20 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
Undercliff quarry in Llewellyn Park. The chief interest centers in 
the groups of radiating columns which form the, upper portion of 
the exposures, the lower portion being divided into vertical columns 
or blocks of larger size. That these two portions of the lava sheet 
belong to one and the same mass is shown, not only by the contin- 
uity of the rock of the upper and lower parts, but also by the mutual 
accommodation of the different sets of columns, which taper off and 
curve in one direction along lines of oblique junction ; and by the 
fact that the positions of the columns are not what they should be 
along the supposed lines of contact. 
The columnar structure in volcanic lavas is unquestionably a 
cracking produced by shrinkage upon further cooling, after the 
mass has consolidated into rock. 
Considering the origin and progress of a crack produced by the 
shrinkage of a homogeneous mass, we see that, starting with a plane 
surface over which forces producing contraction are acting uni- 
formly, the contraction produced on the surface of the mass in a 
given time will be greater than that produced at some depth within 
the mass, and that it will decrease gradually from the surface in- 
ward. As the contraction progresses, the limit of tension in the 
direction of the surface will be reached before that in the direction 
of depth, causing a rapture across the direction of the surface, and 
as the limit of tension for the layer next to the surface is reached 
it will rupture in the same direction as the surface layer did, 
and soon. The direction of the crack is at right angles to that 
of greatest contraction, or normal to the line of maximum strain. 
Moreover the condition of the mass at the moment the limit of 
tension along the surface is reached may be graphically repre- 
sented as in figure 1, the contraction being a maximum in the 
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Fig.1. 
top layer and diminishing successively in each layer beneath to that 
with the initial expansion. The distance of this layer from the 
surface being taken as unity, the maximum contraction at the 
