446 PKOF. J. W. JUDD AND O. A. J. COLE ON THE 



In 1853 Sartorius von "Waltershausen * described similar sub- 

 stances from Iceland under the name of " sideromelane," noticing 

 that they formed the anhydrous kernels of a material identical with 

 his " palagonite." 



In 1868 Petersen f described a glassy variety of the nepheline- 

 basalt of the Rossberg, to which he gave the name of " hydro- 

 tachylyte." 



Zirkel, Vogelsang, Mohl, Borieky, Sandberger, and others describe 

 different varieties of basalt-glass under the name of " tachylyte " 

 (often misspelt " tachylite"); and by this same designation the 

 Scottish examples were referred to by one of us in 1875 in the 

 paper already cited. Mr. Butley, in 1877, applied the same name to 

 a similar rock from County Down, Ireland. 



But in 1872 Prof. EosenbuschJ argued in favour of restricting 

 Breithaupt's name of tachylyte to those varieties which are easily 

 decomposed by hydrochloric acid, and of giving Hausmann's name 

 of hyalomelane to such as are only partially or not at all decom- 

 posed by the same agent ; and this suggestion was adopted by 

 Cohen§, Mohl||, and other writers. 



In 1877, however, Prof. Rosenbusch^f gave up the distinction 

 altogether, having apparently become impressed by the difficulty and 

 uncertainty of the test of solubility in acid as applied to rocks. He 

 was, moreover, influenced in this by the fact that, following the 

 system already adopted by Mohl and Zirkel, he now classified these 

 glassy products not as mineral species, but as rocks — the vitreous 

 form of basalt. * 



Penck**, in 1879, argued with much force in favour of adopting 

 the same course ; and the distinction between tachylyte and hyalo- 

 melane, as well as their inclusion among mineral species, may now 

 be regarded as being by universal consent abandoned. Yarious 

 names have been proposed by different authors for substances of the 

 class, such as " glassy basalt," "vitreous basalt," "basalt-vitrophyre," 

 " basalt-glass," and " basalt-obsidian." 



In his most recently published classification of rocksft, in 1882, 

 Prof. Rosenbusch recognizes only two divisions among the anhydrous 

 basic glasses — basalt-glass (including tachylyte, hyalomelane, side- 

 romelane, and slaggy augite), and hydrotachylyte or nepheline- 

 b as alt-glass. 



2. Distribution and Mode of Occurrence. 

 Basalt-glass, though a widely distributed rock, is one which can- 

 not be considered as by any means of common occurrence. It does 



* Uber die rulkanischen Gesteine in Sicilien und Island, p. 202. 

 t Neues Jahrbuch fur Mm, &c. 1869, p. 32. 

 % Neues Jahrbuch fur Min. &c, 1872, p. 148. 

 § Ibid. 1876, p. 746. 



|| Zusanmienstellung und Beschreibung einer Sammlung typischer Basalte. 

 ^[ Mikroskopische Physiographie, vol. ii. p. 445. 



** Zeitschrit't der deutsehen geologischen Gresellschaft, vol. xxxi. pp. 521, 

 568, &c. 



tt Neues Jahrbuch fur Min. &c. 1882, vol. ii. p. 16.,, 



