On Mineral Veins. 237 
as to present the appearance of a gigantic tesselated pave- 
ment. | 
_ Mr. Hopkins has tried to demonstrate mathematically that 
a series of joints would be formed parallel to an axis of 
elevation, along with a second series crossing the first at 
right angles; but this theory must be put to the test of a 
searching comparison with the observed phenomena in 
numerous instances before it could be accepted as the true 
explanation, and this has not yet been done. In the case of 
the partially-metamorphosed conglomerates described by 
Professor Hitchcock, the bed was crossed by east and west 
joints or divisional planes, cutting smoothly through the 
pebbles; these planes having in some places polished 
faces, although the cut pebbles on each side of the joints 
exactly corresponded, showing that there had been no slip. 
He considered mechanical agency could not possibly 
account for these phenomena, and that we are driven to 
the supposition that some polarising force has been the 
agent. 
It is indeed difficult to conceive how these regular series 
of parallel joints or divisional planes could be produced 
by disruptive forces acting from below, and so far as 
the cleavage planes are concerned, the prevailing opinion 
is that they have been caused by polar forces originating 
with the magnetic currents traversing the earth. It is true 
that laminations similar to cleavage planes may be produced 
by lateral pressure ; but this will not account for the con- 
stant meridional direction of these planes, and has no bearing 
on the formation of divisional planes. 
Mr. Fox’s investigations into this subject are well known, 
and the repetition of one of them by Mr. Hunt of the School 
of Mines, Jermyn-street, appears to bear so exactly upon 
the question, that it may be well to briefly describe it. The 
apparatus used was an oblong box or trough, and the 
material operated on was Stourbridge clay, plaster-of-paris, 
bath-brick, sandstone, and powdered coal. The trough in 
this experiment was divided into two compartments by a 
wall of clay three to four inches thick, and on one side of 
this wall was placed a plate of copper, on the other a plate 
of zinc; the two plates being connected together by a strip 
of copper. The cells were then filled with the exciting fluids, 
that on the copper side being a weak solution of sulphate 
of copper, and on the zinc side of the muriate of soda.. A 
complete circuit was thus formed, the current passing from 
