176 PHYSICAL GEOLOGY. 



crushing, the latter ones taking up heat from that crushed material, and 

 so on. Heat must also be transformed from the work of subsequent 

 friction. Time also must be taken into account ; the shorter the time, 

 the less heat is dissipated; the velocity of crushing would be extremely- 

 great when a rock-mass of great length is being compressed. Another 

 source of heat is the forcing of the fragments that have become viscous 

 through constricted rock-channels of the heated mass. F. D. 



Mallet, R. On the Origin and Mechanism of Production of the Pris- 

 matic (or columnar) Structure of Basalt. Phil. Mag. ser. 4, vol. 1. 

 pp. 122-135, 201-226 ; Amer. Journ. ser. 3, vol. ix. pp. 206-211. 

 (Abstract.) 

 Supposing a homogeneous isotropic mass of basalt resting on a hori- 

 zontal floor and cooling from the upper surface, why should the fissures 

 take the hexagonal form ? The answer is to be found in the ' principle 

 of least action.' A smaller amount of work is required for a con- 

 tracting surface to split into hexagons than into the other two forms 

 (equilateral triangle and square) by which a plane can be divided into 

 equal and similar figures. It is inferred that the splitting temperature 

 of cooling basalt is somewhere between 900° and 600° P. When the 

 contraction due to a lower temperature exceeds the amount of extension 

 (of the still slightly viscous material), rupture takes place ; and the 

 splitting wiU proceed downwards ^ari passi* with the cooling, the rate 

 being much increased by the probable presence of water. Then, the 

 outside of each prism becoming cooled more than the interior, dif- 

 ferential strains will be set up, causing transverse fracture. The 

 distance of the first transverse joint from the cooling surface cannot be 

 less than the diameter of the prisms. The curved form of the trans- 

 verse joints is shown to result from the simultaneous contraction in an 

 axial and a transverse direction, the resultant strain being along a line 

 inclined to the axis ; but this inclination diminishes as the cross fracture 

 proceeds inwards (the width of the remaining contracting material 

 being less) ; and so a cup-shaped surface of fracture is made, the convexity 

 pointing away from the cooling surface. The breaking off of the corners 

 of the hexagonal prisms is also accounted for. The author describes 

 the variations from the theoretical form of the jointed prisms that 

 occur under actual conditions of basalt masses, and concludes by com- 

 bating the theory that the columns were formed by a union of spheroids, 

 maintaining rather that the latter result from the decomposition of 

 basalt in spheroidal surfaces of weakness induced by the occurrence of 

 the joints in the forms described. P. D. 



On Prismatic Basalt. Nature^ vol. xiii. p. 7. 



In answer to a paper by Mr. Scrope {Geol. Mag. 1874), says that the 

 double-concave articulation of the basalt columns in the Geol. Society's 

 hall came from some portion of the mass in which occurred the dividing 

 plane between what was cooled from the top and what was cooled from 

 the bottom. Irregularities in the direction of the convexity may occur 

 a little above or below this plane, from differences in conductivity, &c. 

 The heat-waves may become irregular from various causes ; there would 



