502 PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES, 
To the Dolomite above-mentioned, as occurring pseudo after scalenohedra of 
calcite in the trap tuff at Kinkell, near St Andrews, very unusual interest 
attaches. - Its mode of occurrence and appearance are as follows :— 
The extremely friable tuff of this locality, which from its being wave-washed 
has probably to a considerable extent been exhausted of all constituents soluble 
in water, is rifted and reticulated with shrinkage cracks, which by the process of 
exfiltration are now filled with the above Dolomite. On these “ exfiltration 
veins,” amplifications or ‘“ bunches ” rarely occur ; these being still more rarely 
drusy or cavernous. 
The contents of the druses, arranged in the order of their deposition or 
nearness to the sides of the cavity, are the following :— 
The scalenohedra of Dolomite after calcite of a dull yellow colour ; semi- 
transparent quartz of a pinkish salmon colour, in doubly terminated prisms ; 
“nail-head ” corded aggregations of rhombohedron g crystals of calcite, devoid 
of colour; double six-sided pyramids of colourless quartz. 
The scalenohedra of Dolomite are considered to be pseudomorphs upon the 
following grounds :-- 
1st, Fully developed crystals of the form 7 do not occur of Dolomite, 
though that form occurs as a hemihedral modification of some of its dominant 
rhombohedral forms; while the scalenohedron 7 is one of the commonest forms 
of calcite. 
2d, The cleavage of the crystals, instead of being accordant with a single 
crystal of 7, and continuous across the crystal, is confused, interrupted, and 
evidently in several directions ; leading to the belief that the fracture meets a 
number of crystals lying in every direction. 
3d, The crystals under the lens present every appearance of being pseudo- 
morphic. Upon being broken, the great mass of some is seen to consist of 
unaltered or rather unabstracted colourless calcite ; there being merely an out- 
side layer of the yellow Dolomite: in others a core of calcite, and that fre- 
quently loose, and which rattles when shaken, alone remains within a thick 
casing of Dolomite ; while in a considerable number no trace of calcite is to 
be seen. 
In these last, namely, in those wholly consisting of Dolomite, a central cavity 
invariably occurs; in the specimens which contain a core of calcite, an annular 
vacuity between it and the Dolomite is as unvariably to be seen; while slight 
pressure with the nail will crack off and remove the thin layer mentioned in 
the first of these cases; there being a want of absolute attachment between 
this layer and the invested calcite: 
Ath, The surfaces of the faces of the scalenohedra are commonly rent and 
depressed; in other words, they do not form flat bounding planes, but show 
curved lines, or blunt re-entering angles. 

