576 Correspondence — Bernard Hobson. 



the localities myself to make a fuller study than is possible in the 

 laboratory. In the meantime the following notes may be of interest. 



The nephelite-syenite of the Granitberg was described very briefly 

 by Wagner. It is a foyaite with a very variable amount of nephelite, 

 which may locally make up two-thirds of the rock. The chief dark 

 mineral is a green aegirite-augite. This rock is cut by two dykes of 

 a singularly interesting microfoyaite which contains, in addition to 

 microperthite, nephelite, and pyroxene, smaller amounts of biotite, 

 perovskite, and zircon. The perovskite forms perfect octahedrons up 

 to half a millimetre in diameter. Zircon occurs in skeletons and 

 irregular groups and plates, and often encloses grains and laths of 

 felspar. The dark minerals amount to just under 10 per cent of 

 the rock. 



Prom the neighbourhood of Pomona there are several monchiquites 

 and camptonites, some fresh and others too much decomposed for 

 certain identification. In one of the camptonites, crystals of 

 barkevikite are enclosed by titanaugite with a reaction rim of 

 magnetite separating the two. In all these lamprophyres there are 

 pseudomorphs of a mineral like, iddingsite, apparently replacing 

 olivine. I have also a beautiful segirite-solvsbergite with marked 

 flow-structure, and some typical bostonites and lindoites. Dr. Rogers 

 has also found numerous bostonite dykes in Van Rhyn's Dorp and 

 JNamaqualand. 



I hope to publish a full account of these interesting rocks in the 

 course of time. g j Shand 



Geology Department, Victoria College, 

 Stellenbosch, S.A. 



October 1, 1915. 



BURSTING OF A LAKE BARRIER IN ARGENTINA. 



Sir, — It is not often that one finds anything of geological interest 

 in the report of a railway company, but the following details from the 

 Report of the Directors of the Buenos Ayres Great Southern Railway 

 Company for the year ended June 30, 1915, are interesting. 



" The most sensational, although by no means the most costly, of 

 the long series of mishaps due to this cause [the weather], was the 

 cataclysm in the Rio Colorado Valley in the early days of January, 

 when some thirty-six miles of the Railway were submerged under 

 deep water, and traffic on the Neuquen line beyond Gavietas was 

 entirely cut off for almost a month. This stupendous, and at first 

 inexplicable visitation, was discovered to be the outcome of the sudden 

 release, 350 miles away from the line as the crow flies, of an immense 

 body of water called Lake Carrilauquen, which had been formed by 

 a landslide at a comparatively recent geological epoch. Owing to 

 stress of weather this natural dam suddenly gave way and thus 

 launched 2,800,000,000 tons of water into the valley of the Rio 

 Colorado." 



The lake is nearly 6,000 feet above the sea-level. It was some 

 15 miles long, 1^ miles wide, and over 300 feet deep at the lower end. 



Bernard Hobson - . 



