336 Proceedings of Royal Society of Edinburgh. [sess. 
On theoretical grounds it has been stated that one might expect 
to find that the conversion of the Lead Sulphide, Galena, into the 
Carbonate, would have left some traces of the sulphate, Anglesite, 
seeing that the process involves the oxidation of the sulphur present 
in the original ore of lead. In actual experience this does not 
appear to be in all cases supported by the facts, as Cerussite may 
occur quite commonly without the slightest trace of the sulphate. 
What really happens is, that the solutions arising from the decom- 
position of the Galena pass almost directly into the carbonate, if any 
carbonate of lime happens to be present. Hence Anglesite is rarely, 
or perhaps never, found in any of the lead veins traversing the Car- 
boniferous Limestone areas of the North of England. It is only in 
the veins traversing non-calcareous rocks that one may look for 
Anglesite with any hope of success. In the Leadhills district, 
where the rocks are non-calcareous, both Cerussite and Anglesite 
are found in close contiguity. There is a slight difference in their 
mode of occurrence. Both are confined to metalliferous veins ; 
but the Anglesite usually occurs actually grown upon the Galena 
by whose decomposition it has arisen ; while the crystals of 
Cerussite are more commonly found growing upon other constituents 
of the mineral veins, although in no case at any great distance from 
the parent mineral. It rarely occurs quite alone, but more usually 
in close association with other products of decomposition. In the 
Leadhills mines its common associates are Pyromorphite, Vana- 
dinite, Chrysocolla, Malachite, and Limonite. Some of the rarer 
minerals associated with it are, Leadhillite, Linarite, Caledonite, 
and others. Chalcopyrites, as well as Blende, and one of its 
decomposition products, Hemimorphite, the Orthorhombic Hydrous 
Zinc Subsilicate, are also common in the same veins. 
The history of the Leadhills metalliferous veins appears to be 
somewhat more complex than that of the great majority of mineral 
veins. They occur, as usual, in fault fissures; which appear to 
have acted as channels of communication between the inner 
portions of the Earth’s crust and its surface, at many different 
geological periods. Hence the veins in question have probably 
been filled by the uprise of thermal waters, and subsequently 
depleted to some extent, by the action of surface waters, on several 
different occasions. The varied and complex nature of the 
