168 ON A NEW OPTICAL AND MINERALOGICAL PROPERTY 



interrupting stratum or film abed, Fig. 7. The upper and 

 lower edges, fp->'gq<> of the stratum, are so distinctly crystal- 

 lised, that, by the aid of my reflecting goniometer, their incli- 

 nation to MB, NF, or the angle M/p, was found to be 141° 44'. 

 Now, if we suppose that the fiimfgqp had originally the 

 position of/ g q"p on the upper surface MB of the rhomboid, 

 and was brought into its present position by turning round/, 

 it is obvious, that the angle p'fp must be equal to the acute 

 angle Bfg or BMN of the principal section. But this angle, or its 

 equal M///, is known to be 70° 51 46", as computed from the 

 accurate measures taken by Malus : hence the angle p'fp is 

 also 70° 51' 46 ; and the whole angle Mfp is equal to double 

 of either of these angles, or 141° 43' 32", — a result which co- 

 incides almost exactly with the angle which we obtained from 

 direct experiment. 



Here, then, we have a new fact in Mineralogy, of which I 

 believe there is no other example, viz. That the crystallisation 

 of a vein is regulated by that of the mass which contains it, the 

 axes of the vein being constantly inclined 45° to those of the 

 rhomboid. Nor is this a fact of rare occurrence. The speci- 

 mens of calcareous spar which are intersected with these veins, 

 are as numerous as those which are free from them ; and 

 those which occur in the trap rocks on the east coast of Scot- 

 land, near Montrose, invariably possess this property to such 

 a degree, that they are rendered semi-transparent by the nu- 

 merous veins with which they are traversed. When a candle or 

 a luminous object is viewed through this kind of spar, it is 

 multiplied in such a manner, that the images are heaped toge- 

 ther in the most regular forms, and are tinged with the finest 

 colours, constantly varying with the inclination of the speci- 

 men. 



When these specimens are carefully examined, they exhibit 

 what has never before been observed, two or more sets of 



veins 



