THE ERUPTION OF VESUVIUS IN APRIL, 1906. 241 



surface became covered with a white coating of alkaline chlorides 

 and sulphates that impregnated all the erupted products. Some daj^s 

 later, on account of the persistent rains, the erosive period was 

 inaugurated, with its disastrous floods. 



The phenomena that I have seen in the production and intermix- 

 ture of the chaotic formations, accumulated in the dry way by ava- 

 lanches, with those produced at their expense under the influence of 

 water, and the analogy of structure presented by them after drying 

 and settling, are of great importance for geologists who have to 

 decipher the structure of extinct volcanoes, like those of Auvergne. 

 They explain the difficulties of interpretation, often inexplicable, that 

 one encounters in the study of breccias and conglomerates of trachytic 

 and andesitic origin. 



THE PRODUCTS OF ERUPTION. 



The massif of Vesuvius, comprising Monte Somma and Vesuvius, 

 presents great A^ariations from the petrographic viewpoint, but all 

 the rocks have a family likeness. Thej' are all very potassic and 

 either contain leucite or have a composition potentially leucitic. 



The petrographic character of Monte Somma is more complex than 

 that of Vesuvius, because it contains not only basic rocks with leucite 

 and the leucotephrites, which form dikes and flows, but also types 

 of Avhite acid rocks, which form thick beds of tuffs and breccias. 

 Vesuvius, on the other hand, has been built up by an accumulation of 

 scoriae, ashes, flows, and dikes, belonging only to leucotephrites, 

 which, according to the eruptions, present further variations still 

 imperfectly studied. There are different facies depending uj^on the 

 greater or less abundance of phenocrysts of the predominant min- 

 erals — leucite, augite, olivine. 



I shall take up successively the new magma thrown out as Strom- 

 bolian explosions and poured out as flows, and then the old debris, 

 Avhich constitutes the i^redominant material expelled by the Vulcanian 

 explosions. 



XEW MAGMA. 



The lava has a grayish-black ground mass, with rather abundant 

 ]3henocrysts of leucite and augite some millimeters in diameter. It 

 was particularly interesting to determine if, in the course of the 

 eruption, there w^as any chemical change resulting from differentia- 

 tion between the upper part of the magma, as exhibited in the Strom- 

 bolian explosions at the beginning of the eruption, and the portion 

 poured out last of all. The following analyses show there has been 

 no systematic variation and that the magma has kept a remarkably 



SM 1900 16 



