BASALT-GLASS OF THE WE3TERX ISLES OF SCOTLAND. 447 



not appear to constitute great rock-masses, but is usually found as a 

 local variation of certain types of dolerite and basalt. An excep- 

 tion to this rule exists in the case of the well-known glassy lavas of 

 Hawaii; but this exceptioD we shall discuss in the sequel. 



The most general mode of occurrence of basalt-glass is as a sel- 

 vage (Saalband) to basaltic dykes ; and this is the only condition 

 under which we have found it to occur in the Western Isles of 

 Scotland. Under similar circumstances it has been observed in Ice- 

 land*, Bohemiaf, and many other districts. 



Other modes of the occurrence of basalt-glass which have been 

 recorded are as follows : — Lyell and Murchison found the basaltic 

 lava of Thueyts in the Vivarais, where flowing over gneiss, to be 

 coated on its under surface with a band of glassy material J. Many 

 authors have described the surfaces of lavas and ejected blocks as 

 covered with a glassy crust. Zirkel notices that the lava-stalactites 

 in a cave in a basaltic lava- stream in Iceland have a surface-film of 

 basalt-glass§ ; and a similar film is stated to surround the air-cavities 

 in a basalt of Hellegrund, near Miindenj|. At Bobenhausen in the 

 Vogelsgebirge and elsewhere basalt-glass is recorded as occurring in 

 basalt in nests from the size of a walnut to that of a child's headlf. 

 Sartorius von Waltershausen and others have described kernels of 

 basalt-glass (sideromelane) as being abundant in the basaltic tuffs of 

 Iceland, Sicily, and other districts ; and in these cases the outside 

 portions of the kernels are often found to graduate into palagonite**. 

 Some authors, indeed, regard all palagonite as the result of the hydra- 

 tion and alteration of particles of basalt-glass. 



From a discussion of all the known occurrences of basalt-glass, 

 Mohl ff has argued that it has in every case been formed by the 

 rapid cooling of portions of basaltic lava. 



Besides the two localities at which basalt-glass has been already 

 noticed in the Western Isles of Scotland, we have only succeeded in 

 finding it at three other points, namely Some Point and Gribun in 

 the Isle of Mull and Screpidale in the Isle of Eaasay. The condi- 

 tions under which the rock occurs at these five known localities are 

 as follows : — 



At the Beal, near Portree, in the Isle of Skye, a basaltic dyke 

 has glassy selvages about two inches thick. Murchison, it is true, 

 gives the width as four inches ; but we have never found it exceeding 

 the amount stated. This is by far the most striking occurrence of 

 basalt-glass with which we are acquainted. Its vitreous character 



* Zirkel, Lebrbuch der Petrogi'aphie, toI. ii. p. 303. 



t Boricky, Sitzungsberichte der k. bohiniscken Gesellsohaft der Wissenchaften 

 in Prag, 1873, p. 8 ; also Petrogr. Studien an den Basaltgesteinen Bohmens, 

 p. 182. 



| Edinburgh New Philosophical Journal, April to October, 1829, p. 29. 



§ Lehrbuch der Petrographie, vol. ii. p. 304. 



|| Ibid. 



^1 Neues Jahrb. fur Min. &c. 1841, p. 696. 



** Sartorius v. Waltershausen, ' Ueber die vulkanischen Gesteine in Sicilien 

 und Island,' 1853, p. 202. 



tt Die Gesteine der Sababurg in Hessen (Cassel, 1871). 



