ox THE THEORIES OP ELEVATION AND EARTHQUAKES. Cl 
geneity of ihe mass and the uniformity of the elevating pressure, that tlie 
principal directions would be parallel and perpendicular to the axis of the 
district, the former being that of maximum tension, and the latter that of 
minimum tension, which, in this instance, would be zero. Consequently 
the system of fissures first formed in this case would be longitudinal or 
parallel to the axis of the elevation. Again, if the boundary of the elevated 
district were circular, it is sofficientiy manifest from the conditions of sym¬ 
metry, that one principal direction at any point would lie in a vertical sec¬ 
tion through that point and the summit of the elevation ; and another would 
be perpendicular to that section. If the vertical axis of the elevation should 
be perforated, as in the case of a volcanic mountain by Its vent, it is easily 
seen that, in the higher part of the elevation, the tensions in the latler direc¬ 
tions would be the most efficient in forming fissures, of which a system 
might thus be formed radiating from the central point of the cUvatod mass. 
II the area of the elevated district be bounded by an oval more or less 
eloDgateil, the case will be intermediate to the two former above considered; 
and It is not difficult to see that the two principal directions of tension 
would not geoerally be parallel and perpendicular to the longer axU of the 
district as in the first case, nor would they be exactly like those in the 
second, but would form two systems of lines of an intermediate character. 
This would aUo be true with respect to the resulting systems of fissures. 
Moreover, as in the two first and extreme cases, the two systems of fissures 
(ii both were formed) would coincide, the one with the dip and the other 
with the strike of the elevated beds, at the instant when their formatim com- 
mnesd, 80 the intermediate character of these systems in the intermediate 
of the oval area will render the same rule approximately applicable to 
that case also. 
_ If the irregularities in the line of boundary of the elevated district be great 
in proportion to its whole area, it is impossible to lay down any general rule 
for tlie directions of tho fissures. The influence however of irrecularities 
of the boundary ia not likely to bo very sensible throughout the whole cen¬ 
tral portion of an extensive disturbed area, to which therefore the above rule 
will generally be applicable. 
But here it may be asked, how are the directions of the dip and strike at 
le instant of Incipient fornmiion of tiie fissures to be determined. In 
answer to this it should be observed, that the general directions of the dip 
an strike of the beds at the epocli just mentioned would depend, in the 
Ruined absence of considerable difference of intensity of the elevating force 
' points, on tlie form of the boundary of the elevated area, and 
•' “l'P‘’W'mately detcrmineil when that form, provided it be not too 
vatirtt* ohsiTvii that the continuance of the ele- 
livfal Ibrmalion of the primary fissures, whatever luight be the 
of generally affect in any great degree the mean strike 
eufarirv " hich would always tend to jiarallelism or perpendi- 
of anf- r I primary systems of fissures i-espcctivelv. The formation 
its I riic magnitude of the dip', or might reverse 
preceding role C'reurastances will not affect the application of the 
as an approximate rule of very general 
^ ® riieory we have been devidoping, that tioo 
atr,, w «unj remit from the dniulUineous elevation of an extensive 
direction of the fissitres in one st/stem, at ant/ 
"Lit- “7“’ ^approximately coincide with the mean direction <f the 
^ ^ ^ efet'ofet/ beds at ihe 
