60 
REPORT— 1847* 
direction of the fissure towards the predominant direction of the planes of 
discontinuity. 
22. Formation of Systems of Fismrcs .—I have already remarked that 
from the instant a fissure should commence, a modification of the state of 
tension of the mass in the immediate vicinity would take place almost inOan- 
taneonshj. A similar modification Mould be propagated very rapidly to 
points more remote, but still a finite time would be rerjuisile for this pur¬ 
pose. The formation of the fissure would, in fact, cause a vibration like 
those which will be fully described >ti tlie succeeding section, and the corre¬ 
sponding modification of the tensions would be propagated with the bome 
velocity as the vibration itself. Now let us conceive too fissures to cooi- 
mcijce simultaneously at two different points in a straight hue, Mhicli 
will suppose to coincide with the direction of maximum tension at each 
point. The velocities of propagation of the two fissures, as already stated, 
would generally be very great, exceeding possibly that of a vibration, and 
therefore, if these velocities were nearly equal, either fissure would anive at 
each point through which it would be aucecssivedy jiropagated, before the 
tension at that point could have been relaxed by the formation of the paralid 
fissure. If, on the contrary, the velocity of propagation of one fissure should 
bo considerably greater than that of the other, the tension on eitlier side of 
the one formed most rapidly, might become so relaxed at poiuts through 
which tlio one formed more slowly was tondiug to pass, that iJio propagation 
of the latter fissure might be entirely arrested. If, instead of two fissures, 
any number should commence simultaneously, the same remarks would 
apply to their propagation; and thus we may concedvo the simultaneous 
formation of a system of fissures characterized by a degree oipaTallcUsm to 
each other, dependent ou the parallelism which existed between the direc¬ 
tions of the maximum tensions at different points at the instant previous to 
the incipient dislocation of the mass. 
Immediately after the completion of this system of fissures, it Is manifest 
that all temion in directions pcrpondieular to the plane of a fissure and ia 
iu immediate vicinity, would be destroyed; and tlierofore (nssuniiug the 
absence of yressurc in this stage of the process of elevation) the direction of 
maximum tension there would then become parallel to the fissure. Conse¬ 
quently, if the tension should go on increasing in consequence of a continua- 
tion of the elevation, till it became greater than tlie cohesion, another system 
of fissures might be formed, such that each fissure should be approximately 
perpendicular to each fissure of the previous systmu at the point of their iu- 
terscction. ^ 
We may here remark that the formation of systems of fissures as above 
described, would be most likely to take place in those cases in which, at the 
instaiit of the formation of the fissures, the elevation should be as rapid as 
should be consistent with a cunthiuousy as distinguished from an exptosire 
action of the elevating force. ° ^ 
23. Application of the Iheory .—The most defective part of the tlieor)-, 
considered as a mechmucal theory, consists in our inability to determine by 
P«-‘“c«Pa» directions of tension for 
any but the most simple forms ol the boundary of the elevated district In 
po'nt of view, however, the defect is of ntdo importance; for 
owing the principal direcUons in certain simple cases, we cau obtain them 
y mference in such more complicated cases as usually present themselves, 
aufficicut for all geologicid purposes. Thus 
V*’- two p^rallef straight^es, and to 
indefinite length. U is mamtest, assuming as heretofore the homo- 
