ANNIVERSAKY ADDRESS OP THE rRESIDENT. 121 



they are bluish grey to dark iron-grey, or even black. They break 

 with a splintery or almost conchoidal fracture, and show on a fresh 

 surface an exceedingly fine-grained, tolerably uniform texture, with 

 minute scattered felspars. 



One of their most striking features is the frequency and remark- 

 able development of their flow-structure, ^ot merely as a micro- 

 scopic character, but on such a scale as to bo visible at a little 

 distance on the face of a cliff or crag, this structure may be fol- 

 lowed for some way along the crops of particular flows. The 

 darker and lighter bands of devitrification, with their lenticular 

 forms, rude parallelism, and twisted curvature, have been compared 

 to the structure of mica-schist and gneiss. One aspect of this 

 structure, however, appears to have escaped observation, or, at least, 

 has attracted less notice than it seems to me to deserve. In many 

 cases it is not difficult to detect, from the manner in which the len- 

 ticles and strips of the flow-structure have been curled over and pushed 

 onward, what was the direction in which the lava was moving while 

 still a viscous mass. By making a sufficient number of observations 

 of this direction, it might in some places be possible to ascertain 

 the quarter from which the several flows proceeded. As an illus- 

 tration, I would refer to one of the basement-felsites of Snowdon, 

 which forms a line of picturesque crags on the slope facing Llan- 

 beris. The layers of variously devitrified niatter curl and fold over 

 each other, and have been rolled into balls, or have been broken up 

 and enclosed one within the other. The general push indicated by 

 them points to a movement from the westward. Turning round 

 from the crags, and looking towards the west, we see before us on 

 the other side of the deep vale of Llyn Cwellyn, at a distance of 

 little more than three miles, the great dome-shaped 3Iynydd-mawr, 

 which, there is every reason to believe, marks one of the orifices of 

 eruption. It might in this way be practicable to obtain information 

 regarding even some of the vents that still lie deeply buried under 

 volcanic or sedimentary rocks. . 



That these felsites were poured forth in a glassy condition 

 may be inferred from the occurrence of those minute perlitic and 

 spherulitic forms so characteristic of the devitrification of once 

 vitreous rocks. Mr. Rutley was the first who called attention to 

 this interesting proof of the close resemblance between Palaeozoic 

 felsites and modern obsidians, and other observers have since con- 

 firmed and extended his observations*. 



* Quart. Journ. Geol. Soc. toI. xxxv. (1879) p. 508. 



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