32 The Iron Ores of Great Britain. [Jan., 
In attempting any classification of Iron ores which shall be 
regulated by the conditions under which they are found, we are at 
once met with the difficulty of finding many similar varieties of 
ores occurring in lodes or veins, and also amongst those which are 
evidently sedimentary deposits. It will not, therefore, be attempted. 
There can be no doubt that the most ancient of the Iron formations 
are those Oxides of Iron which occur as lodes or veins in the slate 
and granite rocks. Of this kind we have some examples in Corn- 
wall, Devonshire, Wales, and Scotland. Generally speaking, how- 
ever, these veins are not sufficiently extensive to be worked. The 
Tron Mine at Restormel, near Lostwithiel, in Cornwall, which has 
been wrought for many years, and which still produces considerable 
quantities of ore, and those near St. Austell, in the same county, 
are amongst the most remarkable. At Hennock, on Dartmoor, there 
occurs an immense vein of Micaceous Iron ore, but this of late years 
has not been employed. All these are evidently the result of the 
ageregation of ferruginous particles, often under the influence of 
erystallogenic force, mechanically separated from water holding iron 
in solution, which has flowed through the rock fissures probably 
within a limited period after the formation of those fissures. The 
Tron lodes differ in no respect from those of the other metals, except 
in the nature of their contents, and they have been formed under 
analogous conditions, all of them indicating the influences of an 
elevated temperature, of an electrical disposing power, and those 
mechanical agencies which are still obscure, but which are bemg 
gradually developed under the general appellation of Osmose forces. 
Magnetic Iron Ores.—Magnetic Oxide of Iron or Magnetite is 
a compound ore of the Sesquioxide of Iron and the Protoxide of 
Iron. Amongst other adventitious matters, this ore usually 
contains manganese and sometimes tin. In Western England 
several lodes of magnetite are known, one near Penryn and another 
not far from St. Austell, in Cornwall, has been worked, but not 
extensively. Near the Haytor rocks, on Dartmoor, a much more 
extensive deposit of this ore is found; but it is difficult im this case 
to determine satisfactorily, whether this is a set of veins or of beds 
interstratified with slate and sheeted masses of greenstone porphyry. 
At Brent, which is only a few miles distant from the Dartmoor 
deposit, we find magnetic ore covering, like a shell, an immense 
boss of trappean rock. In nearly all cases, magnetite is associated 
with, or is found in proximity to, some igneous rock, and to the 
action of this on either Oxide or Carbonate of Iron is no doubt due 
the magnetic character which distinguishes this ore. If the 
Spathic Carbonate is exposed to a regulated heat, it is converted 
into a Magnetic Oxide of Iron, and manufactories have been 
established for its production, to be used as @ paint for work which 
is much exposed to the action of the weather. 
