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 pitchstone, 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. 
