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



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 the greater depth the mass assumes a more 

 lithoidal structure, and then becomes more and more stony as we de- 

 scend, 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 exhib- 

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

 moving current. The fragment cools instantly, and we find the sur- 

 face covered with a vitreous coat ; while the interior, although ex- 

 tremely 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 volcanoes of the Sandwich Islands, accord- 

 ing to the observations of Mr. Dana : and in this case the refrigera- 

 tion 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 pressure of a column of melted matter in a fissure greatly exceeds 

 that in an ordinary stream of lava ; and pressure checks the expan- 

 sion 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 

 divisions which give rise to the prismatic structure are at right angles 

 to the cooling surfaces. (See above, p. 61 V.) 



