204 Physical Notices on the Bay of Naples. 



The cellular lava, properly so called, is, as far as I know, 

 nearly of one description in every situation, and it is widely 

 distributed through the volcanic regions of the globe. In 

 composition it probably does not differ from the compact spe- 

 cies, but owes its name to the cavities distributed through its 

 mass, which render it extremely light, and balls of it are often 

 discharged from the crater along with the sand or ashes. In 

 this condition it is frequently found in the ruins of Pompei. 

 Its characters seem to indicate that it forms the upper portions 

 of the streams of lava where the air-bubbles, disengaged by 

 pressure from the lower part, are collected and prevented 

 from escaping by the rapid cooling of the cells containing 

 them when near the open air. In structure the cellular lava 

 precisely resembles some of our amygdaloidal trap rocks, where 

 the zeolites have been removed by the weather from the cavi- 

 ties. Not unfrequently this lava is coated with a green salt 

 of copper. I have an example of this from the " Cratere del 

 Francese." 



The decomposed lavas have very different characters from 

 the two preceding kinds. Their appearance is so dissimilar, 

 that an uninformed observer would find it difficult to recog- 

 nize the hard basaltic mass of fresh lava in the soft clayey 

 bed which it forms when decomposed. The action is in two 

 ways, either by the simple weathering of the ingredients, or 

 when this agent is assisted by the gaseous emanations with 

 which volcanic countries abound. The chemical and other 

 details we cannot pursue ; but in the works of Breislak * and 

 Dr Daubeny -f they will be found considered. The general 

 fact, however, is sufficiently simple, that those rocks in which 

 felspar predominates are reduced in proportion to its consti- 

 tuent quantity to clay-stone, and the potash with which many 

 of these rocks abound makes the soil thus produced extreme- 

 ly fertile. In the crater of Mount Vesuvius, as well as at 

 the Solfatara, the decomposed lavas are very extensive, and 

 generally pure white. As they are accidentally mixed with 

 more or less sulphur and red orpiment, the shades of colour 

 are beautifully and infinitely varied. See my remarks on Ve- 



* Campanie, toni. ii. 96- t Volcanos, p. 167 and 376. 



