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 ineli- 
nation to MB, NF, or the angle Mfp, was found to be 141° 44% 
Now, if we suppose that the film fgqp had originally the 
position of f g' q p' on the upper surface MB of the rhomboid, 
and was brought into its present position by turning round f, 
it is obvious, that the angle p' fp must be equal to the acute 
angle Bég or BMN of the principal section. But this angle, or its 
equal Mfp’, is known to be 70° 51 46", as computed from the 
accurate measures taken by Matus: hence the angle p'f p 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 
