Washington — Submarine Eruptions of 1831 and 1891. 139 



The feldspar basalt scoria therefore is an an.dose (IL5.3.4), 

 and it may be noted that calculation of the norm of Foerstner's 

 analysis also leads to the same result. As the rock is highly 

 vitreous, its mode is indeterminate. 



Variation in the magma. — The second analysis by Abich 

 (IV) was made of the light-colored pumice which was washed 

 up on the shore near Sciacca during the first days of the erup- 

 tion, while the other (III) was made of the scoria which formed 

 the island itself and which dates from the later period of the 

 eruption. They are of special interest, because, in spite of their 

 crudities, they indicate that the material was not uniform in 

 chemical character, but that the magma underwent a change in 

 chemical composition during the progress of the eruption. 



Abich, of course, does not give any description of the micro- 

 scopic characters, and Foerstner's description of the light gray 

 pumice (p. 392) which he also assumes to be that of the early 

 phase, shows rather indefinite characters. According to him it is 

 composed of flakes made up of minute, mostly colorless crystals, 

 which have little action on polarized light, with redder, rust- 

 brown particles and magnetite grains. The gray pumice 

 contains fragments of material resembling the black basalt 

 which he analyzed. He expresses the opinion that the light 

 color may be due to the action of acid .vapors. But his descrip- 

 tion, taken in connection with the analysis by Abich, which 

 shows a marked variation in the alkalis, much lower lime,, 

 magnesia and iron oxides, and higher silica and alumina, as 

 compared with the basalt, indicates that this light-gray rock is 

 a pumiceous trachyte, presumably described from a differen- 

 tiation of the magma. 



The marked change in composition is commented on by 

 Abich (p. 74), who suggests as a cause, either a gradual change 

 in the depths from a silica-rich rock to one lower in silica with 

 the addition of much lime (a vague forecast of the modern 

 notion of differentiation), or the action of the volcanic, highly 

 heated gases on pre-existing rocks, which gave rise to the gray 

 pumice, his opinion favoring this latter interpretation. 



In view of the short duration of the eruption the fact of 

 such a decided change in magmatic character as is indicated by 

 Abich's analyses is very striking. But its greatest interest lies 

 in that the change is remarkably like that shown in the succes- 

 sion of magmas on the neighboring island of Pantelleria, where 

 the earlier eruptions furnished highly salic and alkalic trachytes, 

 rhyolites, and pantellerites, followed at the close by basalts 

 very similar to that of Graham Island and the eruption of 1891. 

 This would indicate for both localities a similar original magma 

 and a similar course of differentiation. This matter will be 

 discussed at greater length in a forthcoming paper on the rocks 

 of Pantelleria. 



