ON THE THEORIES OF ELEVATION AND EARTHQUAKES. 89 
plosive action would be comparatively small, as would be also, therefore, the 
compressioD of the fluid in the immediate vicinity of the centre of explosion. 
In such case the intensity of the vibratory wave of compression, and the 
pressure propagated by it, would be comparatively small, while the surface 
ware would be comparatively large. On the contrary, the crust being heavy 
and rigid, the compression of the fluid would be great, and a pressure of 
great intensity would be propagated by tlie resulting vibratory wave. Now, 
the velocity with which such a wave is propagated is such that an cxtrciuelj' 
short spee of time might suffice to eejualizu the pressure in every part of 
the fluid, and if a sensible surface wave were not produced before the expi- 
raflon of this short time, it nianifostly could not be produced at all. TJuis 
suppose explosive action to arise from the rapid production of elastic 
Tapour within the fluid, and along a median line from one extremity of the 
fluid mass to the other. Also, suppose the process of generating the elastic 
vapour to be such as to require a certain number of seconds (s\ Lich we may 
denote by w) before the clastic force became sufficient to cause incipient 
elevation of the erq^t. In water the velocity of propagation for a vibratory 
wave of compression is about 4800 feet per second; in fluid lava it would 
probably be considerably more. Assume it to be one mile per second. 
•^m! ” seconds the vibratory wave, and with it an Increase of pressure, 
will have travelled n miles on each side of the median line of action. This 
rapidly the additional pressure produced by the compression of 
the fluid, would bo propagated to distant points, and how inconceivably rapid 
must be the generation of clastic vapour to produce an elevating force suffi¬ 
cient to cause a considerable elevation along the median line of its action, 
not exceeding a few miles in breadth, and before the remoter portions of 
he superincumbent crust should be subjected to the action of the elevating 
orce. If the crust were so thin and flcxiblo as to require but little force to 
elevate it, such might possibly be the manner in which the elevation would 
commence; but that an clastic force capable of elevating a mass of the thick¬ 
ness tt-hich many elevated portioiw of the earth’s crust must have had, should 
ered'hl ” instantaneously generated, appears, I think, altogether in- 
If however this narrow linear elevation were to bo thus instantaneously 
I**®!*'movfinpnt would doubtless bo propagated on either 
-• eto the remoter portions of the crust reposing on the fluid beneath, ex- 
c y analogous to that of a great sea wave, except that the height of the 
•ve Would proljably diminish rapidly in its progressive movement, on 
*^j**\*^ inflexibility of the solid crust. Each portion of the crust 
tA h k<-‘lcvated successively in a great surface wave, and fissures 
whol^ produced, not by the general and simultaneous tension of the 
in the theory previously considered, but by the local tension 
of the dificrent portion# as they were succcssivelv raised into 
their wave-like form. ^ 
eisi-^T? *he>uodo of action attributed to the subjacent fluid by some geolo- 
L “nflerslaud their viowa on the subject corn'ctly; but it appears to 
,1 . theory can be deemed admissible which involves the hypothesis 
generation of elastic force to the enormous amount re- 
Ir nn ,1 “'y continuous previous generation of a similar force. 
8 Hnf>r!« * ccf'trary, we allow the continuous generation of force, till the 
^ nneumbent mass should be brought into a general state of tension 
marv owwwrfion, it must be to the tension so produced that the pri- 
principally due, whatever might be the 
O' producing the small additional elevating force which would be 
