338 Proceedings of Royal Society of Edinburgh. [sess. 
dissolution of any part of the Cerussite. Certain of the contours 
of the crystals seem at first sight to be rounded ; but this feature 
is, on closer examination, seen to be due to the co-existence of a 
number of adjacent faces, usually brachy domes, which form but 
small angles with each other. It is possible, of course, that these 
may represent later crystalline growths upon a surface rendered 
convex by the action of solvents. 
Scottish Cerussites are usually colourless, or, at the most, of a 
pale cream-colour. A few are tinted green, but whether the 
colouring matter in this case is due to admixture with compounds 
of Vanadium, or of copper carbonate, has not yet been determined. 
Some Cerussites are lead grey, ranging to sooty brown. The 
colouring matter in this case may prove to be lead dioxide. (A 
compound or mixture largely consisting of lead dioxide has been 
stated to occur at Leadhills. Some mineralogists have regarded 
this as a definite compound, and have elevated it to the rank of a 
separate mineral species, under the name of Plattnerite. It seems 
to me to be a decomposition product, of indefinite composition, 
which may have arisen through the action of surface waters upon 
what has been called plumbo-calcite.) It is remarkable that so 
little of the Cerussite shows any trace of coloration by the 
Vanadium compounds, although these have imparted brilliant 
yellow, green, and other tints to the associated Hemimorphite and 
Pyromorphite. The Vanadium in the Leadhills mines may 
possibly have originated through the decomposition of the ferro- 
magnesian silicates present in the Arenig lavas there, as small 
traces of Vanadium can usually be detected in the fresh minerals 
in question. Possibly the phosphoric acid, to which the Pyro- 
morphite is partly due, may have arisen from the decomposition of 
the Apatite crystals in the same rocks. 
The point • of attachment of the Scottish Cerussites appears to 
be usually c {001}. The direction of greatest elongation varies in 
different crystals, and does not appear to follow any definite law. 
But elongations parallel to a {100} and to c {001} are commoner 
than others. The subjoined figures (10 to 17) will illustrate the 
diversity of habit referred to. 
Traces of a kind of polysynthetic growth in directions parallel 
to b {010} are so very general that they may be regarded as of 
