OLD RED SANDSTONE VOLCANIC EOCKS OF SHETLAND. 385 



tains hardly any crystals of quartz and felspar. The sections from Laxa Burn, 

 near Sand Church, in Sandsting, display similar characters. 



From the foregoing descriptions it cannot be doubted that this well-marked 

 fluxion structure is due to movement of the molten magma which originated 

 in the fissures radiating from the intrusive sheets. 



The evidence seems to point to the conclusion that the materials originally 

 existed in a glassy form, and that the rock when it consolidated was a rhyolite. 

 Since that period a process of devitrification has been in operation, which has 

 converted the glassy matrix into the micro-felsitic matter. The fact that only 

 portions of the denser bands remain persistently dark under crossed nicols, 

 while the coarser bands transmit the light, would seem to indicate that the 

 devitrification has advanced so far as nearly to destroy all traces of the original 

 glass. Nay more, in one instance (65 e), from near Aith Ness, in Aithsting, 

 the fluxion structure is hardly traceable, save with polarised light. This section 

 shows a crystalline felsitic ground mass, with occasional crystals of orthoclase 

 and quartz ; the latter being minutely distributed in the ground mass, though 

 much of it appears to be of secondary origin, filling cracks in the section. 

 The whole mass has been much kaolinised, and is darkened by the presence of 

 minute specks of ferrite. Indications of fluxion structure are observable in 

 the neighbourhood of the isolated felspar crystals, but the rest of the ground 

 mass has lost all trace of this structure. It seems reasonable to infer, there- 

 fore, that this example displays an extreme type of devitrification, and that 

 possibly many of the quartz-felsites belonging to the Old Red Sandstone forma- 

 tion of Shetland may represent what originally were rhyolitic rocks, though all 

 traces of the original structure have disappeared. 



We have been favoured with the following note from Professor Eenard, 

 who has kindly examined the typical sections showing fluxion structure from 

 Shetland: — " L'examen que j'en ai fait a porte" sur deux points: 1. Je crois 

 qu'il n'y a pas lieu de clouter que les echantillons, specialement les sections 

 typifiques 2, 3, 5, 59, montrent avec beaucoup d'evidence une structure fluidale. 

 2. Je ne vois pas d'inconvenient a'nommer cette roche rhyolite ancienne, et je 

 crois en efiet comme vous, que c'est une rhyolite dont la base s'est devitrifi^e, 

 et que ne contient guere d'el^ment vitreux apres les modifications auxquelles 

 elle fut soumise. Si on admet cette maniere cle voir, d'apres la classification 

 que nous suivons sur le continent on la grouperait peut-e"tre parmi les felsit 

 pechsteine, mais c'est la un terme de nomenclature sur laquelle il y a beaucoup 

 a distinguer. Vous savez, mieux que je ne puis vous le dire, que les felsit 

 pechsteine repr^sente les rliyolites anciennes et apres tout l'un nom vaut 

 l'autre." 



On referring to the table of chemical analyses, it will be seen that the 

 members of the acidic series — both the intrusive sheets and the branching 



