524 Correspondence — Br. H. J. J. Laws. 



the magma had remained in contact with aquiferous strata, gradually 

 absorbing water, and consequently the impossibility of the outflow 

 of lava until all the water-saturated magma has been ejected. In 

 relation with this the presence of the three divisions of the ejecta- 

 menta ; the lowest or first being a very vitreous pumice, except for 

 the presence of pre-eruptive minerals such as the felspars, amphibole, 

 mica, and magnetite of first consolidation ; the second or middle 

 division being far more microlithic, with the presence of pyroxene or 

 other eruptive or post-eruptive formed minerals ; and the third or 

 upper division consisting of ash due to the loss of cohesion within 

 the magma in consequence of the advanced stage of conversion of 

 the glassy part into " formed " or individualized matter. 



In many tufas, and especially those formed from the third or upper 

 division of the products of a plinian eruption, or of any pumiceous 

 ash, there are an abundance of little pisolitic concretions of great 

 perfection. Scrope has suggested drops of water falling in the dust as 

 the cause of their production. To me they seem rather a segregation 

 similar to what occurs in the felspatbic and siliceous glazing cream 

 in pottery manufacture ; but there is still much doubt, and observation 

 would be of much value if made on the new ash, as we do not know 

 whether they are formed immediately, or after long exposure and 

 soaking by moisture. One thing should be remembered, and that 

 is, that a few small ones may be inclosed in a larger one, which is 

 not compatible with Scrope's theory. 



The next point is the vesicular structure in ashes, which often so 

 well mimics that of lava, that in old decomposed rocks much doubt 

 may exist as to whether a given mass may be a lava or a tuff. 

 I think the drops of rain, suggested as an explanation of pisolites, is 

 the real influence at work in producing these vesicular cavities. 



Next, it would be interesting to know whether new cones (or more 

 properly crater-rings) have been formed within the craters of 

 explosion towards the end of the eruption. 



Observations on the proportion of the essential, accessory, and 

 accidental ejectamenta should be made in each of the subdivisions of 

 the deposits, and at different distances from the centres of explosion. 



Observations of the green and dry wood buried in the pumice 

 would clear up the question as to whether the peculiar lignitization 

 or carbonization to be seen at Pompei, not only in wood, but also in 

 bread, fruit, cloth, grain, etc., is due to burning, baking, or subse- 

 quent decomposition. In Pompei neither glass nor lead was fused 

 where buried by the falling pumice. This I have shown to be due 

 to the low temperature of the pumice from the loss of heat in 

 converting liquid or dissolved water into vapour on the relief 

 of pressure during the eruption. 



Lastly, search should be made for fulgurites, which have been met 

 with at Pompei. 



H. J. Johnston-Lavis. 



