IS 13.] Mineralog'ical Observations on Cornwall. 31!> 



for the mines are all situated in the clay slate, frequently at no 

 very great distance from the granite. Now when these mines go 

 deep, it sometimes happens that they penetrate through the 

 whole of the clay slate, and continue on in the granite ; thus 

 affording a demonstration that the granite lies under the clay 

 slate. This is the case, for example, in the mine of Huel Unity, 

 two miles east from Redruth. 



This clay slate, in every part of the county that I ohserved it, 

 as for example at Truro, at Bodmin, at Launceston, and in the 

 mines extending about ten miles cast and west from Redruth, 

 has the characters of transition clay slate. It has not the conti- 

 nuous lustre which distinguishes primitive slate ; but is almost 

 dull, glimmering only from the specks of mica which are scat- 

 tered through it. The colour is most commonly dark blue, or 

 purple. This rock is uniformly distinguished in Cornwall by the 

 name of killas. Hence, if my opinion be well founded, the 

 Cornish term killas is synonymous with transition slate. I have 

 every reason to believe that it is never applied to greyicacke. 

 Hence the substitution of it for greywacke, as has been proposed 

 by Mr. Playfair, and practised by Mr. Allan and Sh J James 

 Hall, would be the introduction of an error, and would uni- 

 formly mislead every mineralogist who drew his notions of grey- 

 wacke from the kilias of Cornwall. 



In the parish of St. Stephen's, about five miles from St. Austle, 



there occurs in the granite range a curious rock, which is wrought 



in many places, and constitutes the material from which the 



porcelain clay for the Staffordshire potteries is obtained. This 



rock consists of a white and very soft felspar basis, which 



crumbles to powder between the fingers, and falls down in the 



state of clay in water. Small transparent quartz crystals are 



thick scattered through this basis. 1 could perceive no mica, 



and have reason to believe that the rock contains none. This 



rock is sent to Staffordshire in two states. It is quarried out in 



lumps, and sent in that state without any preparation. These 



lumps are ground down in Staffordshire, and the quartz powder 



acts the essential part of counteracting the too great tendency 



which the clay has to contract. The second mode in which it 



is sent to Staffordshire is quite different. The rock is exposed 



to a current of water, which, washing off' the fine clay particles, 



runs milk white into pits prepared for the purpose. Here the 



water gradually evaporating leaves the line white clay, which is 



dug out in squares, and when dry packed up in barrels, and sent 



to the sclj. hore. Only the purest water can he used for this 



purpose, and great care is taken that the instruments used are of 



■UCn a nature as not to communicate any metallic stain, which 



would render the clay unsaleable. 



The excellence of this rock consists in the almost total ah- 



