THE MINING DISTRICTS OF CORNWALL. 341 



formed at moderate temperatures becomes probable from certain 

 phenomena observed at a selfatara, known as the Sulphur Bank, 

 near Borax Lake, California. Here one of the fumaroles, which is 

 alternately filled with water at a temperature considerably below 

 the boiling-point, or serves as a channel for the egress of various 

 gases and vapours, has its sides coated with gelatinous silica, beneath 

 which is a layer of chalcedony resting on crystalline quartz. On 

 drying, the gelatinous deposit assumes the appearance of ordinary 

 chalcedony ; and the mixture was found on analysis to contain no 

 less than 7 per cent, of mercury in the form of cinnabar. The 

 quartz-crystals of this deposit are seen under the microscope to con- 

 tain the usual fluid-cavities, in which the bubbles bear the customary 

 varying proportion to the enclosed liquid. It contains, in addition, 

 numerous gas- and vapour-cavities : and many of the fluid-cavities 

 have the form of well-defined negative crystals. 



The fissures enclosing mineral veins have evidently been subjected 

 to a series of repeated widenings, such as would result from any 

 movement of their irregular surfaces, but which may also sometimes 

 be the result of mechanical forces developed by the crystallization 

 of the various included minerals. In many cases the original 

 opening would appear to have been a mere comminuted fracture of 

 the rock in a given general direction, between the several planes of 

 which a deposit of mineral substances has subsequently taken place 

 through chemical agency. The final result, in such cases, will be a 

 brecciated veinstone of the kind so constantly met with in mineral 

 districts. 



The examination of a very large number of sections made from 

 Cornish veinstones shows that the quartz, which is usually more or 

 less crystalline, contains numerous gas- and fluid-cavities, and en- 

 closes ores of tin, copper, zinc, and other metals. Capels are most 

 frequently composed of a quartzose base, through which crystals of 

 schorl are very thickly disseminated either in the form of spheroidal 

 aggregations radiating from various centres, or as acicular crys- 

 tals crossing one another in all directions. Sometimes, particularly 

 when they occur in slates, capels are a mixture of quartz and 

 chlorite ; in others, both chlorite and tourmaline are present. They 

 also often contain innumerable small fragments of the country rock, 

 and are traversed by narrow strings of quartz into which project 

 hair-like crystals of schorl, which are generally attached to the sides 

 of the enclosing fissure. 



In addition to the foregoing, capels frequently enclose crystals 

 and crystalline groups of quartz (traversed by belonites of schorl) 

 which sometimes appear to have been broken, by contraction or 

 otherwise, and afterwards repaired by a growth of schorlaceous 

 matter within the crack. 



Fig. 5. Represents a patch of crystalline quartz in a capel from 

 Botallack, magnified 15 diameters, which has the appearance of 

 having been thus broken, and the fissure afterwards filled with 

 tourmaline. From the way in which the needles of schorl penetrate 

 into the substance of the enclosing quartz, it would almost appear 



