

 granitic belt of rock which cuts gneiss near Struay Inn, Ross-shire. 



It here occurs in jet-black crystals of some inches in length along with muscovite, 

 orthoclase granular pink, and microcline of a dove blue, garnet and beryl. Its specific 

 gravity is * .In powder it is brown. 



* The blank was in the MS. Professor Qeikie informs me that the specific gravity lies between 3-1 and 3-24 ; 

 Scottish examples being nearer to the latter than to the former value.— P. G. T. 



