THE ERUPTION OF VESUVIUS IN APRIL, 1906. 245 



The essential character of all these modifications is the partial or 

 entire disapjjearance of leiicite, which maintains its geometric form, 

 but becomes transformed into sanidine, sodalite, and especially micro- 

 sommite. frequently accompanied by basic plagioclase. The forma- 

 tion of new minerals does not take place necessarily in the places occu- 

 pied by the old minerals, but may proceed in adjacent cavities. 

 These colorless minerals, feldspars and feldsi^athoids. are accom- 

 panied by numerous other minerals such as augite, hornblende, biotite, 

 hematite, magnetite, and more rarely melanite, sphene, and olivine; 

 in some cases I have observed recrystallization of leucite. These 

 minerals present definite associations depending upon the character 

 of the rock at the expense of which they are formed, and on the con- 

 tlitions governing their formation. The brown hornblende, for exam- 

 ple, and its associate, magnetite, appear to be formed in a reducing 

 medium, whereas aegirite-augite, alwaj^s accompanied by abundant 

 crystals of hematite, has crystallized in an oxydizing medium. 



The new minerals are not only found as beautiful crystals in 

 druses ; they also impregnate the rock, inclosing the normal minerals. 

 The augite and amphibole and at times the mica may become oriented 

 upon the original pyroxene. This is especialh^ true of the augite. 

 The primary augite becomes colored progressively yellow and takes 

 on the optical properties of aegirite-augite. 



The mechanism of these transformations can be established with a 

 certain probability. We know that the eruptive period terminated 

 by the recent disaster has had as a principal result the filling of the 

 crater formed by the eruption of 1872. The frequent flows from the 

 flanks of Vesuvius since 1875 show that melted magma has been for 

 this whole period in intimate contact with the walls of the subter- 

 ranean channels. It has raised their temperature above 500° C. and 

 furnished them with emanations, the nature of which we know. The 

 transformations, which are to be attributed to these emanations, have 

 been produced by the chlorides and alkaline sulphates, especially by 

 those of sodium that impregnate all the samples studied. It is strik- 

 ing to note the constant substitution of sodic or calic minerals for 

 leucite. The two most common acid minerals are sodalite and micro- 

 sommite. 



The general interest that these transformations present is, then, to 

 show with perfect clearness the influence in contact metamorphism 

 of volatile products emanating from a magma, and the fixation of 

 some of them by the transformed rock. 



The leucitic magmas occupy a special place among eruptives by the 

 intensity of the contact metamorphism they exhibit. I have shown 

 in other instances by consideration of their inclusions, that they are 



