SS a a ag NR a a i ga la te ae pct Sg a ere og SU) 
R&R. Mallet— Origin of the columnar structure of Basalt. 207 
coacting with enormous external pressures, the origin of which 
is left perfectly vague, or to some play of successive and joint 
actions of all these various forces. 
Professor James Thomson, in a paper read some years ago, 
concretionary spheroids pressed together. Professor Thom- 
son’s views, however, are still far from complete, and the mode 
the salient phenomena of the prismatic and jointed structure of 
basalt, as observable in nature, can be accounted for upon the 
admitted laws of cooling, and contraction thereby, of melted 
rock possessing the known properties of basalt, the essential 
conditions being a very general homogeneity in the mass cool- 
ing, and that the cooling shall take place slowly, principally 
om one or more of its surfaces. 
Thus, taking the simple case of a tabular mass of molten 
basalt, whose top surface is level, the depth being great, and 
the two other dimensions indefinitely greater than that, and 
assuming the material at one temperature initially, and homo- 
geneous and isotropic, and that cooling takes place from the 
top surface only, he, on these data, proceeds to consider the 
phenomena that will successively result by contraction in 
cooling. 
While the mass remains at its upper part still plastic by heat, 
contraction will be met by internal movements and subsidence 
ol the top surface, and no cracking or splitting can take place 
until the material there has become rigid enough to break 
under tensile strain. He points out that this degree of rigidity, 
or “splitting temperature,” is not reached until the top surface 
has fallen to between 900° and 600° Fahr. 
At this temperature the cooling surface begins to separate 
by fracture, penetrating perpendicularly to it into smaller sur- 
faces. These must be similar and equal in area, and such 
that their edges in contact can make up a continuous superfi- 
cies. ‘T'o relieve the orthogonal strains in the cooling surface, 
and to meet the above conditions, only three geometric figures 
or the separating surfaces are possible, namely, the equilateral 
triangle, the square, and the regular hexagon. 
Am, Jour Ser. Turrp sues Goes IX, No. 51.—Manrcu, 1875. 
