HOLWELL CAVERN. 27 
of which might have been produced by central or volcanie 
heat, or immense pressure, added to the other requisites. 
The last thing to consider is—From what source does the 
required electric action arise? Now in answer to this, as 
far as we know, it most probably arises from one of the fol- 
lowing causes ; first, from terrestrial electrie eurrents, caused 
by permanent magnetic action passing at right angles to 
them ; or secondly, from similar electric currents exeited by 
the union of vast strata of dissimilar rocks in contact 
with subterranean waters ; or thirdly, from similar currents 
either excited or aided by a central or volcanic heat, 
perhaps coming under the laws of thermo-electricity ; 
or fourthly, and lastly—by the local electric action before 
alluded to, and which, I presume, is always the case as 
respects the formation of insulated cerystalline matters 
which are found scattered in all directions—often at a 
great distance from metallic and earthy lodes or veims. 
These latter are probably formed by the constant action 
of definite terrestrial eleetrie currents. In the case of 
Holwell Cavern, the electrie power is probably exeited by 
the contact of the clay slate and limestone strata ; or 
there might be a definite current passing along, or at right 
angles to the main fissure, occasioned by terrestrial electri- 
eity, (the existence of which is certain) and on the negative 
portion of the roof and sides of the fissure, the arragonite 
would most certainly be deposited. I once formed some 
beautiful regular and irregular erystals of arragonite in a 
very simple manner, viz. by filling a common salting 
pan with spring water, and placing the two halves of a 
brick upon the bottom of the pan, at a small distance from 
each other, and resting a whole brick upon them, in the 
form of abridge. Each end of this upper brick I con- 
netted by platinum wires with the opposite poles of a 
D3 
