FID. I 2 



210 Mr. R. Mallet on the Origin and Mechanism of 



been poured, and where the mass, whether from being relatively 

 thin, or from having been poured out over a surface of badly 

 conducting material, is mainly cooled from the top surface only, 

 then the prisms may be all vertical and straight throughout the 

 mass, the stratum of broken fragments being at the bottom but 

 varying in thickness, that being greatest about the summits of 

 the inequalities of the bottom. But where (as in fig. 12) the 

 entire thickness of the mass is 

 great, the top surface being, 

 as before, nearly level, but the 

 bottom surface presenting con- 

 siderable inequalities, and cool- 

 ing taking place from the top 

 and also from the bottom, then 

 the upper range of prisms will 

 still be vertical, straight, and parallel to each other; but the lower 

 range of prisms formed will be alternately divergent and conver- 

 gent and present more or less irregularities, and be separated from 

 the upper range by a curved stratum of irregular fragments, the 

 general contour of which will have a certain resemblance to the 

 curvilinear forms of the bottom upon whicb the mass was poured 

 out. If the basalt be poured forth beneath, or prior to its cool- 

 ing be covered over by a thick stratum of detrital or other badly 

 conducting material, so that the cooling takes place almost wholly 

 from the largely irregular bottom surface (as in fig. 12 bis), then 



AX 



will the prisms be alternately convergent and divergent through- 

 out the entire thickness of the basalt, their uppermost portions 

 being separated from the detrital covering by a layer, variable in 

 thickness, of irregular fragments of basalt Should the molten 

 mass have been forced up through a wide vertical or more or less 

 inclined fissure of large size, which it fills to the surface but does 

 not overflow (as in fig. 13), then cooling from both sides of the fis- 

 sure will produce two ranges of prisms straight and perpendicular 



i 



