162 NATURAL HISTORY 
called deads. Thefe ftreams are of different breadths, feldom lefs 
than a fathom, oftentimes fcattered, thongh in different quantities, 
over the whole width of the moor, bottom, or valley, in which 
they are found ; and when feveral fuch ftreams meet, they often- 
times make a very rich floor of tin, one ftream proving as it were 
a magnet to the metal of the other. 
In the tenement of Douran, in the parifh of St. Juft, (Penwith) 
in the year 1738, there was a very Angular ftream of tin difcovered; 
the ore was pulverized, betwixt one foot and one foot and a half 
in depth or thicknefs, of various breadth. In the moory ground, 
where it was firft difcovered, it had a back of foil and gravel over 
it, only two feet high, but, as the ftream advanced farther to the 
eaft, it had ftill a higher covering, till at laft it had all Douran hill 
(which may be about forty feet perpendicular) over it, the ftream 
continuing ftill its horizontal pofttion. 
That this ftream was collected from the adjacent Jlrata , and then 
fpread in this equable manner by the force of waters, is extreamly 
probable; but how it fliould become covered with fuch a large 
heap of rubble, clay, and gravel, as compofes the hill of Douran, 
is not fo eafy to decide : there are indeed ftrong proofs, in feme of 
the adjoining cliffs, of large heaps, very little inferiour to this in 
quantity, which were moft likely laid where we find them by the 
waters of the deluge. But whether this remarkable pofttion of are- 
naceous tin is owing to the waters of the flood (which indeed is a 
moft fertile folution of fubterraneous difficulties, but I fear too often 
recurred to) may be well queftioned. It feems to me, that where- 
ever there is an horizontal extended vacancy betwixt the Jlrata , and 
at the bottom of that vacancy an even hard floor, either of ftone, 
clay, or gravel, into which the waters cannot readily fink, and 
lodge their depofits in chinks and crevices, there the waters will 
fpread their contents horizontally. Suppofe then the Jlrata of 
the hill of Douran to be well impregnated with this arenaceous tin, 
the waters percolating thro’ the hill would by degrees congregate and 
wafh it forwards till it met with fuch an horizontal floor as has been 
mentioned, which the tin, not being able by its weight to pene- 
trate, muft confequently fpread itfelf upon the furface in breadth 
and thicknefs anfwerable to the vacancy which receives it ; and this 
is moft likely to be the cafe, and may happen under the higheft 
and moft rocky mountains which flood unfhocked by the flood, as 
well as here at Douran under a gentle rifing. 
There are feveral Jlreams of tin in St. Stephen’s Branel, St. Ewe, 
St. Blazy, and other places, but the moft confiderable ftream of 
tin in Cornwall is that of St. Auftel moor, which is a narrow valley 
about a furlong wide, (in fome places fomewhat wider) running 
near 
