244 THE ERUPTION OF VESUVIUS IN APRIL, 1906. 



conditions of metamorphism which prevail at the depths from which 

 a part of the material has been derived. 



These rocks can be referred to two groups : First, those of volcanic 

 ©rigin, and second, limestone and its metamorphic derivatives, which, 

 while important from the metamorphic standpoint, furnish no infor- 

 mation other than that already available from the study of the tuffs 

 of Monte Somma. 



The blocks of volcanic origin include fragments of rocks, in places 

 broken and ejected by the explosions (these were already consoli- 

 dated, but of a high temperature), and also fragments that had seen 

 the light one or more times as old products of ejection, torn from beds 

 of tuff or from breccias of former eruptions. All are leucotephrites, 

 but vary in chemical and mineralogical composition and in texture. 

 Some of them are of types comparable to those thrown out in the 

 historic flows of Vesuvius; others, much more crystalline with a 

 doleritic texture, are like types prevailing in the tuffs and breccias 

 of Monte Somma, being fragments of intrusive flows or dikes crys- 

 tallized at considerable depth under different conditions than the 

 preceding types. 



The study of these different rocks permits of tracing the varia- 

 tions of the magma of this volcanic massif and the influence of the 

 conditions of cooling upon the nature of the rocks derived from it; 

 but the metamorphic modifications observed in many of them have a 

 more general importance. 



These modifications are of two kinds. The one kind is due only to 

 the action of heat and is produced in blocks carried up by the fluid 

 magma or falling into it after ejection. These changes can be of 

 recent origin and be accomplished in the course of the eruption. The 

 fusible minerals are melted, forming a glass enveloping the other 

 constituents and frequently containing new minerals. 



Much more important are the other modifications, which have 

 been produced at a temperature beloAv the melting point of the most 

 fusible of the constituents. They are not the work of the recent 

 eruption, but have resulted from a previous long-continued process. 

 The intensity of the modification is variable, though in some speci- 

 mens the microscopic appearance is not changed. A speciuien orig- 

 inally vesicular or scoriaceous, has its ca^dties lined with newly 

 formed crystals; in the case of a breccia the fragments become coated 

 with new minerals. The attack is often more profound, ciwstals 

 of leucite becoming by corrosion like small geodes, the cavities 

 enlarge, the texture changes, the rock becomes porous and very crys- 

 talline, and large crystals, particularly microsommite, appear simu- 

 lating phenocrysts. 



