306 MR. G. A. J. COLE ON SOME ADDITIONAL 



fragments at Snead are soft, and devoid of vitreous lustre ; but 

 sections go far to prove that they were formerly more distinctly 

 glassy than the tachylytes of Mull or Skye. They are, indeed, from 

 their associations, referable rather to andesite-glass, like the matrix 

 of the well-known rock of Eskdale, than to the rarer and more basic 

 group of tachylytes. 



Their colour under the microscope varies from a warm brown to 

 yellow, and the relation of this material to the included porphyritic 

 crystals affords very satisfactory evidence. The plagioclase felspars, 

 of the broad type common in andesitic lavas, are corroded and 

 perforated by the yellow matrix ; this matrix, moreover, though it 

 now polarizes in streaky irregular areas, has given rise only to 

 minute brown aggregations resembling the magnetite- and pyroxene- 

 microlites formed during the consolidation of basic glass. Some 

 particles of the yellow substance are pumiceous ; and similar material 

 occupies the interstices between the other ejected fragments in the 

 ash (PI. XI. fig. 5). 



A comparison with the tuff of Aci Eeale, Sicily, in which " sidero- 

 melane " and palagonite fragments abound, greatly aids one in 

 assigning a vitreous origin to the soft dark particles of Snead. If, 

 further, we examine the andesites associated with this Ordovician 

 tuff, we find an anisotropic substance, clear yellow in thin section, 

 taking the place that in modern examples is occupied by the glassy 

 matrix. Thus in the Corndon area, and, to take another example, 

 in the andesites of the Carneddau Hills near Builth, the cavities of 

 the corroded felspars, and the interstices of the crystalline mesh- 

 work of the ground-mass, are filled with this yellow alteration- 

 product. The interesting augite-andesites of Iceland and the Paroe 

 Islands, with their areas of yellow-brown residual glass, form an 

 admirable series for comparison. 



It may be claimed, then, that the tachylytes thus brought 

 together add in some measure to our knowledge of the spherulitic 

 forms ; while we have evidence of the persistence of similar types 

 from early Palaeozoic up to Tertiary days — evidence, indeed, of the 

 detailed similarity of causes operating at various periods, however 

 far apart in time. I would, in conclusion, call attention to a rock as 

 yet, I believe, unrecognized in Britain, the Yariolite of continental 

 authors. Little need be added to the full and excellent discussion of 

 its characters given by Prof. Eosenbusch*. The well-known 

 pebbles of the Durance, which are to be found in most old collec- 

 tions, are referred by M. Lory f to the selvages of the euphotides of 

 Mont Genevre, the age of their intrusion being later than the 

 Infra-lias. Delesse X ^^^ analyzed the included spherules, and 

 regarded them as a form of triclinic felspar ; while M. Levy § 

 has determined that the fibres of which they are composed are 

 elongated crystals of oligoclase. He points out, moreover, that the 



* Mikro. Pbysiog. der massigen Gesteiae, 2te Auflage, p. 227, &c. 



t Descript. geol. du Dauphine, p. 577. 



J Coniptes EenduB, tome xxx. (1850), p. 741. 



§ Bull, de la Soc. geol. de France, 1876-77, p. 238. 



