THE MEDITERRANEAN NATURALIST 
137 
to say they are due to the “convulsions of nature” 
— a fraudulent explanation that used to satisfy our 
forefathers. The sinking of the crust, if it be an 
explanation of the forcing up of mountain ranges 
by lateral pressure, fails to explain normal fault- 
ing, which is a phenomenon of an opposite 
nature. 
Yet normal faulting is one of the most important 
factors in geology. Mountain ranges do not exist 
everywhere ; there are always extensive level and 
little-disturbed plains in connection with them, 
such as the plains of Central Europe and the 
American Prairies. It would however, be difficult 
to find a geological map in which the observer has 
had the opportunity of actually seeing the struc- 
ture of the earth, which is not full of faults. Con- 
torted regions, as a rule, possess fewer of these 
characteristic fractures than the more level plains. 
It may safely be said that the absence of faults in 
a geological map is more due to their obscuration 
by drift coverings than to their non-existence. 
Geological mappers, of course, do not put in what 
they cannot see or reasonably infer. In proof of 
this it is only necessary to point to the wonderful 
system of fracturing which is always delineated in 
coal regions, where the opportunities of examining 
the strata are far fuller than in other regions. 
The distinguishing feature of normal faulting 
is that the “hade” or slope of the fault is in the 
direction of the downthrow. An earthslip in a 
cutting gives one a good notion of what has taken 
place, but faulting is, as a rule, sharper in the 
fracture and more regular. Many geologists have 
pointed out that it is “as if the strata had been 
drawn apart.” If they have been drawn apart it is 
obvious it cannot have been done by the same 
cause which previously squeezed them together 
and ridged them up into mountains. Normal 
faulting is posterior to folding. The student of the 
subject may take up almost any geological section 
of folded strata, and he will find that normal 
faults cut through the folds, and that, as a rule, if 
the downthrow side were lifted up to the extent of 
the throw, the curves of the fold would be conti- 
nuous. 
It seems to me as plain as anything can be that 
the cause producing the folds could not produce 
the normal faults. What, then, do they result 
from? 
The only cause we know of capable of drawing 
the strata apart is contraction by cooling. Here we 
have what we were looking for, namely, the effects 
of the sinking of the isogeotherms as opposed to 
the compression and mountain upheavals produced 
by their rise. 
I have already explained that the upheaval of a 
mountain range by lateral pressure resulting from 
cubic expansion ends in the actual transference of 
materials to the locus cf the range. When the 
internal energies have for the time exhausted 
themselves, and the isogeotherms sink cubic can- 
traction takes place. The weight of the portion of 
t he earth affected will allow of the existence of no 
cavities beyond a certain depth, consequently the 
strata have to accomodate themselves to the new 
conditions of space, which they do by shearing 
into wedge-like forms which, by gravitation, fit 
together again and close up the opening. A very 
interesting illustration of the truth of these prin- 
ciples has lately been observed 'and described by 
me. (1) A banded silty clay in the Glacial Drift 
at Neviri Carnarvonshire just the sort of deposit 
to exhibit and mark any break of continuity 
which might occur was divided by a system of 
Normal Faults reproducing in minafcure all the 
characteristics of such Faulting including the hade 
to the downthrow, the dropping in of wedge-siiaped 
blocks and the occasional curve of the bands down- 
wards on one side of the Fault and upwards on 
the other. These Faults were as if cut with a 
knife and fitted closely; the whole being like a 
diagram of a Faulted Coal Field. The cause was 
evidently loss of bulk by loss of moisture or cu- 
bical shrinkage. Substitute shrinkage by loss of 
heat and the parallel becomes exact. It has 
been pointed out already that expansion by rise 
of temperature under sedimentary areas was 
conceived to be a geological agent by Babbage, 
Herschell, and Scrope, but as an explanation of 
mountain building it was left by them in a very 
crude form. The dynamical effects were not fully 
traced, and cubical expansion was uu thought of. 
Great praise is due to Captain Hutton for the 
impetus he gave to the theory by supplying 
elements that were previously lacking in it. I am 
not aware, however, of the existence of any 
(T) A minature illustration of Normal Faulting. 
Geo. Mag. November 1891 y>. 487. 
