276 Mr. Hopkins’s Abstract of his Memoir on Physical Geology. 
follows that however small the cohesion may be between two 
successive lamin or strata, this will produce no effect on the 
position of the fissure. In such case then its horizontal di- 
rection will still be accurately determined by our equation. 
‘This is important, because in a stratified mass the cohesion 
between different beds must probably be often much less than 
that between the constituent particles of each bed. The same 
conclusion will hold with respect to any accidental planes of 
less resistance which do not deviate too much from horizon- 
tality ; but if they be vertical, or nearly so, they will produce 
‘the accidental and partial deviations which have already been 
noticed. 
In forming a judgement of the probable extent of these 
planes of less resistance, we must be careful not to be too 
much influenced by the impressions produced by the exami- 
nation of a disturbed district, since we are now speaking of 
the existence of these planes in the undisturbed mass. I would 
also observe, that we are only concerned with this kind of 
discontinuity in the cohesive power, so far as it depends on 
local and irregular, and not on general causes, since, as already 
stated, I exclude those cases in which any regularly jointed or 
laminated structure may be supposed to have existed in the 
mass previously to its elevation. Now as far as the planes we 
are speaking of might be caused by accidental circumstances 
in the constitution or deposition of the mass, it would seem 
necessary to suppose them irregular in position and partial in 
extent; in which case, as we have seen, partial deviations 
only would be produced by them in the vertical or horizontal 
directions of the fissure. 
It appears then from what has preceded, that the equation 
above given will accurately determine the direction of a fissure 
at any proposed point, produced by tensions such as we have 
supposed, not only in a homogeneous mass, but also in a mass 
in which there may be any number of planes of less resist- 
ance, provided they do not deviate too much from horizon- 
tality, and notwithstanding any variation in the cohesive power 
of the mass depending on the difference of position of one 
point and another. From the interpretation of the equation, 
it appears that the fissure (or rather its intersection with a 
horizontal plane) will in general be rectilinear only in the 
, &c. are the same 
fi, fa 
F 
particular case in which the ratios : r? 
for every point through which it passes, supposing the direc- 
tions of the tensions at one point respectively parallel to those 
at another. There is, however, one important exception, viz. 
