Geological Society of London. 235 



beach. A probably analogous band of flints has been noticed at Eed 

 Hill, Berks, by Prestwich. The direction of the currents wearing 

 away tbe clay bands and depositing the galls and sands was suggested ; 

 and these observations were offered as further materials in working out 

 the hydrography and history of the Lower Tertiaries. 



3. " On the Origin of Slickensides, with remarks on specimens from 

 the Cambrian, Silurian, Carboniferous, and Triassic formations." By 

 D. Mackintosh, Esq., E.Gr.S. 



This paper was founded on specimens, a selection of which was 

 exhibited. The author stated that his observations led him to believe 

 that true slickensides are produced by the movement of one face of rock 

 against another, accompanied by partial fusion. He indicated that in 

 many cases the slickensided surfaces are not only polished and striated, 

 but also hardened, and that there is an imperceptible gradation from 

 this hardened film to the ordinary structure of the rock. 



II. — March 10, 1875. — John Evans, Esq., V.P.R.S., President, in 

 the Chair. — The following communication was read : — 



" The Bocks of the Mining Districts of Cornwall, and their relation 

 to Metalliferous Deposits." By John Arthur Phillips, Esq., M.I.C.E., 



E.Gr.S. 



In this paper the author adduced numerous facts observed by him in 

 the examination of the rocks of the mining districts of Cornwall, which 

 led him to the following conclusions: — The clay-slates of Cornwall 

 differ materially in composition, but no re-arrangement of their consti- 

 tuents could result in the production of granite. Some of the " green- 

 stones " of the Geological Survey Map are volcanic rocks contempo- 

 raneous with the slates among which they are found, whilst others are 

 hornblendic slates, diorites, etc. Granites and elvans having a similar 

 chemical and mineralogical composition, were probably derived from 

 the same source ; but the volume of the bubbles in the fluid-cavities of 

 both having no constant relation to the amount of liquid present, do 

 not afford any reliable data from which to calculate the temperatures 

 at which these rocks were respectively formed. The stone-cavities of 

 elvans, and probably of some other rocks, are often the results of the 

 irregular contraction, before the solidification of the base, of imbedded 

 crystals of quartz. In rocks having a glassy base, glass-cavities will 

 be produced. The vein-fissures of the tin- and copper-bearing lodes of 

 Cornwall were produced by forces acting after the solidification of the 

 elvans, but in the same general direction as those which caused the 

 eruption of the latter ; and these fissures were afterwards filled with 

 minerals deposited by chemical action from water and aqueous vapours 

 circulating through them, but not necessarily at a high temperature. 

 How far these deposits were produced by water rising from below or 

 influenced by lateral percolation cannot be determined ; but the effects 

 produced on the contents of veins by the nature of the inclosing rock, 

 and the occurrence of deposits of ore parallel with the line of dip of the 

 adjoining country, lead to the conclusion that lateral infiltrations must 

 have materially influenced the results. Contact-deposits and "stock- 

 werks " have been formed by analogous chemical action, set up in 

 fissures resulting from the junction of dissimilar rocks, or in fractures 

 produced during the upheaval of partially consolidated eruptive masses. 

 The alteration produced in stratified deposits in the vicinity of eruptive 



