242 



THE ERUPTION OF VESUVIUS IN APRIL, 1906. 



uniform composition. Analyses of more numerous examples would 

 show perhaps greater differences. 



A. Scoria ejected at beginning of tlie erviption coUectod hy Mattencci near the observa- 

 tory. 



B. Lava of April S collected near the cemetery of Torre Annunziata. 



C. Lava of 1631. Analysis by Washington. 



D. Lava of 1S72. Analysis by Washington. 



This composition is characteristic of the Vesuvian lavas, and is 

 particularly remarkable for the high percentage of potassium. These 

 rocks belong to the type Vesuvose (II 8.2.2) of the quantitative 

 classification, and the older rocks of Somma differ from the recent 

 lavas by less potash and a smaller ratio of KoO : NaoO. 



I have followed the development of crystallization by comparing 

 forms of cooling less and less rapid. The scoriae of the Strombolian 

 explosions are very rich in brown glass, the specimen analyzed 

 being fragile and crumbling easily under pressure of the finger. 

 Another specimen collected at the fissure of 1,200 meters altitude is, 

 on the contrary, very resistant. The scoriae contain in their glass 

 large crystals of augite, leucite, titanomagnetite, a small proportion 

 of basic plagioclase, with a little apatite and olivine. There are few 

 or no microlites of feldspar, but those of augite are abundant. The 

 normal and tranquil process of crystallization was interrupted, and 

 cooling was so rapid that crystals were not developed. 



A form of consolidation less rapid is observable on the front and 

 upon the superficial parts of the flows. The rock still contains much 

 brown glass and it contains the same phenocrysts as the scoriae, but 

 they appear more abundant, and biotite is not as rare as a pheno- 

 cryst. There is also a large amount of microlitic leucite, augite, and 

 plagioclase. 



The most crystalline type is found in the interior of large blocks 

 broken by contraction from the surface of the flows. The rock, 



