734 ON THE VOLCANO OF TAAL, 



I mean the theory which he propounds as to the order in 

 which volcanic products make their appearance. According to this 

 author, the first erupted rocks are those of intermediate composi- 

 tion known as Andesites. These Andesites, which are especially 

 characterized by the nature of their felspar, sometimes contain free 

 quartz, and are then known as quartz-andesites or dacites from 

 their abundance in Transylvania, the old Roman province of Dacia. 

 Richthofen sugo-ests that another class of volcanic rocks to which 

 he gives the name of " propylites " were in every case erupted 

 before the andesites, and in support of his views adduces the fact 

 that in many instances propylites are found underlying andesites. 

 But the propylites are, in chemical composition, identical with the 

 andesites, and, like them, present some varieties in which quartz 

 occurs, and others in which that mineral is absent. In their 

 microscopic characters the propylites difter from the andesites and 

 dacites only in the fact that the former are more perfectly crystal- 

 line in structure, beingindeed in many cases quite undistinguishable 

 from the diorites or the plutonic representatives of the andesites. 

 The propylites also contain liquid cavities, which the andesites and 

 dacites as a rule do not, and the former class of rocks, as Prof. 

 Szabo well points out, are usually much altered by the passage of 

 sulphurous and other vapours, in consequence of which they 

 frequently contain valuable metallic ores. The extension of these 

 andesitic lavas is sometimes accompanied, and sometimes preceded 

 or followed, by eruptions of trachytic lavas — that is, of lavas of 

 intermediate composition which have a different kind of felspar 

 from that prevailing in the andesites. In the final stages of the 

 eruptive action in most volcanic districts the lavas poured forth 

 belong to the classes of the rhyolitic or acid, and the basaltic or 

 basic lavas.* 



The author from which the above is taken, goes on to tell us 

 that this law is admirably illustrated in the Lipari Islands. The 

 great central volcano of this group, now in a ruined condition, is 

 composed of andesitic lavas. The other craters disposed on three 



* Judd, Volcanoes, op. cit., p. 199. 



