THE MEDITERRANEAN NATURALIST 
Now let us fix our attention on the prism F, and 
consider the forces that act on it. Suppose the 
pressure of the rock above to be equal to p tons 
per square foot. Then this pressure acting on the 
prism E will cause it to exert a lateral pressure 
on the prism F. tending to thrust it forwards 
towards A. This lateral pressure must be less 
than p, but it is proportional to it and consequently 
may be represented by ap in tons per square foot, 
where a is a coefficient depending on the plasticity 
of the clay. 
On the other hand, the tyancl of clay between A 
E is resisted in sliding forwards by the friction of 
the clay on itself at the upper and lower surfaces 
of the stratum. This resistance is proportional to 
the superincumbent pressure, p, and to the surface 
over which the sliding takes place. 
Let us new consider what will happen to a slice 
of the stratum, l feet long, measured along the 
escarpment, d feet wide from A to E, the thickness 
of the stratum being t. 
The force urging the slice forward from 
behind is aplt ; and the friction which resists the 
motion is bpld where b is the coefficient of on the 
upper and also on the lower surface of the stra- 
tum, or bpld in all friction of the clay on itself. 
When 2 bpld is less than aplt , motion will ensue, 
but when the former expression is the greater 
there will be no motion Suppose we make aplt— 
2 bpld 
at. 
Thenc?= 
2 b 
This last equation gives the greatest distance from 
the face of the escarpment at which sliding can 
take place for given values of a and b. For plastic 
clay a = 0'59 and b = 0'27, consequently d — IT t. It 
appears from this that the extent to which the clay 
could be squeezed out would not much exceed the 
thickness of the stratum, 
It will be noticed that the thickness of the 
overlying rock does not affect the result which is 
independent of vertical pressure. For, although 
the force urging the clay forwards increases with 
the pressure, the frictional resistance increases in 
exactly the same ratio. 
The coefficients a and b are not independent. 
They are both functions of the “angle of repose,” 
or the natural slope which a heap of material 
369 
assumes when left to itself. This angle has been 
determined to be equal to 15°. Then, 
I — sin 15° 
a — , and b — tan 15°. 
. I + sin 15° 
We may conclude that the effect which mecha- 
nical pressure has in causing a thinning out of a 
soft stratum is inappreciable. 
Gavin J, Burns B. Sc. 
In the above cases Mr. Burns has considered the 
question from one point of view only viz that in 
which the layer of plastic material is homogeneous 
throughout, and is interstratified between two 
horizontal layers of rock the upper of which is of 
an uniform thickness. In such a case the pressure 
would be a constant and would always be exerted 
in a direction at right angles to the horizontal 
surfaces of the overlying rock masses. 
Such physical conditions are not, however, to be 
met with among the Maltese strata; and Mr. 
Burn’s formulae therefore applies only to the 
hypothetical case which he has quoted. 
The dip that the Maltese strata have; the 
different thicknesses of the formations that rest 
upon the Clays, and therefore the variability of 
pressure which is exerted; the irregularity of 
outline of the upper and lower surfaces of the 
intercalated clay-bed; the non-homogeneous cha- 
racter, and the consequent variability in the 
porosity of the clays; and lastly the extent and 
direction of the faultings which have imparted to 
the upper strata of the plateaux a tendency to 
slide, and thus to squeeze out their plastic 
foundations; these constitute a few of the more 
important of the factors which must be taken 
into consideration before it will be possible to 
arrive at a satisfactory solution of the problem by 
the aid of mathematical formulae. 
J. H. Cooke. 
The World’s Coal Supply. 
In the course of an valuable article on the above 
subject Mr. Lavard the editor of the Goal Trade 
Journal gives the general public some interesting 
details having reference to this most important 
subject. 
