from near Bertrich, in the Eifel. 243 



lava which can be thus so easily traced down to Bertrich is part of 

 a great stream which in all probability commenced at some point in 

 the Miillischwiese, and followed the course of the Uessbach, filling 

 its channel to a considerable height. On its way it forms the well- 

 known " Cheese Grotto." about half-a-mile above Bertrich. It is 

 difficult to determine the exact spot in the Miillischwiese at which 

 the flow commenced, for the valley is now all under cultivation and 

 much overgrown in places, and no traces of the flow can be seen 

 except at the quarry to which I have referred, which is at the 

 junction of the two valleys. Excavations have now been commenced 

 immediately above the quarry by the side of the high-road ; and the 

 lava which is there being exposed is exactly similar to that seen 

 below in the large quarry, only it is not so hard and compact, and 

 presents a somewhat slaggy appearance. 



It was in the Miihlrech quarry that I found many specimens of the 

 dark-green glass to which I have alluded. It occurs in irregularly- 

 shaped patches, and apparently forms a thin lining to cavities in the 

 lava. The latter is a hard, dark-coloured rock of basic composition, 

 and consists mainly of crystals of augite and olivine imbedded in 

 a dark-brown, glassy ground-mass, which is rich in magnetite. I 

 have found no traces of felspar, leucite, or nepheline. Zirkel ' 

 thinks that the last may occasionally be present, although its out- 

 lines are indistinct, and also sanidine. 



As far as I am aware, but little attention has been drawn to this 

 curious feature of the Bertrich lava. Roth 2 attributes the origin of 

 the green vesicular glass found in the scoriae and lavas of Bertrich, 

 the Falkenlei, and Hiistchen, another of the thi - ee craters to which 

 I have already referred, to the melting down of the augite present in 

 the lavas. " An alteration," he says, " of the olivine is nowhere to 

 be observed, a result of the difficulty with which it fuses." Von 

 Dechen 3 thinks that Roth's explanation is unsatisfactory; for were it 

 correct, the glass would undoubtedly be found in lavas and scorire 

 from other parts of the Eifel, since augite occurs so abundantly in 

 them all. " Probably," he adds, " the melted augites would not lie 

 next to the unaltered ones, and, moreover, would not have preserved 

 the granular form of the latter while enclosed in the thick lava, 

 whereas in reality longitudinal sections of lenticular bodies are to be 

 found there." i 



At first sight the colour and lustre of the glass led me to suppose 

 that it was melted olivine ; and no satisfactory proof having 



1 "Basaltgesteine," p. 180. Bonn (1870). 



2 E. Mitscherlich, " Ueber die vulkauischen Erscheinungen in der Eifel und iiber 

 die Metamorphie der Gesteine durch erhobte Teruperatur " : Im Aui'trage der 

 Konigl. Akademie der Wissenscbaften zu Berlin berausgegeben von J. Roth, I860, 

 p. 29. 



3 " Geognostiscber Fiihrer zu der Yulkanreihe der Vorder-Eifel," p. 26. Bonn 

 (1886). 



4 Possibly be refers to tbe streak-like form sometimes taken by the glass ; but bis 

 meaning is not clear to me. His words are : " Walnscbeinlicli wiirdeu die gesch- 

 molzenen Augite uicbt ueben den uuveranderten liegen und deren Kornerform in der 

 dichten Lava eingeschlossen bewabrt baben, sicb liuigliche Durehschnitte liusen- 

 formiger Korper darstellen." 



