THE OCT 1 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. VI. 



No. I.— JANUARY, 1899. 



a:Etx(jrXj<rj^iL, ^^isticleis. 



L — Fulgurites from Tupungato and the Summit of Aconcagua. 



By Professor T. G. Bonnet, D.Sc, LL.D., F.E.S., V.P.G.S. 



rpHE members of Mr. E. A. FitzGerald's expedition to the Chilian 

 1_ Andes in 1896-7 ' made a rather large collection of rock 

 specimens from the higher regions, which has been entrusted 

 to me for examination. The descriptions will be published in 

 Mr. FitzGerald's forthcoming book, but as two are exceptionally 

 interesting I wish them to appear in the Geological Magazine. 

 Permission for this has been kindly granted by him and by his 

 publishers, Messrs. Methuen, to whom I return my cordial thanks. 



The specimen of greatest interest is a block of andesite with 

 remarkably fine fulgurites, taken from the actual summit of 

 Tupungato (20,260 feet) by Mr. S. M. Vines, a member of the 

 expedition, who made the first ascent in company with the noted 

 Swiss guide Zurbriggen. The mountain-top is a curving plateau, 

 from which three peaks rise up to a maximum height of about 

 300 feet. The crater has disappeared, but a little scoria was 

 collected, from which I conjecture that some traces of it may still 

 be found. 



The first specimen bears this label: "Found loose in quantities 

 on the top of highest of the three heaps or peaks at summit of 

 Tupungato." It is a block rather irregular in shape, one surface 

 being somewhat convex and the corresponding one concave, but the 

 dimensions may be roughly given as 6x6x4 inches ; it is 

 slightly weathered on the older surfaces (some being fresh fractures). 

 The rock is compact, except for a few minute vesicles ; in colour 

 a dull purplish brown or madder, and spotted with small white 

 felspars, the largest of which vary from about one-tenth to one-sixth 

 of an inch in diameter. Under the microscope we find the minerals 

 of early consolidation to be (a) iron-oxide ; (6) brown hornblende, 

 not seldom fairly idiomorphic, very variable in size, and sometimes 



1 For a summary of the "^vork, see the Geographical Journal, vol. xii, p. 469. 



DECADE IV. VOL. TI. — NO. I. 1 



