Mr. Hopkins’s Abstract of his Memoir on Physical Geology. 279 
then it is easily seen that if the velocity of propagation of the 
first fissure should be such as to continue it from Q to P, in 
less time than the relaxation of the tension would be commu- 
nicated from Q to Q’, (Q Q' being parallel to P P’,) the con- 
tinued formation of A! Q’ would not be arrested. Now I have 
shown* that the velocity of propagation will be extremely 
great, so that the distance Q! P’ may be large, and all physi- 
cal impossibility is therefore entirely removed. Let us sup- 
pose, for instance, a system of parallel fissures to begin simul- 
taneously along the lower surface of the elevated mass, and 
to be propagated upwards +. If the mass be nearly homoge- 
neous, the velocity of propagation will be nearly infinite; and 
if the fissures be not too near together, it is very possible that 
the time requisite for the relaxation of the tension to be com- 
municated from one fissure to the distance of the next, may 
be greater than that which is necessary to propagate the fis- 
sures to the upper surface of the mass. In this case it is 
manifest that every fissure will necessarily be continued to 
that surface. It seems most probable, however, that in actual 
cases, similar to that just stated, a part only of the fissures 
commencing below would reach the higher portion of the 
mass. If its thickness should be very great, the fissures 
reaching the surface would probably be at a proportionally 
greater distance from each other. 
In this manner, then, the formation of systems of parallel 
fissures presents no difficulty. Adopting this view of the sub- 
ject, we are immediately led to the conclusion, that the whole 
of any disturbed district, characterized by a continuous system 
of parallel dislocations, must have been elevated simultaneously. 
It is not, however, here meant to be asserted that the whole 
elevation must have taken place at once, but that that move- 
ment which determined the positions of the principal and cha- 
racteristic dislocations by causing the commencement of their 
formation, must have been a great movement, and must have 
extended at least as far as such dislocations may be observed 
to follow continuously the same law. Subsequent efforts of 
the elevatory forces might take place in any number, but it is 
evident that they would have but little effect in producing new 
fissures parallel to the former, (since the mass would generally 
yield along the old ones,) though they may be very instru- 
* Memoir, p. 22. 
+ | have shown (Memoir, p. 43) that fissures must generally commence 
in the lower portion of the mass; and we may remark, that according to 
our hypothesis respecting the rapid increase of intensity of the elevating 
force, from the instant the elevation commences, the formation of the fis. 
sures must begin almost accurately at the same instant, 
