124 NEWER PLIOCENE PERIOD. [Ch. X. 



or trap dikes are as regular in shape as those of Somma. The 

 solution, therefore, of this problem, in reference to the modern 

 dikes, is most interesting, as being of very general application 

 in geology. 



Varieties in their texture. Having explained the origin of 

 the parallelism of the sides of a dike, we have next to consider 

 the difference of its texture at the edges and in the middle. 

 Towards the centre, observes M, Necker, the rock is coarser 

 grained, the component elements being in a far-- more crystal- 

 line state, while at the edge the lava is sometimes vitreous and 

 always finer grained. A thin parting band, approaching in its 

 character to pitch stone, occasionally intervenes on the contact 

 of the vertical dike and intersected beds. M. Necker mentions 

 one of these at the place called Primo Monte, in the Atrio del 

 Cavallo ; I saw three or four others in different parts of the 

 great escarpment. These phenomena are in perfect harmony 

 with the results of the experiments of Sir James Hall and 

 Mr. Gregory Watt, which have shown that a glassy texture is 

 the effect of sudden cooling, and that, on the contrary, a crys- 

 talline grain is produced where fused minerals are allowed to 

 consolidate slowly and tranquilly under high pressure, 



It is evident that the central portion of the lava in a fissure 

 would , during consolidation, part with its heat more slowly 

 than the sides, although the contrast of circumstances would 

 not be so great as when we compare the lava at the bottom and 

 at the surface of a current flowing in the open air. In this 

 case the uppermost part, where it has been in contact with the 

 atmosphere, and where refrigeration has been most rapid, is 

 always found to consist of scoriform, vitreous, and porous lava, 

 while at a greater depth the mass assumes a more lithoidal 

 structure, and then becomes more and more stony as we descend, 

 until at length we are able to recognize with a magnifying 

 glass the simple minerals of which the rock is composed. On 

 penetrating still deeper, we can detect the constituent parts by 

 the naked eye, and in the Vesuvian currents distinct crystals of 

 augite and leucite become apparent. 



