THE MIKING DISTRICTS OF CORNWALL. 323 



have become filled by crystallized transparent quartz. The struc- 

 ture of this rock is exceedingly indistinct ; but when examined be- 

 tween crossed prisms, the mass breaks up into a sort of shadowy 

 mosaic of variously coloured patches, without definite outline. 



II. Killas from Polgooih Mine, Eighty-fathom level. — This speci- 

 men is harder than that obtained from the adit level, and is of a 

 grey colour, marked by reddish-brown spots. The appearance of 

 sections of this rock, when placed under the microscope, does 

 not differ materially from that of those of the slate nearer the 

 surface. 



III. Killas from Polgooth Mine, One-hundred-fathom level. — The 

 clay-slate from this depth is rather harder than that found at shal- 

 lower levels. Sections prepared from specimens from this locality 

 very closely resemble those from the eighty-fathom level. 



IV. Killas from Polmear Mine, St. Austell, forty fathoms below 

 the surface. — This specimen is much contorted, and is readily divided 

 into curved laminae with glassy surfaces, on which are numerous 

 wavy lines resembling minute ripple-marks. Its colour is grey, in 

 places slightly tinged with yellow. Sections of this rock, when exa- 

 mined under the microscope, exhibit a crypto crystalline structure, 

 through which numerous small grains of quartz are disseminated. 

 A greenish tint is imparted by flocculent microliths. 



V. Killas from " Sanctuaries," St. Austell. — This, in common with 

 many other clay-slates lying above the natural drainage-level of the 

 country, is very light in colour, the prevailing tints being grey and 

 buff ; and it bears the appearance of alteration. 



VI. Killas from Dolcoath Mine, Camborne, Two-hundred-and- 

 Jlfteen-fathom level. — This rock is exceedingly hard, has an imperfect 

 cleavage, and is of a dark grey colour. Under a low power, it is seen 

 to be an aggregation of quartzose granules, through which patches 

 of a greenish tint are disseminated. When a power magnifying 400 

 diameters is employed, small grains of magnetite are distinguished, 

 from which, as a centre, indistinct fan-like aggregations, of perhaps 

 some variety of chlorite, diverge in all directions*. It also contains 

 some broken and rounded plates of mica, and a few fragments of a 

 brown semitransparent mineral. The two last are mechanically 

 imbedded in the slate. 



VII. Roofing -slate, Delabole, Camelford. — This slate is of a grey 

 colour, and cl javes readily into thin laminae. The surfaces indicating 

 planes of bedding are frequently covered by a thin crystallized film 

 of calcite. Sections of this rock, under a 1-inch objective, do not 

 exhibit any very distinct structure ; but they are seen to be dotted 



* It may be objected that rocks affording by analysis " traces " only of mag- 

 nesia cannot contain an appreciable amount of chlorite; it must, however, be 

 borne in mind that when sections are examined under the microscope the green 

 mineral is seen disseminated in a flocculent form, and only in sufficient quantity 

 to impart a greenish tint to certain patches. It is also necessary not to lose sight 

 of the fact that some chloritic minerals contain but a very small proportion of 

 magnesia, while chloritokl is frequently altogether without it. 



