in Cornwall and’ Devonshire. 419 
the rock, and-covered’ by an overburden, as the streamers call 
it, which is sometimes:from 20 to 70 feet thick. The tin is in 
rounded fragments, sometimes as large as walnuts, but more ge- 
nerally in the state of sinall gravel, and even of fine sand ; it is 
imbedded in loose matter, composed of the detritus of the rocks 
from which it may be supposed to have been separated. 
The principal peculiarity of stream tin is the: absence of any 
other metallic mixtures, except nodules of hematitic iron ore, 
which sometimes accompany it. This circumstance fits it for 
producing a very pure metal. This is not the place to speculate 
on the causes which have so completely freed these ores from 
substances with which they were. in all probability originally 
combined, or to inquire whether it is to be attributed to mecha- 
nical action, or whether it has been effected by decomposition ; 
but it may be remarked that, besides the hematite already men- 
tioned, only the indestructible metals, and the oxide of tin, are 
now discovered existing in deposits of this nature. 
The operations of dressing stream tin are simpler than those 
for mine tin. It is smelted also in a different manner, and pro- 
duces a superior metal known by the name of grain tin, which 
is principally used by the dyers, and for the finer purposes. 
The processes for dressing mine tin are in many respects the 
same as are used for all other ores, but are subject to some varia- 
tions, which are attributable to the following peculiarities. 
1. Being for the most part found intimately dispersed through- 
out the matrix, the whole is necessarily pounded down to a very 
fine state, to admit of the perfect separation of the ores. 
2. That heing unalterable by moderate degrees of heat, it ad- 
mits of calcination, by which the specific gravity of the sulphu- 
rets or arseniats with which it is mixed, may be lessened, and a 
mode obtained of rendering them more separable. _ 
3. That the weight of tin ore being greater than most others, 
it is less liable to waste in the processes of washing, and, there- 
fore, may be dressed so as to be nearly clean from all substances 
not actually adhering to it. 
From the first of these peculiarities it follows, that all tin 
mines must be furnished with stamping-mulls of sufficient power 
to bruise down the ores raised, which is generally done so as to 
produce a minute division of the whole, and on this account, 
formerly, the quantity and fall of water that could be applied ‘to 
this purpose usually limited the quantity of ore that could be re- 
turned from a mine, or the whole was frequently carried to some 
spot favourable to the erection of water-wheels to be applied to 
this purpose. Within a few years steam-power has been applied 
to stamping-mills, and has tended to increase the supply of tin 
ores, Engines for this purpose, of considerable power, are 
3G2 working 
