256 «/. W. Judd — On Volcanos. 



down to us are of a very meagre character; that of Pontanus having 

 been written in 1538, and that of Marenta in 1559. 



The products of this last eruption in Ischia present many features 

 of the highest interest to the geologist. The rock of the Arso 

 lava-stream is decidedly more basic in character than any other 

 rock in the island. It does not, however, fairly fall within the class 

 of lavas intermediate between the trachytes and basalts, to which 

 Abich gave the name of " trachy-dolerite," and to which more 

 modern writers apply the term "andesite." The basis of this lava 

 is of a very dark grey, almost black colour, and consists of an 

 amorphous dark-coloured magma, in which crystals of orthoclastic 

 and plagioclastic felspars are imbedded. Scattered in great abund- 

 ance through the mass are large crystals of sanidine, which, as 

 Fuchs has shown, present some very anomalous characters. With 

 these occur, sometimes in great abundance, crystals of hornblende, 

 augite, mica, and magnetite, and irregular grains of olivine. A very 

 similar rock occurs, as we have seen, in several of the Lipari Islands. 



In illustration of the composition of the peculiar lava of the Arso, 

 I may cite the analyses made of it by Abich and Fuchs, and that 

 of the cinders of the Cremate by the latter author. 



Silica 



Alumina 



Ferric Oxide 

 Ferrous Oxide 

 Lime ... ... .. 



Magnesia 



Potash 



Soda 



The specific gravity of the Arso rock is according to Fuchs 2-61. 

 Abich estimated the mineralogical composition of the Arso lava to 

 be as follows : — 



Felspar 84*45 



Olivine 3 - 61 



Augite 9-22 



Magnetite _ 2-72 



Ischia contains a great number of hot and mineral springs, at 

 many of which those baths have been erected for which the island 

 is famous. Some of these springs also give off large quantities of 

 carbonic acid and other gases, and a number of stufe from which dry 

 steam issues also occur. These numerous hot springs, etc., testify 

 •to the activity of the forces still at work beneath the island ; so 

 that a new outburst of these forces, which have now been otherwise 

 dormant for nearly 600 years, may at any time take place. 



The effects of subterranean forces in changing the elevation of 

 different parts of the island within periods which are geologically 

 very recent are manifested in the raised beaches of Lacco, Punta St. 

 Alesandro and Punta dell' Imperatore. All of these yield great 

 numbers of shells of the species still existing in the Mediterranean, 

 which are found up to heights of 130 feet above the sea-level. 



Alike in the composition of its lavas and tuffs, and in the periods 



Arso lava. 



The same. 



Cinders of 



Abich. 



Fuchs. 



Cremate. Fuchs, 



. 60-80 



57-73 



54-83 



, 17-21 



17-85 



20-17 



. 3-55 



4-44 



4-77 



. 1-29 



3-90 



3-86 



1-43 



3-65 



4-12 



, 2-07 



1-77 ■ 



1-93 



. 7-77 



7-65 



7-38 



4-64 



3-77 



3-04 



