which form the opposite Walls of Cross-veins. 455 
distinct ones, or whether they may be branches of the same 
which unite and form one vein at other places, the physical 
necessity that their heaves of the same lode must be in oppo- 
site directions will still exist, provided the only mass of rock 
moved is that contained between them; for theory makes the 
extent of the heaves dependent on the inclination of the lode 
and the distance of the motion, as we have already so often 
seen. 
Now the spaces at present occupied by the cross-veins are 
supposed to have been fissures originating in the movements 
which produced the heaves of lodes, although their ingredients 
may have been introduced at a subsequent period. 
at Wheal Union (4) we have every particular necessary to 
determine whether the same degree and direction of motion 
will produce fissures of the same width as the branches of the 
cross-vein, and at the same time heave the lode to the ex- 
tent, and in the directions, observed at the different levels. 
At 22 fathoms deep the trawn heaves Wheal Gwens lode 
36 feet, and at 30 fathoms 25 feet; but below this level it 
divides into three veins, of which one heaves the lode 1°5 foot ; 
the second, 8 feet; and the third, 24 feet: in all cases towards 
the right-hand. At the upper edge of the wedge-shaped 
mass of rock included between the eastern and middle branches 
of the trawn, the former of them is 6 inches and the latter 3 
feet wide. : 
Now in order to the prodyction of fissures of these respect- 
ive dimensions by the subsidence of this included mass of 
rock, in a direction op, (Pl. IV. fig. 14) parallel to a line 
uniting its present vertex with the point of the cavity at 0, 
which it must, on this assumption, be presumed to have ori- 
ginally filled, the extent of the subsidence must have been 
about 3°75 feet. 
Again, in order that a subsidence of this wedge-shaped por- 
tion, to that extent, should produce at 43 fathoms deep the 
observed heave of 1°5 foot in the lode by the eastern branch 
of the trawn, the direction of the fall must be in a line which 
dips northward about 72° from the horizon. 
But this amount and direction of motion, instead of the ob- 
served results of 25 and 36 feet respectively at 30 and 22 fa- 
thoms deep, will only produce heaves of 1:5 foot and 1°3 foot. 
But if the direction and extent of the motion be the same 
in all cases, the mechanical effects will be the same whatever 
is the width of the cross-vein, or whether it consists of a single 
vein, or of several. 
Let us, then, assume a movement in the direction already 
named, dipping northward 72°, but of a sufficient magnitude 
