THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE III. VOL. III. 



No. VI.— JUNE, 1886. 



OlilG-IlNr^.Xj ABTICLES. 



I. — On Marekanite and its Allies. 



By Professor John W. Judd, F.E.S., 

 President of the Geological Society. 



IN almost every collection of minerals there may be found speci- 

 mens of the curious little glassy balls, which, from the locality 

 of their occurrence — the great Marekanka, near Okhotsk in Siberia — 

 have received the name of Marekanite. These glassy balls are more 

 or less perfectly rounded in form, they vary in tbeir colour, through 

 different shades of smoke-grey to orange-brown, while in size they 

 range from the dimensions of a pea to those of a walnut. Most 

 mineralogical treatises still continue to recognize Marekanite as a 

 mineral, and class it either as a variety of Obsidian or of Pearlstone. 



While totally disallowing the claim of Marekanite to rank as a 

 mineral-species or even variety — the petrographer recognizes in this 

 substance a type of rock, which proves to be by no means devoid of 

 interest, when its properties are carefully studied, nor destitute of 

 suggestiveness, when its mode of occurrence is thoughtfully considered. 



Occurring as it does on a spot frequently visited by travellers, the 

 appearance and surroundings of Marekanite have been very fully 

 described ; among others by Steller, Laxman, Allegretti and Pallas ; 

 it was the last-mentioned naturalist who appears to have made the 

 rock widely known by sending specimens from St. Petersburgh to 

 most of the Museums of Europe. 



The first mineralogist who seems to have expressed an opinion on 

 the nature of Marekanite was Sewergin, who in 1796 described it as 

 a " Glasszeolite." * To this conclusion he was probably led by 

 studying its behaviour with the blowpipe. 



Lowitz and Gmelin made analyses of the substance, 2 and these, 

 though very imperfect, as might be expected from the early date at 

 which they were undertaken, showed the rock to contain a very high 

 percentage of silica with some alumina and a small proportion of the 

 alkalies and alkaline earths. 



The author to whom we are however indebted for most of our 

 knowledge of the properties of this curious substance is Klaproth, 



1 Nova Acta Acad. Petropol. T. xii. (1801) p. 237. 



2 Neue Nord Eeitrag, vol. ii. p. 299. 



DECADE III. — VOL. III. — NO. VI. 16 



