188 MR. G. A. J. COLE ON THE ALTEEATTON OF 



tint, averaging -01 of a millimetre in diameter, together with a 

 number of minute greenish prisms, which give vivid colours under 

 crossed nicols. The latter are probably pyroxenic. 



The results of the foregoing observations may be briefly summarized. 

 Leaving out of count the matter added to an altered spherulitic rock 

 by infiltration, such as the quartz which has, in lavas above the 

 Conwy Palls, produced veins some six or seven inches wide, there 

 is an evident tendency in such masses for the constituents to 

 rearrange themselves, probably under the influence of heated waters 

 as in the classical experiments of Daubree *. The silica thus 

 accumulates locally, and crystallizes out in nodular forms determined 

 by the original spherulites, while materials of much more basic 

 character than might have arisen from primary devitrification are 

 developed in layers and patches, and prepare the rock for future 

 physical change. It is rash to speculate on the final condition of 

 such a rock, were it to come under the influence of mountain- 

 building processes. Grreat " eyes " of quartz, however, resolvable 

 under polarized light into granular aggregates, agaiust which wisps 

 of soft micaceous matter are pressed and bent, are features not 

 unfamiliar in highly siliceous schistose rocks ; and it seems probable 

 that this differentiation and grouping of constituents would be closely 

 imitated by further metamorphism of the lavas of Conway and 

 Digoed. 



The consideration of the changes undergone by our own coarsely 

 spherulitic rocks leads one inevitably to the conclusion that the bulk, 

 at least, of the " pyromerides " t and nodular porphyries of the 

 continent were also originally glassy rocks. Prof. Bonney has 

 practically expressed this view in dealing with the petrosiliceous 

 structure in his Presidential Address for 1885 $. While it is of 

 doubtful advantage to theorize respecting rocks with which one has 

 no field-acquaintance, a few points of evidence may perhaps be 

 mentioned here. Vogelsang § states that in a dyke of " Kugel- 

 porphyr " at Ozani, Corsica, the spherulites are accumulated at the 

 sides, an observation that may be paralleled on a minute scale in 

 the glassy selvages even of basaltic rocks, as, for instance, in a vein 

 near Tolly more, County Down. Delesse |i, again, insists strongly on 

 the connexion between an excessive proportion of silica and the 

 development o£ " globular " or coarse spherulitic structure ; and it 

 need scarcely be pointed out that this same predominance of silica 

 characterizes the most glassy of modern volcanic rocks. The 

 following analysis by Delesse ^ of a spheroid from the pyromeride 

 of Wuenheim, in the Yosges, may be compared with that of the rock 



* Etudes synthetiques de geologic experimentale, p. 159, &c. 



t The name Pyromeride (" only in part fusible") is due to Haiiy, and refers 

 to the different behaviour of the constituents, quartz and " felspar." (Monteiro, 

 Journal des Mines, tome xxxv. (1814), p. 359.) 



J Proc. Geol. Soc. 1885, p. 95. 



§ Js'eues Jahrb. fiir Min. &c. 1863, p. 102 (Niederrhein. Gesellsch. tur 

 Natur- und Heilkunde, Aug. 6, 1862). 



II Memoires de la Soc. geol. de France, 2me serie, tome iv. pp. 325, &c. 



^ Bull, de la Soc. geol. de France, 2me serie, tome ix. p. 176. 



