234 Geological Society: — 



excavated to their present depth, and forest-growth took place upon 

 the old marine plain. The forests were then gradually circumscribed 

 by the encroaching sea and diminishing rainfall, which also led to 

 changes in the streams ; and finally the sea entombed the forests and 

 swamps on the coasts, and produced the present cliff-line. The 

 results of this period are the submarine forests, most of the river- 

 valley gravels, and alluvial tracts bordering the present river- 

 courses. 



January 9, 1878.— Prof. P. Martin Duncan, M.B., F.R.S., 



President, in the Chair. 



The following communications were read : — 



1. "On the Great Plat Lode south of Redruth and Camborne." 

 By Dr. C. Le Neve Poster, B.A., F.G.S. 



The author described the mode of occurrence of the stanniferous 

 deposit known as the Great Plat Lode, the mines worked in which 

 extend for a distance of 3| miles, and furnish about one eighth of 

 all the tin raised in Cornwall. The mines in question are Wheal 

 Uny, South Carn Brea, "West Wheal Basset, South and West Wheal 

 Prances, South Condurrow, and Wheal Grenville ; and in all the 

 lode dips at a much less angle than the average of Cornish veins, 

 the dip at Wheal Uny being only about 46° S. Throughout the 

 lode contains a small leader, usually only a few inches wide, 

 occupying the space due to the shifting of the two sides of a fissure, 

 and filled partly mechanically, partly chemically. Above, or below, 

 or on both sides of this there is a mass of stanniferous schorl rock 

 from 4 to 15 feet wide; this contains from 1 to 3 per cent, of 

 cassiterite, in little grains, or in strings or veins. Schorl rock, very 

 poor in tin (locally called capel or greybacJc), separates the lode from 

 the surrounding granite or killas, but passes on one side into the 

 lode, and on the other into the granite or killas, so that no ivall is 

 recognizable. Prom these characters the author inferred that the 

 lode and the capel are merely altered rocks, the fissure now occupied 

 by the leader having served to bring up vapours or solutions which 

 have entirely changed the rocks on both sides of it. In support of 

 his opinion, the author adduced other instances of the change of 

 both granite and killas into schorl rock ; and further stated that, 

 both at South Condurrow and Wheal Grenville, he has found in the 

 schorl rock cavities as large as a pea, agreeing in form with crystals 

 of orthoclase felspar. 



2. " On some Tin-mines in the Parish of Wendron, Cornwall." 

 By Dr. C. Le Neve Foster, B.A., P.G.S. 



The mines described in this paper are called Balmynheer, The 

 Lovell, and South YVendron. In the former the stanniferous de- 

 posit consists of a large irregular mass of rock 30-50 feet thick ; its 

 dip is 1ST., at an angle of about 30°, and its strike E. 32° jN\, along 

 which it has been traced for 36 fathoms. The tinny rock is sepa- 

 rated from the granite above by a slide or vein of white clay, with 



