60 
THE MEDITERRANEAN NATURALIST 
consider that the earth throughout as a mass is 
declared by such good physicists as Sir William 
Thomson and George Darwin to be as solid as 
steel from the surface to the centre, it would seem 
ather foolish to search for specially weak places 
under sedimentary areas. If, however, it’could be 
shown that lateral pressure consequent upon the 
shrinkage of the under-layers of the earth’s crust, 
and acting only to the depth of a few miles below 
the surface — that is, to the level-of-no-strain — is 
capable of piling up the crust into mountain 
ranges, the explanation is a feasible one, as no 
doubt the upper layers of mere sediment would be 
weaker than the surrounding buttresses of old 
rocks. 
The structure of all great ranges, as known 
through the labours of geologists the world over, 
negatives this supposition, and shows that we 
must seek for a deeper-seated force than that 
derivable from the secular contraction of the globe. 
What can this force be? It was shown long ago by 
Babbage, Scrope, and Herschell that the laying 
down of beds of sediment must produce a rise of 
temperature in the rocks below. Borings, well 
sinkings, and mining have shown that as the earth 
is penetrated the temperature rises, and tin's is 
true of new deposits as well as of old rocks. Many 
observations have been made of the rate of increase 
of temperature, which show that it varies to the 
extent of 1° Fahr. in 281 feet at Anzin, in the 
North of France, to 1° .in 157 '2 feet in lie Minas 
Geraes Mines in Brazil. There is a regular but 
slow outflow of heat taking place from the earth, 
so that when a sedimentary layer or covering 
accumulates at any locality on the earth’s surface, 
all the layers of that sediment eventually get 
heated to the normal temperature due to the depth 
and the conducting power of the rock. 
Thus, for example, if the rate of increase is V in 
60 feet, a deposit 6,000 feet thick would become 
eventually 100° hotter on the under side or base 
than at the surface, and all intermediate depths 
would take their proportionate temperature. 
Planes of equal temperature in the earth’s have 
been called iscgeotherms. It is not an attractive- 
looking word, certainly, but the authorities having 
adopted it all we have to do is to follow suit, 
especially as the word expresses an exact thought. 
It will, thus be seen that the laying down of sedi- 
ment first affects the temperature of the immediate 
underlayers, but eventually it is felt to profound 
depths. Now what will be the effect of this rise 
of temperature on the portion of the earth’s crust 
so affected? Everyone knows that heat expands 
and loss of heat contracts substances, but in vary- 
ing degrees. To determine the amount I made a 
series of experiments on various rocks, and I found 
that the mean linear expansion was 2.77 feet per 
mile for every 100° Fahr. In small pieces of rock 
very exact and minute measurements are necessary 
even to detect any alteration of dimensions, the 
change of bulk being proportionately small; but 
when the dimensions are magnified so as to include 
sections of the earth’s crust the change of bulk is 
measured by cubic miles. This, like many other 
things, is all a matter of relation , and it takes time 
! and thought for the tyro to accustom himself to 
think that great effects can from little causes spring. 
If, however, there is one thing which geology 
teaches the patient investigator it is not to despise 
apparently small forces. We see this every day, 
yet we learn not. Every one-hundredth part of an 
inch of rain means one ton per acre. A ton seems 
a great deal when collected together, whereas one- 
hundredth of an inch seems beneath notice. So it 
is with the forces of expansion; by alteration of 
temperature they pass unnoticed until some large 
structure is dependent upon these forces being 
effectively provided for. A hot sun on one side of 
the Menai or Britannia tube will twist it three 
inches laterally and two and a-half inches verti- 
cally where it is free to move. During the very 
hot summer of last year the rails of many railways 
had to be taken up and shortened, although it is 
customary to allow for expansion at each joint, 
and the holes of the fish-plate bolts binding them 
together are made slightly oval to meet the 
difficulty. No large engineering work is properly 
designed unless provision is made for meeting the 
changes of bulk caused by changes of temperature. 
Metal plates for roofing, such as in lead gutters 
and flats, have to be laid in sections so as to be 
free to move, for, if soldered together in one length, 
they will soon tear themselves to pieces. 
Notwithstanding these precautions, all old lead 
gutters and fiats are full of wrinkles and ridges, 
produced by infinitesimal changes, which end in 
the forcing up of these ridges. Each expansion by 
