68 



Observations on a remarkable Deposit of Tin-Ore at the 

 Providence Mines, near St Ives, Cornwall. By William 

 Jory Henwood, Esq., F.R.S., F.G.S., Member of the 

 Geological Society of France, &c. Communicated by the 

 Author.* 



The Providence Mines, in the parish of Lelant, comprise the mines 

 formerly known as Wheal Speed, Wheal Laity, Wheal Comfort, 

 and Wheal Providence, long worked on the eastern side of the hill 

 which slopes from Knill's monument to the sea. 



(a) Observations on the eastern workings in the slate, and on the 

 western within the granite formation, have already appeared in the 

 Royal Cornwall Geological Society's Transactions, "f The interme- 

 diate tract, now to be described, is wholly in granite, of which the 

 upper beds are composed of a basis of greyish felspar and quartz, 

 imbedding medium-sized crystals of white felspar, as well as numer- 

 ous small groups of schorl in radiating crystals : but near the produc- 

 tive parts of the lodes the rock is mostly rather coarse-grained, its basis 

 is greenish- grey felspar, black mica, and quartz ; and the included 

 porphyritic crystals of felspar are either of a pale buff, a pink, or a 

 reddish-brown hue. 



(6) These veins are — 



The Cross-course or Trawn, which bears about 22° W. of N., and dips E.J 



Wheal Comfort lode „ 15° W. of N., „ W. 



and Wheal Laity lode or lodes „ 17° S. of W., „ S. 



Connected with the Wheal Comfort lode there isa " Carbona,'^ 

 to which further reference will be made presently. 



It may be here stated generally, that the Cross-course is from 

 one foot and a half to two feet in breadth, and is composed of disin- 

 tegrated fine grained granite, divided by numerous joints parallel to 

 the "walls;" as well as by many other curved and irregular ones 

 which intersect each other in every imaginable manner, and are 

 filled with oxide of iron, and closely but unconformably striated. 



The Wheal Comfort lode varies in width, from a few inches to 

 more than six feet. At a distance from the Wheal Laity lodes it 

 is of granite, very thinly impregnated with tin-ore ; — the remainder 

 consists of quartz, schorl-rock (capclj, brown iron-ore, and greenish 

 and brownish felspar, in some places, — near the Wheal Laity lodes, 

 — abounding in tin-ore. 



* For the Paper in full vide vol. vii. of Transactions of the Royal Geological 

 Society of Cornwall. 



t Vol. v., pp. 16-20; Tlate ii., fig. 7; Tables 21 and 22. 



| The " directions " have reference to true north, the " dips" are from the 

 horizon. 



§ I have already described a similar though a much smaller formation in one 

 of these mines. Corn. Geol. Trans., v., Table 22. 



