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west corner of our island has long been famous. I am not this evening going 
to enter into a treatise on mining ; but am going to give a few facts drawn either 
from my own observation, or from conversations with the practical miners on 
the spot, many of whom are men of great intelligence. Perhaps of all practical 
operations mining is that which has least been reduced to a science, and indeed 
I must confess myself mostly ignorant of the probable conclusions drawn by 
students of that branch of Mineralogy ; still, unless I am mistaken, the very 
elementary question, How did the ore get into those veins where it is found ? 
has been much vexed and little answered. 
“‘T must, however, begin by premising one word about the geological structure 
of the County itself. It will be sufficient to say that it is an instance of a 
simple linear upheaval—the central ridge or back-bone running very nearly 
N.E. and S.W., from which the land slopes’ off pretty evenly both ways. 
This central ridge or back-bone consists of granite in many parts, otherwise of 
gneiss and mica slate, flanked up by clay slate which extends in most parts to 
the sea, except where an eruption of trap, such as serpentine or hornblende has 
burst its way through the overlying strata. The elevation reached is never 
great, in no place I think exceeding 1700 feet. ; 
‘*Tt is in the clay slate, usually not far from its juncture with the granite that 
the metalliferous mines occur ; the metals worked being chiefly tin and copper, 
though not unfrequently lead with a small alloy of silver is produced. 
*¢The metal lies in veins or /odes, which are noticed as differing in structure 
from the surrounding rock, several often running more or less parallel to each 
other, and usually inclined to the surface. Running obliquely across a number 
of these lodes is nearly always a cross course or alvine course (vide section, ) 
composed often of quartz, and not usually productive of metal. 
“‘These lodes are supposed to have been a long time geologically back the 
subterranean water-courses of the county, carrying away the water which 
filtered through the surface strata. Similar courses occur now in every part of 
the world, as ordinary watercourses running in a great variety of directions. 
It is moreover held essential to the production of metal that the lodes shall run 
more or less N.E. and S$. W. (vide section.) This at least is a rule followed by 
practical miners, who all assured me that a lode was quite useless if running in 
any other direction. The practical miner also insists very strongly on the 
existence of the cross or alvine courses, crossing obliquely the more jor less 
parallel veins. ' 
‘« Where crossed by an alyine course, the lode is found to be shifted a few feet 
in one direction or another, and its metalliferous character is often entirely 
changed. Occasionally when the alvine course is metalliferous it contains 
metal quite different to that on either side of it. 
““These cross courses seem to suggest the occurrence of subterranean disturbance 
along the line of the present alvine course, blocking the free flow of water 
through the channels, which were then filled up by matter filtering in; and that 
“the metal must also have filtered in and crystallized in its present form. Tin, 
however, can scarcely have filtered through in the form of binoxide, as that is 
insoluble, both in water and all known acids. 
“In the earliest times tin was not obtained from the lodes, but in the form 
known as sfream tin. This form naturally occurs where a stream of running 
water happens to intersect a lode cropping up near the surface. The stone is 
disintegrated by the action of the water, the lighter and finer particles of sand 
are carried away far down the stream, the heavier crystals of binoxide of tin 
soon sinking to the bottom, where they are found mixed with sand in the stream’s 
bed. The ore can then be obtained pure by repeated washings. This was 
probably the only form of working known in the days of the Romans, and 
many of the mines worked originally by them are now open. In some cases 
the workings are merely in the refuse left by former workers, which can now 
be worked to advantage owing to constantly economizing methods of separation 
of the ore, 
