JOINTED STRUCTURE. 
301 
for these phenomena ; but future observations are required, before any satisfactory hypothesis 
can be advanced. 
From all the facts observed, there seems reason to believe that these joints are due to 
galvanic or magnetic agency which has pervaded all the strata ; but that certain subterranean 
influences, operating at certain periods or under certain circumstances, may affect this con¬ 
dition. Where such an agency has acted more powerfully over a limited area, it may have 
produced a greater development of this symmetrical structure. The fact that these joints for 
the most part follow a uniform direction, points to some universal law like galvanism, or 
pervading heat, as the primary cause of their production. The uniform direction of metallic 
veins is not more persistent than is the direction of these joints ; and if we refer the cause of 
the former to galvanic agency, why should we not refer the latter to some influence equally 
universal ? 
The tendency of our ordinary clays to separate into laminas parallel to the lines of deposition, 
and also by vertical planes into rhombic masses, indicates the same structure, and the operation 
of the same laws as those producing more extended effects upon our slate rocks. 
The shores of our lakes, the banks of rivers, and the high cliffs bordering the ravines of the 
southern counties of the district, present these joints in great perfection. The whole effect 
produced by them, the sliding down, the undermining and destructive action of the weather, 
is picturesque in the highest degree. In some places, their great number and regularity 
suggests the idea that they may have been produced by causes not unlike the symmetrical 
divisions in the trap and basaltic rocks. This effect, though not as regular as the latter, is 
often nevertheless constant in certain rocks, and as equally governed by certain laws, and we 
may not unaptly term it mountain crystallization; yet, although we cannot prove that the 
ordinary laws of crystallization have had the slightest influence, so neither can we in the 
columnar basalts. In these we find the direction of the columns at right angles to the cooling 
surface; and in the jointed structure, we find, with few exceptions, that the direction is at 
right angles to the plane of deposition.* 
From the smoothly vertical faces of these joints where the rock is of a uniform composition, 
we are compelled to ascribe the cause to something beyond that which contraction on desicca¬ 
tion would produce ; indeed, from analogous cases, this latter kind of structure is not produced 
in right lines, but in variously curved and undulating directions. Another objection to this 
latter view is presented in the fact that some of the most argillaceous masses, or those which 
on desiccation would contract most, are nevertheless almost or entirely free from these sym¬ 
metrical divisions. Limestone, on the other hand, which in many cases we must suppose was 
* Sandstone in conjunction with trap, often becomes columnar for a distance from its point of junction with the trap ; and I 
have seen fragments of furnace hearthstones, which had become columnar in like manner. In these cases, we cannot doubt that 
this columnar arrangement has resulted from the action of the heat upon the sandstone. The jointed structure is often very 
analogous to this ; and in the vicinity of slight upliftings of the strata, the joints cross each other so closely as to give the mass, 
where partially broken or removed, a columnar appearance. Granite or gneiss often takes upon itself this columnar aspect so 
perfectly, that at a little distance it might be mistaken for columnar basalt. 
