ON THE DETRITAL TIN-ORE OF CORNWALL. 197 
Op The tin-ground, throughout its entire width, is divided—obliquely . 
both to its surface and to the (shelf) rock ‘it rests on—by a thin 
seam of clay impervious to water; and on opposite sides of this 
it is of remarkably different appearance. 
The upper part consists of angular and subangular masses of slate, 
of quartz, and of various (Shodes) vein-stones, and of smaller 
bodies and granules of crystalline tin-ore bearing traces of 
fracture or of abrasion, all imbedded in bluish clay. 
i the lower portion pebbles of slate still prevail, and nodules of 
felspathic porphyry (Hivan) are not uncommon; but quartz 
occurs less frequently and in smaller masses; the tin-ore rarely 
appears unmixed, but—contrariwise—is diffused through ma- 
trices natural to it in the slate-series. The interstices are filled 
with tough clay 6f reddish-brown hue. The deepest are not 
always the richest parts of the deposit...... 00000060 aq 00000 9 feet. 
The works are within a mile anda half §.H. of the granite, yet they 
afford no trace of granitic matter. 
The tender and fissile (Shelf) slate beneath—often little other than 
laminated-clay—rapidly softens on exposure; and the perceptible disinte- 
gration and consequent increase of volume, give—so to speak—a yeasty 
appearance to the semi-fluid mud. 
Small quantities of Stream-tin-ore have been obtained also at 
Penrose in Sennen, Tregadgwith in Saint Burian, and in some 
other glens which open to the sea between the Land’s-end and 
Penzance ; but—even at the high prices which have lately prevailed 
—the proceeds have not sufficed to pay the workmen. Near the 
confluence of the streams from Cold-harbour-moor* and Tregilsoe,+ 
at the head of Marazion marsh,} an inconsiderable deposit of this 
* Ante, p. 196. 
¢ Ibid, p. 196. 
t «The open sea (as I have seen in the Mount’s Bay) throws in [tin- 
ore | to us in a pulverized state.” [This] ‘‘ comes probably from some lodes, 
‘which, lying bare to the sea, have their upper parts fretted off, and by 
‘‘storms thrown in among the sands.” Boruasrz, Natural History, p. 164. 
‘The sand of the eastern or Marazion green affords tin-ore sufficient 
‘to pay in some measure for its streaming, which process on a small scale 
‘‘ig here in actual operation.” 
Boasz, Cornwall Geol: Trans: iii, p. 178. 
‘A bed of stream tin-ore of very inferior produce, some 20 or 30 feet 
‘‘ above the sea-level near Newtown, on Marazion green, has for many years 
‘afforded employment to a few persons; and in the vale between Wheal 
“ Darlington and the Marazion Mines a thin and poor bed of tin-ground rests 
‘“‘on the shelf at about the sea-level; on this reposes a bed of vegetable 
‘‘ matter containing the trunks and branches of oak, willow, hazel, and per- 
‘haps other trees, as well as nuts in abundance; this is again covered, to 
‘the level of the slimy soil which bears the present vegetation, by sea- sand 
‘‘ mixed with shells.” Hrnwoop, /bid, v, p. 34. 
Carne, [bid vi, p, 233. 
