418 EDGAR HALL. 
and had forced a layer of schist outwards into the drive. The 
crystals were of a brilliant blue colour, and of course were very 
brittle, but in other respects the resemblance to fibrous malachite 
as seen in the schist at other places in the mine was complete. 
The resemblance at once suggested the origin of fibrous mala- 
chite, namely, that it is a pseudomorph after sulphate of copper. 
The ordinary text-books of mineralogy seldom hint at the method | 
of formation of minerals, and in the case of malachite the writer 
has so far been able to find only one explanation of its formation. 
Frank Rutley' suggests that the mineral ‘“‘has in most cases 
resulted from the percolation of water through copper-bearing 
rocks, and the subsequent deposition of the dissolved carbonate 
in fissures and cavities.” This explanation seems improbable in 
view of the insolubility of copper carbonate. Watt? states that 
the basic carbonate requires a pressure of four to six atmospheres 
for solution in water containing carbonic acid. Such pressures 
are impossible under natural conditions at the short distance from 
the surface within which malachite is found. The deposition from 
solution also presupposes the formation of the carbonate from the . 
chalcopyrite which must have formed its starting point, and this 
presents equal difficulties. 
The production of sulphate of copper from cupreous pyrites is 
the first and simplest result of oxidation, and from sulphate of 
copper any soluble carbonate will, at ordinary temperatures and 
pressures, produce a basic carbonate of the composition of mala- 
chite. Verdigris, the product of slow oxidation of metallic coppet 
in moist air at ordinary temperatures, also has the composition of 
malachite, but it is hardly likely that the alteration of cupreous 
pyrites will follow that route, 
Given the oxidation of cupreous pyrites by surface influences 
and the formation of fibrous crystals of sulphate of copper - 
cavities of a lode during a prolonged period of dry weather, it is 
easy to understand that during an ensuing period of wet weather, 
Co 
1 Mineralogy, p. 211. 2 Dictionary of Chemistry, New Edition. 
