R. Mallet— Origin of the columnar structure of Basalt. 209 
so the fracture Rowe a transverse joint takes place oblique 
to the sides of the prism; the obliquity beta less as the 
fracture advances toward the axis of the prism, so that when 
of at ie of these joints, and the modifications of their 
curvature produced by varied conditions in the coo 
It is further shown that the partial or complete deiadlianeed 
of certain fragments, frequently observed to be partially or 
wholly detached from the cusps of the concave side of these 
joints at and near the solid angles of the hexagon, is a conse- 
ort necessarily resulting from the mode of production of 
e joints themselves. The author then points out that in the 
case of very slender prisms other (and mechanical) conditions 
besides those of differential cooling enter into the production 
of the cross joints, which are at more considerable and irre- 
gular distances apart, and in planes of fracture often nearly 
transverse to the axis of the prism. 
He also discusses modifications produced in the prisms 
themselves, and in their cross joints, by ee, in the 
mass of basalt itself— er for example, by a more or less Late 
ously developed cleavage in the basalt in planes transverse to 
minimum tcc: Hs of pet 
The conditions producing greater or less interspaces between 
the prisms, which may vary from point to point of the same 
are pointed out, as also those which cause the spaces be- 
tween successive joints in adjacent prisms to esi g in succes- 
sive planes, transverse to their axes, or the contrary. 
The author then proceeds to discuss the various positions in 
ways normal to successive isothermal couches or planes at ache 
splitting temperature, taken in succession within the mass. 
