210 R. Mallet—Origin of the columnar structure of Basalt. 
If the mass be tabular, as already assumed, and cooling take 
place only from the top surface, the prisms will be straight and 
vertical, extending from top to bottom nearly of the tabu- 
lar mass, and being separated from the bottom on which it rests 
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diameter partially or wholly then commencing. The converg- 
rature. : 
The author then proceeds to develo and illustrate by dia- 
. . . : < are 
sive isothermal couches, taken in descending order, are not pat 
allel to the original cooling surface, as they are in all cases 0 
straight and parallel prisms, but divergent gradually from the 
cooling surface and from each other, then the lines of opt 
of the prisms, always normal to these couches, must be cw'v 
