300 
GEOLOGY OF THE FOURTH DISTRICT. 
are readily split in a direction parallel to the natural faces. These vertical divisions are of 
immense importance in quarrying, particularly in the limestones. I have seen quarries 
wrought to the depth of twenty or thirty feet, and for several rods in length, the back formed 
by a single plane, or by the salient and reentering angles of the lines of these joints. Owing 
to this circumstance, a great amount of labor and expense is saved over what would be neces¬ 
sary if the whole length and depth was to be wrought down by blasting. 
In cases of slight disturbances or upliftings of the strata, as there have been in some places, 
the slaty cleavage seems more perfect, and the joints more numerous as well as better defined. 
116 . 
In this instance, from some cause or other, the joints are exceedingly abundant; and they 
are not all vertical, but cross diagonally from one to another ; neither are the planes all parallel 
in their horizontal direction, frequently meeting, so that the block between them wedges out 
at a very acute angle. 
The same formation of rock has, in one place, a more perfect jointed structure than in 
another, though there has been no perceptible influence exerted. Still, however, it may be 
due to similar causes, and only at a greater distance from the centre of influence. 
These joints, though usually perpendicular to the planes of deposition, or slaty cleavage, 
(which are coincident,) are not invariably so; they are sometimes oblique to this direction, 
and even curved, leaving a spheroidal surface upon the mass below as the upper becomes 
removed. 
Notwithstanding that joints are far more numerous in the vicinity of slight uplifts, still I 
would not infer from this fact that they are due to the same cause. The manifestations of 
disturbing influences at such points may be owing, as suggested by Prof. Phillips, to their 
presenting less resistance than others; and for this reason, though the force were applied 
equally under the whole area, that point where joints were most numerous would yield first. 
It becomes a subject of interesting inquiry, why certain shaly masses are destitute of this 
symmetrical division, while others of the same age possess it in a high degree ; for so far as 
external appearances are concerned, they are as susceptible of such effects as those where this 
structure is perfectly developed. Several modes of explanation offer themselves to account 
