388 THE OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 



EXPLANAT[ON OF PLATES XLV. AND XLVI. 



Fig. 1. Porphyrite, Rooeness Voe, Northmavine, showing minute columnar crystals of plagioclase fel- 

 spar, abundant magnetite, with a green decomposition product. A fine ground mass is 

 interposed between the felspar crystals. This is a typical example of the porphyrite lavas of 

 Old Red Sandstone age in Shetland (30 diameters). 



Fig. 2. Diabase, Clouster Voe, Aithsting. A contemporaneous volcanic rock seen with polarised ligbt. 

 The crystals of plagioclase felspar are well preserved and abundant, the augite bas been con- 

 verted to a large extent into chlorite. Magnetite is very abundant throughout the section 

 (70 diameters). 



Fig. 3. Diabase, Skeld Hill, Sandsting. An intrusive rock seen with polarised light. This section 

 shows the marked distinction between the interbedded and intrusive diabase rocks. The 

 plagioclase felspar crystals are larger than in the preceding section, but much decomposed. 

 The augite has been largely replaced by a green fibrous mineral, but fresh crystals are recog- 

 nisable in the section. Magnetite is also present (20 diameters). 



Fig. 4. Rhyolite, Papa Little. An intrusive rock showing dense bands of micro-felsitic matter, with a 

 prism of orthoclase lying obliquely across the direction of the flow. This section exhibits 

 very perfect fluxion structure, the movement being from right to left (20 diameters). 



Fig. 5. Rhyolite, Papa Little. An intrusive rock seen between crossed Nicol prisms, showing wavy 

 bands of micro-felsitic matter, which partly remain dark and partly transmit a faint light. 

 Lines of quartz and felspar are arranged more or less parallel with the wavy bands of vitreous 

 matter. The quartz is elongated in the direction of the flow (20 diameters). 



Fi<* . 6. Spherulitic felsite, Papa Stour. An intrusive rock seen with polarised light, composed of reddish- 

 brown felsitic matter, arranged in well-marked spherules, in which the fine radial fibres are 

 prominently developed. In some parts of the section the diverging fibres have been sup- 

 planted by a fine grained ground mass. Nests or cavities filled with a secondary quartz are 

 visible. The ground mass is stained with minute ferruginous particles (70 diameters). 



Fi". 7. Felsite, Dales Voe, Sandness. An intrusive rock seen with polarised light, showing a fine 

 felsitic ground mass, with large irregular grains of quartz containing fluid inclusions. Strings 

 of quartz also traverse the section. A well-marked prism of orthoclase forms a prominent 

 feature near the centre of the drawing, and minute grains of magnetite are evenly distributed 

 through the mass (30 diameters). 



Fig. 8. Binary granite or aplite, Heads of Grocken, Northmavine, seen with polarised light, consisting 

 of-reddish-brown orthoclase, which is much kaolinised, and quartz with fluid inclusions (20 

 diameters). 



