which form the opposite Walls of Cross-veins. 388 
The centre of motion must be either at the surface or at 
some point beneath it. 
If at the surface, there must necessarily be a simple inter- 
section at that spot (a, Pl. IV. fig. 6); whilst at every inferior 
station there must be a heave, and the distance of the heave 
will increase directly as the depth. 
The same state of things. but in a reversed order, must also 
take place should the centre of motion be at some point be- 
neath our reach; for in stich a case the heave must be a maxi- 
mum at the surface, and diminish as we approach the centre. 
In either case all the heaves will be in the same direction 
at all depths. 
If, however, the centre of motion be at some point below the 
surface (but one which has been reached by mining), as at 2, 
Pl. IV. fig. 7, then at that point will there be a simple in- 
tersection, and at every other a heave. Here, also, the ex- 
tent of the heave must of course increase the further it is 
situated from the centre of motion, whilst all the heaves above 
that point will be in one direction, and all those below it in 
the other. Thus, if we suppose the portion a', b! of the lode 
to be further from us than a 6, then the heave above x will be 
towards the right-hand, and that below it towards the left. 
It has been already stated that we possess but three ex- 
amples of the heaves of the same lode by the same cross-vein, 
which are in opposite directions at different levels: two of 
these (7, v) have been observed at only one spot above, and 
another below, the neutral point (v): in the third, Wheal 
Providence, although the direction of the heave is reversed 
between 30 and 40 fathoms deep, yet at 58 fathoms its extent 
is the same (10 feet) as at 48 fathoms. 
Thus, of the three facts (7, «, v) to which alone this solution 
can possibly apply, two do not furnish sufficient evidence to 
determine its application, and the third is inconsistent with it. 
(5.) What then must be the conditions of dip and motion, 
in the heave of a single lode by a single cross-vein, that will 
occasion simple intersections at some levels and heaves at 
others, whilst at the same time all the heaves shall be in 
the same direction ? 
We have seen that neither a horizontal (1.), vertical (2.), 
nor curvilinear (4.) motion, nor one coincident with the mean 
dip of the lode (3.), will satisfy the conditions demanded by 
the greater number of facts. Let us now assume a motion 
which, in a single case at least, seems likely to comply with 
them all; although there seems no reason for presuming on 
its existence, but that, in the case of individual intersections 
taken singly, it will comply with many of the facts. 
