Vol. 59.] IN GLASSY IGNEOUS ROCKS. 443 



the amount of resistance offered to the mineral which takes the lead 

 in crystallizing. 



EXPLANATION OF PLATE XXVI. 



Fig. 1. Obsidian Cliff. The feather-like crystals characteristic of the porous 

 patches or spherulites. Crossed nicols. X 20. 



2. Vicart Cliffs (Jersey). Example of primary devitrification, in which 



the early crystallisation of superfluous silica has apparently been 

 followed by an eutectic. Ordinary light. X 20. 



3. From Cadhat Plain (Sokotra). Shows slight fluxional, with consequent 



modification of spherulitic, structure : the latter in parts of the slice 

 passing into a more or less root-like or speckled structure (^small 

 scattered grains of quartz and felspar). A compact quartz-felsite, 

 which probably was so from the first. Crossed nicols. X 20. 



4. From Tan-y-maes (Caernarvonshire). Spherulitic structure, often 



rather tufted and growing round felspar- crystals (less definitely 

 connected with the quartz). Small spherulites occur in the matrix 

 of the rock, which sometimes exhibits a more or less root-like struc- 

 ture, apparently connected with the other, and sometimes seems to 

 be simply speckled, but occasionally approaches micrographic. A 

 spherulitic quartz-felsite. Crossed nicols. x 20. 



5. North-western end of Girgha range, near Hadibu Plain (Sokotra). The 



matrix exhibits a sort of ' vermicular ' structure, which here and 

 there (as in the north-eastern corner of the drawing) is grouped into 

 a rather irregular spherulite with root-like branches. A compact 

 quartz-felsite. Crossed nicols. X about 70. 



6. Base of the cliff, Corriegills shore (Isle of Arran). This rock (see 



G-eol. Mag. 1877, pp. 506-509) is here and there slightly spherulitic, 

 and one some distance away, probably corresponding with it, is 

 markedly so. It is also slightly fluxional, and exhibits conspicu- 

 ously the root-like structure shown in the figure. The rock is from 

 the base of a rather compact felsite, softened or locally melted by an 

 intrusive pitchstone. Crossed nicols. X about 100. 



Discussion. 



The Chairman (Mr. Teall) said that he was extremely glad to 

 see attention directed to the work of those specialists who were 

 engaged in the study of alloys. Many of their results would 

 probably be found applicable to rocks. At the same time, caution 

 was needful in drawing conclusions from one set of phenomena 

 to another. Messrs. Haycock & Nevill, whose results were not yet 

 fully published, had shown that, after complete solidification, very 

 important changes took place in copper-and-tin alloys, so that the 

 structures and the compounds produced at earlier stages of con- 

 solidation disappeared, to be replaced by later products. It was 

 not improbable that similar changes would be found to have taken 

 place in many rocks, even in such as differed totally in composition 

 from felsites or pitchstones. 



Prof. G. A. J. Cole congratulated the Authors on continuing to 

 add new observations to those brought forward on devitrification 

 during the last thirty years. The question of eutectic association 

 of various minerals might possibly be pushed too far, since the con- 

 ditions prevalent in the earth's crust might allow one pair of minerals 



