64 
REPORT—184/. 
neath. Tlie general elevated mass would thus tend to descend by fls own 
w’ejght, but would manifestly be unable to subside Into its original position 
on account of the relative displacements of its wedge-like portions, if the 
fluid pressure should be entirely removed, and the whole mass, together 
with its lateral abutments, should be sufficiently firm, it would be supported 
on the principle of the arch. In this case an enormous horizontal pressure 
would be produced. Generally', the fluid pressure might probably be only 
partially destroyed f the more it should approximate to the extreme case just 
mentioned, the greater would be the horizontal pressure, which might thus 
become much greater than the vertical pressure to which any portion of the 
uplifted moss could possibly bo subjected. 
If the elevation should be continued to a much greater extent than sup¬ 
posed in the above case, the fissures would be widened, and afl'ord room for 
far more irregular and varied modes of subsidence after the great effort of 
the elevating force had ceased, than would be possible in cases of smaller 
disturbance. Let fig. 4 Represent a transverse section of a disturbed district 
across three primary longitudinal fissures, nearly at the instant of its greatest 
elevation, and therefore previous to any considerable subsequent subsidence. 
Fig. 4. 
If the intumescence of the subjacent fluid, and consequently its supporting 
power, were immediately afterwards dirainisiied by the escape of elastic 
vapoure, there would be an immediate subsidence, wliich, in cases of vio' 
lent disturbance, would probably bo governed in a great degree by acei- 
Thus it might happen that the two central portions 
should be so brought in contact with the neighbouring lateral portions about 
the points N ami N', as to be prevented from subsiding except bv turning 
rouud those points as pi>"ots, the lateral portions receiving .some accidenta 
support not aflorded to the contra! ones. In such case the two central 
masses would be brought into contact at M. If the masses should be nearly 
equal, similar, and similarly situated, the resultant pressure between them 
would be approximatuly horizontal. Let MU be the direction of the pres¬ 
sure on the mass i'S, and NQ a perpendicular on MU. Then the support 
jug power of this force w'otild be measured by the product of the force and 
xtS' 9 ^ gravity of the mass PS, GH vertical, 
and NK perpendicular to GII, the tendency of the weight of the mass to 
cause It to subside by, turning round the point N, will be measured by tkr 
pioduct of the weight and the line NK; and so long as this product should 
Oe greater than the supporting power of the pressure at M (omittiDg any 
derived from the subjacent fluid), the masses 
^ continue to subside. The further they should subside 
y I of equilibrium, the greater would be the 
sent t f? pressure at M would acquire. Suppose fig. 5 to rep^' 
sent the final state of subsidence. The weight of the mass PS would he 
