528 POST-PLIOCENE VOLCANIC ROCKS. [Ch. XXX. 



ally intervenes, on the contact of the vertical dike and intersected beds. 

 M. Necker mentions one of these at the place called Piirao Monte, in the 

 Atrio del Cavallo ; and when I examined Somma, in 1828, I saw three 

 or four others in different parts of the great escarpment. These phenom- 

 ena are in perfect harmony with the results of the experiments of Sir 

 James Hall and Mr. Gregory Watt, which have shown that a glassy tex- 

 ture is the effect of sudden cooling, while, on the contrary, a crystalline 

 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 near the bottom a»d at the surface of a current flow- 

 ing 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 Yesuvian currents 

 distinct crystals of augite and leucite become apparent. 



The same phenomenon, observes M. Necker, may readily be exhibited 

 on a smaller scale, if we detach a piece of liquid lava from a moving 

 current. The fragment cools instantly, and we find the surface covered 

 with a vitreous coat ; while the interior, although extremely fine-grained, 

 has a more stony appearance. 



It must, however, be observed, that although the lateral portions of 

 the dikes are finer grained than the central, yet the vitreous parting 

 layer before alluded to is rare in Vesuvius. This may, perhaps, be 

 accounted for, as the above-mentioned author suggests, by the great heat 

 which the walls of a fissure may acquire before the fluid mass begins to 

 consolidate, in which case the lava, even at the sides, would cool very 

 slowly. Some fissures, also, may be filled from above, as frequently 

 happens in the volcanos of the Sandwich Islands, according to the obser- 

 vations of Mr. Dana; and in this case the refrigeration at the sides 

 would be more rapid than when the melted matter flowed upwards from 

 the volcanic foci, in an intensely heated state. Mr. Darwin informs me 

 that in St, Helena almost every dike has a vitreous selvage. 



The rock composing the dikes both in the modern and ancient part of 

 Vesuvius is far more compact than that of ordinary lava, for the pres- 

 sure of a column of melted matter in a fissure greatly exceeds that in 

 an ordinary stream of lava ; and pressure checks the expansion of those 

 gases which give rise to vesicles in lava. 



There is a tendency in almost all the Vesuvian dikes to divide into 

 horizontal prisms, a'phenomenon in accordance with the formation of 

 vertical columns in horizontal beds of lava; for in both cases the divi- 



