OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 379 



indicate a sporadic outburst during the deposition of the flaggy series of 

 Bressay and Noss. 



The discharge of these highly basic lavas and tuffs and the deposition of 

 the associated sediment, were followed by the injection of three great sheets of 

 highly acidic rocks. The relations of the sheet in Papa Stour to the basic rocks 

 plainly show that the eruption of that mass was later than the ejection of the lavas. 

 Similar evidence is supplied by the granite mass in Sandsting, and there is every 

 probability that the great Rooeness plateau was erupted at a later date than 

 the basic lavas of Northmavine. Numerous veins of granite, of quartz-felsite, 

 and of rhyolite radiate from these intrusive sheets, which doubtless belong to the 

 same period of intrusion. The last phase was characterised by the eruption of 

 a series of highly basic rocks, consisting of diabase, which traverse alike the 

 ancient crystalline rocks, the Old Red strata and the great intrusive sheets. 



II. Microscopic Characters. 



The microscopic examination of the bedded lavas proves that there is a con- 

 siderable difference between the so-called porphyrites and the diabase rocks, 

 which both occur in the series. A typical example of the former group, taken 

 from the neighbourhood of Ockren Head in Rooeness Voe, shows that it is 

 composed mainly of very minute columnar ciystals of plagioclase felspar, which, 

 as a rule, are much altered, and only occasionally show traces of the twin 

 striation. Between these closely aggregated crystals there is a fine ground 

 mass, and some of the minute interspaces are also occupied with a bright green 

 decomposition product, which may be green earth. After the felspar, however, 

 magnetite is the most abundant mineral, — so much so, indeed, that it might be 

 grouped with the class of felspar-magnetite rocks described by Dr Archibald 

 Geikie in his paper on the " Carboniferous Volcanic Rocks of the Basin of the 

 Forth."* 



The sections prepared from the lavas in the altered Old Red area, between 

 Clouster and Aith Voes, show that plagioclase felspar is the chief constituent. 

 Augite, however, is also present, though it is only occasionally recognisable, by 

 far the larger portion having been converted into chlorite. In one instance 

 where this alteration has been considerably developed the augite has a granular 

 appearance ; but in other sections a few larger crystals of augite remain which 

 are quite distinguishable. The magnetite, which is also largely distributed, has 

 been converted to a great extent into limonite. This type is a true diabase, and 

 were it not for the great alteration which has taken place in the pyroxenic 

 mineral, it might be compared with some basaltic lavas of later palseozoic age. 



* Trans. Roy. Soe. Edin., vol. xxix. p. 508. 



