138 Scientific Proceedings, Royal Dublin Society. 



up by stating that pumice is filled by vesicles of all sizes, but 

 mostly small, and approaches the vitreous state, whilst scoria only 

 contains vesicles of large size, and approaches a crystalline structure. 

 The ejectamenta during strombolian action is a true scoria, being 

 dependent upon borrowed steam that rises in the magma column, 

 and forms the vesicles. 



In lavas the presence of vesicular cavities is no proof of the 

 actual amount of original vapour, for the latter will be allowed to 

 more easily escape in a microcrystalline mass, such as that of 1631 

 of Vesuvius, which is a very compact rock, yet gave forth enormous 

 quantities of vapour as almost to resemble the explosive type of 

 eruption. The lava of 1858, which is rich in large leucite crystals 

 and much interstitial glass, is a remarkable spongy structure, 

 because its viscid nature prevented the escape of the few included 

 bubbles of vapour, which, compared with others, was remarkably 

 small in quantity in that eruption, affording compact types of lava 

 surface. This escape of vapour may so separate the constituent 

 minerals of a scoria surface as to leave it in a perfectly incoherent 

 and pulverulent state. This I have seen in some of the trachytes 

 of Ischia, and of the Solfatara (Monte Olibano). 



Another fact is, that lava as it pours out that portion which is 

 nearer the surface will, in all probability, be the richest in water, and 

 will produce a stream thickly covered with scoria. But as the portion 

 which comes from greater depths rises it will have been exposed 

 for less time to aquiferous rocks, and in consequence, containing 

 less water, will produce a smoother-surfaced stream. This was 

 remarkably the case in the Yesuvian eruption of 1855 ; l the first 

 streams that issued were much rougher than those at a later date. 



The conditions under which the composition of Igneous Hocks is 

 modified. — One of the most vexed questions in geology is undoubt- 

 edly that of the variation in an igneous rock, and more especially 

 with regard to its chemical than its mineral composition. Space 

 forbids here to enter fully into the theory of stratification of mag- 

 mas, as represented by Yon Eichlhofen and others. No distinct 

 division can be drawn between rocks derived from the most acid, 

 and the ultrabasic magmas, showing that they can mix in all pro- 

 portions. Then again, whatever be the silicates we may fuse 



1 Memoiria s. Incend. Vesuvio, 1855, G. Guarini, L. Palmieri, and A. Scacchi. 



