production of the Prismatic Structure of Basatt. ] 33 



tween highly heated bodies and liquids such as water, as 

 shown by Boutigny's experiments, here come into play to re- 

 sist the entrance of the water fully into the fissure ; for, as we 

 have seen, the splitting of the mass cannot commence except 

 in those portions of it which have fallen to 600° Fahr. The 

 temperatures of the sides of the prisms already split, and thus 

 much below 600°, would therefore exercise no repulsion to 

 the water which descended into these fissures, but would be for 

 the most part at such temperatures as have been found the most 

 favourable for the rapid communication 'of their heat to the 

 water, and the production from it of steam at a pressure due to 

 the superincumbent head. The conjoint action, therefore, of 

 steam so produced demanding avast supply for latent heat, and 

 of rapid and circulatory movements given to the sheets of water 

 within the fissures, would tend most powerfully to lower the tem- 

 perature of the prisms, and that with an energy the greater as 

 they became longer by the fissures extending more deeply into 

 the mass. 



It has been urged by some authors that water cannot have 

 entered the fissures between the prisms, inasmuch as these are 

 frequently found so close as not to admit the blade of a 

 table-knife; but even if this closeness were universal, water 

 would freely enter and ooze through such fissures at a mode- 

 rate pressure. The actual water-pressure, however, to which 

 many amygdaloidal basalts have been exposed must have been 

 very great ; for it has forced the sea-water and its saline contents 

 in solution through the substance of the prisms as if through a 

 filtering-stone, and so enabled the cavities of such basalt to be- 

 come gradually filled with hydrous minerals, zeolites, chalcedo- 

 nies, &c. Besides this, the thinnest film of water within the 

 fissures must tend to widen these by hydrostatic pressure. It 

 may also be remarked that an overflow of basalt covering a level 

 surface must, by heating that surface, raise it slightly into a con- 

 vex umbo, whence the fissures and prisms, when formed, must be- 

 come slightly divergent ; but after the whole has cooled com- 

 pletely, the umbo will disappear by contraction down to the ori- 

 ginal level surface, or even somewhat below it ; and so all the 

 prisms will approach each other towards their upper extremities, 

 and thus in an after age be found to grip one another and be in 

 closer proximity than at the time of their first formation. Re- 

 turning now to the production of cross joints, we have assumed 

 the tabular mass of basalt to be isotropic, its coefficient of con- 

 traction being thus the same in the vertical direction, or parallel 

 to the axes of the prisms, as in the two others orthogonal thereto 

 or horizontal. Were the temperature in any given section of a 

 prism transverse to its axis the same throughout (?". e. the axial 



