98 
AMERICAN GEOLOGY. 
efficient force by bringing the particles near to each other. 
Pressure unaided by this force, is insufficient to develop planes 
of any kind. 
The production of the planes of cleavage*or lamination, has 
been attributed, by Prof. H. D. Rodgers, to an electro-galvanic 
or electro-thermal agency. This, to be sure, must be regarded 
as one of the universal properties of matter, or a force made 
sensible through the medium of matter. The theoretical ex¬ 
planation is founded on the supposition that the slate planes, 
after flexure, are alternately hot and cold, and hence are the 
generaters of an electrial power analogous to that of the 
thermo-electric pile. It is difficult, however, to understand how 
these arrangements operate in the production of planes; more¬ 
over, it requires the preexistence of planes of some kind, 
in order that the analogies may be made out. It seems, 
that in this explanation the effect preceded the cause. If 
each molecule of matter has polarity, all we have to do to secure 
a symmetrical arrangement is, to bring the particles composing 
a rock within the sphere of each other’s attraction. Adjust¬ 
ments will then take place. Compression or pressure operates 
in this way. 
The existence of planes of lamination or cleavage in gneiss, 
mica slate, etc., receives an explanation at least partly from 
the foregoing principles. The efficient cause may be stated in 
different words. We may recognize the polarity of the mole¬ 
cules of matter, or we may use the word crystallization, and in 
some cases molecular forces; for in the use of the latter term, it 
seems we recognize a greater change of the particles in space, 
even entire strata are formed by this force. Concretions, too, 
are gathered or formed from similar particles, and from com¬ 
paratively wide spaces; where the matter is insufficient to 
form a stratum, for in the mass of mud which ultimately forms 
slate, the lime and silex were intermingled without order; 
but now we find the lime in nodular bands, or distinct nodules, 
as in septaria, which could not have been in that state in the 
original deposition. We call it then a concretionary or mole- 
