Professor T. G. Bonney — Fulgurites from the Andes. 3 



be a layer of imperfectly fused rock, tlie transition from this to the 

 perfect glass being very rapid. It is more or less opaque, but signs 

 of felspar microliths can be detected. Evidently the rock was not 

 materially affected by the lightning for more than about a hundredth, 

 of an inch beyond the fused part.^ 



The next specimen is from the actual summit of Aconcagua, which, 

 was also collected by Mr. Vines. In the rock itself there is nothing 

 exceptional, but it is interesting as being from the highest point yet 

 certainly reached. This, according to a trigonometrical measurement 

 by Dr. Giissfeldt, is 22,867 feet above the sea.^ The summit is 

 a square plateau, 75 paces each way, sloping slightly to the north. 

 But few specimens were brought from the uppermost part of the 

 mountain, none of which are scoria. This rock is no doubt 

 from a dyke or a fragment of a flow, so that the crater has 

 probably disappeared even more completely than from Tupungato. 

 Photographs show that Aconcagua has been greatly sculptured by 

 meteoric agencies ; at least as much, though on a grander scale, as 

 the Puy de Sancy in Central France. The specimen bears this 

 label : " Loose fragment from the summit of Aconcagua ; all the 

 neighbouring rock of the plateau, about 75 yards square, appears to 

 be the same." It is a rather triangular piece of rock, measuring from 

 5 to 6 inches along the side and about 2^ inches in the thickest part. 

 Its colour is a warm purplish-grey ; the old surfaces are rather 

 speckled and have weathered to a reddish or yellowish brown tint ; 

 small whitish felspars and a black pyroxenic mineral, with possibly 

 some iron-oxide, can just be detected by the unaided eye, and a few 

 vesicles are present. These larger minerals prove on microscopic 

 examination to be : (a) Felspars rather variable in size, often about 

 one-fiftieth of an inch long, much as described above, probably 

 andesine. (6) Hornblendes, fairly regular in outline, but occasionally 

 with blunted angles or a corrosion border, sometimes even rounded ; 

 pleochroic, changing from pale greenish-brown to a deep sienna- 

 brown; in one or two cases they form a kind of skeleton, owing to 

 the inclusion of small irregularly-shaped felspar crystals, (c) A few 

 grains or crystals of augite of a pale-brown tint inclining to green. 

 {d) Iron-oxide. The base in which these crystalline grains are 

 embedded is sprinkled with opacite and clouded with ferrite, being 

 apparently a slightly decomposed glass ; it is crowded with micro- 

 liths of plagioclase, which give rather small extinction angles. 

 Hence the summit rock of Aconcagua is a hornblende - andesite. 

 This determination is fully confirmed by a chemical analysis, which, 

 has been made for me in the Chemical Laboratory of University 



^ Fulgurites from Mont Blanc and from Monte Viso (collected by Mr. J. Eccles) 

 have been described and figured by Mr. F. Rutley (Quart. Journ. Geol. Soc, 

 vol. xli, p. 152, and vol. xlv, p. 60) ; also (in a serpentine) by Miss E. Aston and the 

 present writer (id., vol. lii, p. 452). 



■^ Two peaks of greater elevation in the Himalayas are said to have been climbed, 

 but in each case much doubt exists as to the accuracy of the identification. Pending 

 the determination of the heights lately reached in the Bolivian Andes by Sii- 

 M. Conway, and perhaps in any case, Aconcagua is the highest peak which has 

 been certainly scaled. 



