244 Prof. J. W. Judd — On Marekanite and its Allies. 



states that Marekanite loses on ignition from one to four per cent, of 

 its weight. 



The last point to which I have to direct attention with respect 

 to Marekanite is its microscopic structure. This is found to differ 

 in a remarkable degree in the several varieties. 



Some of the Marekanite -balls are almost colourless and perfectly 

 translucent. These, as pointed out by Zirkel, 1 are among the purest 

 of natural glasses, and are comparable to Fulgurite and Boutellen- 

 stein. 2 From these translucent varieties we found every gradation 

 to the more opaque types which are of deep smoke-grey and orange- 

 brown tints, their opacity being evidently due to primary devitrifi- 

 cation. 



Porphyritic crystals are rare, and of small size in Marekanite. 

 Those observed appear to be referable to hornblende, magnetite, and 

 a brown mica. Black trichites and reddish or yellowish-brown 

 globulites abound, the latter being usually drawn out by the move- 

 ment of the glass into cloud-like wisps, and the rock being very dis- 

 tinctly banded by the disposition of these in the mass. 3 In many cases 

 the commencement of a pumiceous structure is exhibited through 

 the abundance of cavities drawn out in the direction of the flow. 

 The Marekanite-nodules exhibit all the characters of a volcanic glass 

 undergoing incipient devitrification and exhibiting flow structure, 

 with occasionally the beginnings of pumiceous distension. 



Now the peculiar characters of Marekanite seem to be capable of 

 simple explanation when we take into account its mode of occurrence, 

 which has been so clearly described by Erman and other writers. 4 



According to the information supplied from these different sources, 

 there occurs in the district around Okhotsk a considerable tract of 

 highly acid, igneous rock (rhyolite or quartz-andesite), which is 

 sometimes vesicular, at other times compact, and appears to be some- 

 times intrusive in strata containing Devonian plants. This rock is 

 generally stony in texture, but sometimes passes into " hornstone- 

 like " varieties ; while locally, as on the banks of the great Marekanka 

 brook, it assumes a more or less pefectly glassy condition. At the 

 locality mentioned, the igneous mass forms a cliff between 200 and 

 300 feet in height, the rock varying in colour from snow-white to 

 reddish-brown, and its different varieties constituting wavy bands, 

 which, as is so commonly the case with the vitreous lavas, simulate 

 the appearances presented by many crumpled, foliated rocks. 5 This 

 banded, glassy rock is seen to be traversed by numerous concentric 

 joints, so that globular masses breaking up like onions accumulate 

 in immense numbers at the foot of the cliff, forming a talus which 

 reaches half-way to its top. The nuclei of these concentric masses, 



1 Mikroskop. Beschaff. der Mineralien uud Gesteine (1873), p. 351. 



2 See Wichmann, Zeitsch. d. d. Geol. Ges. xxxv. p. 849 ; Diller, Amer. Journ. Sc. 

 xxviii. p. 252 ; Rutley, Quart. Journ. Geol. Soc.vol. xli. (1885), p. 152. 



3 Compare Zirkel, Mikroskop. Beschaff. der Mineralien und Gesteine (1873), p. 361. 



4 Erman, Archiv. f. d. Wissenseh. Kunde Busslands, iii. (1843), p. 175 ; Herter, 

 Zeitsch. d. d. Geol. Ges. xv. (1863), p. 459. 



5 See Geol. Mag. Dec. II. Vol. II. pp. 68 and 69, Fig. 9. 



