250 Dr. Mac Culloch on a 



ing to have been most separated where the heat has acted most 

 immediately ; though from this appearance it is not unlikely 

 that this separation has been produced by mechanical causes. 

 Their structure being tender, it is easy to imagine that the in- 

 cessant motion of so heavy a fluid as iron along the surface of 

 the hearth, had worn the sides of the prism so as gradually to 

 permit it to insinuate itself among them, and thus to cause a 

 separation towards the upper extremities. It is essential to 

 attend to this circumstance, becauses it involves an important 

 question respecting the columnar trap rocks. In the desicca- 

 tion of clays containing much water, a division of the mass 

 takes place, which causes it to separate into parts of an irre- 

 gular prismatic configuration. This is the effect of drying ; 

 and, in consequence of the diminution of bulk thence resulting, 

 the prisms are separated from each other by considerable inter- 

 vals. In the columnar trap rocks, on the contrary, the prisms 

 remain in perfect contact ; proving that the configuration does 

 not, in this case, depend on a general diminution of the bulk of 

 the original mass. In the case of the sandstone in question, it 

 is certain that it can undergo no loss of water sufficient to 

 produce a separation of the mass from that cause ; while the 

 absolute contact of the prisms, where they have not been ex- 

 posed to causes of waste, proves that the separation into these 

 forms is the consequence of circumstances of some other nature. 

 On examining with a magnifying glass the texture of this 

 sandstone in its present state, the most remarkable change is 

 the want of angularity in the particles of the sand. It is true 

 that this may have been the original character of the stone, but 

 it is accompanied by an occasional vitreous splendour in some 

 of them, which seems to prove that their surfaces have been 

 slightly fused. It does not follow, however, that this is a fusion 

 of quartz ; it is more likely to be a glass produced by a combi- 

 nation of the argillaceous earth, where the two have been in 

 contact. The small opaque particles which seem to be inter- 

 mixed with the grains of quartz, may possibly consist of the 

 supposed argillaceous earth of the original stone ; but they are 

 so difficult to determine, that I am even inclined to doubt if 

 this specimen ever contained any clay, and whether it is not a 

 simple sandstone. It has certainly undergone some change j 



