SOME ANCIENT AND MODERN VOLCANIC HOCKS. 393 



31icrosco_pic examination (Plate XVII. figs. 1 & 2). — The base con- 

 sists of imperfectly crystallized prisms of sanidine, with some larger 

 ones porphyritically imbedded in it. Scattered throughout are nu- 

 merous crystals of magnetite ; and every here and there are others, 

 probably both of orthoclastic felspar and augite, altered into a brown 

 mineral showing bands parallel to the sides of the crystals. This 

 product of alteration forms also grey and brown streaks between 

 the small felspar prisms. In fig. 2 an irregular crystalline plate 

 still shows some parts not wholly altered, while the compound crystal 

 on the other side of the figure indicates the manner in which the 

 original mineral has been replaced in regular bands parallel to the 

 outer walls ; this structure is also well seen in the oblong crystal of 

 fig. 1 *. 



With regard to the order in which crystallization has taken place, 

 it seems certain that the magnetite was the first, the large crystals 

 the next, and the small imperfectly formed felspar prisms the last to 

 crystallize. In fig. 1 an example is seen of a magnetite crystal 

 entirely surrounded by one of the brown pseudomorphs ; and the 

 way in which the minute felspar prisms have flowed round the 

 larger crystals, invariably setting tangentially to the sides of each, 

 is particularly well seen when viewed in polarized light, as in fig. 2. 

 This structure of the base is especially interesting when viewed in 

 connexion with the bulky buttress-like appearance of the lava-flow ; 

 and Scrope speaks of its remarkable mineral character as accounting 

 " for its extremely imperfect fluidity." I shall, however, have 

 occasion to refer again to this same plastic nature of a crystalline 

 base when dealing with lava -flows of Lower Silurian age. 



The brown pseudomorphic mineral would seem largely to consist 

 of iron oxide ; and the rock is in places reddened by it, 



2. Leucitic Basalt, Torre dell' Annunziata, Naples. 



The rock to which the following notes relate belongs to the 

 Vesuvian lava-flow of 1631. Its position may be thus described : — 

 Walking towards 'Naples from Torre dell' Annunziata along the sea- 

 shore, the cliffs are at first seen to consist of nearly horizontal beds 

 of somewhat coarse ash, the fragments partly rounded, while further 

 on white lines and layers of sandy mud appear in some places. Just, 

 however, where the railroad enters a cutting, a bed of close green - 

 spotted lava lies upon this ash, its lower part, for about a foot or 

 two, being very cindery and scoriaceous. It is quarried at various 

 points along the beach, and retains the same general character as 

 far as a large old square fort, where it is more scoriaceous. 

 Beyond this point the upper part of the cliff-section shows loose 

 scorise, and the lower part compact lava, which, however, passes in 

 places into a sandy micaceous rock, presenting indications of partial 

 bedding., This coast-section represents the seaward end of the lava- 

 streams of 1631, some of which flowed for a distance of five miles. 



* It is probable that many of these pseudomorphs are after augite; but there 

 has been so much alteration, and there are so few good crystalline forms to 

 examine, that their nature cannot always be determined with certainty. 



