THE MINING DISTRICTS OK CORNWALL. 321 



them, varies considerably in colour, structure, and composition, and 

 has in some places been subjected to such an amount of metamorphism 

 as to render it essentially a crystalline rock. Its colour is most 

 frequently grey, greyish blue, or blue ; but it has sometimes a green, 

 brown, purple, or violet tint. It is usually lamellar and fissile, but 

 it is occasionally intersected by numerous minute veins of white 

 quartz, by which its cleavage is impaired. 



Intercalated bodies of hornblendic slates occur in masses, which 

 are sometimes of considerable extent, as at Penzance and in the 

 neighbourhood of St. Ives. 



There can be no doubt that the slates were once deposits of 

 clayey mud, intermixed with variable proportions of siliceous and 

 other sands, resulting from the disintegration and partial decompo- 

 sition of previously existing rocks. Their chemical composition and 

 mineralogical constitution will consequently vary in accordance with 

 the character of the original rock, the nature and extent of the 

 decomposition they have experienced, and the amount of meta- 

 morphism to which they have been subjected subsequently to their 

 deposit. 



It has been shown by Daubree and others that felspathic rocks 

 subjected to prolonged trituration in presence of pure water undergo 

 a decomposition by which alkaline silicates are obtained in solution, 

 while the rock is reduced to the state of mud or sand*. Decompo- 

 sitions of this nature must have been effected on an extensive scale 

 during the transformation of such rocks into clay-slates ; and the 

 composition of the resulting sedimentary beds may therefore be ex- 

 pected to differ considerably from that of the rocks from which they 

 were originally derived. 



With the view of determining the composition of these slates, and 

 of ascertaining the nature of the changes produced in them by meta- 

 morphism, I have, at various times, subjected a great many different 

 varieties to analysis, and have cut and examined, under the micro- 

 scope, a much larger number of sections. Two concurrent analyses 

 were made of each rock ; and the mean results are embodied in the 

 following Table f (p. 322). 



I. Killas from Polgooth Mine, St. Austell, Adit level. — Has a 

 decidedly argillaceous smell, and adheres slightly when applied to 

 the tongue. Colour generally light grey, marked with darker 

 shades of the same tint, and occasionally stained with yellow or 

 brown. Thin sections of this slate, when examined under the 

 microscope, are observed to be traversed by numerous fissures, which 



* " Le feldspath en fragments soumis a une lcngue trituration en presence de 

 l'eau distillee, et dans des cylindres en gres, subit une decomposition notable 

 accusee par la presence dans l'eau de silicate de potasse qui la rend alcaline " 

 (Rapport sur les Progres de la Geologie Experimentale en France, p. 49). 

 Similar results were obtained by Kenngott, " Ueber die alkalische Reaction 

 einiger Minerale," in the 'Neues Jahrbuch ' for 1867, pp. 302, 429,769. 



t In preparing rock-specimens for analysis, several unweathered fragments 

 from each locality were first crushed on a steel anvil ; the resulting coarse 

 powder was subsequently well mixed, and a sufficient quantity for all the 

 various estimations was afterwards pulverized in an agate mortar. 



z2 



