372 SIR A. GEIKTE ON THE TERTIARY [May 1 896, 



a rule, it is merely their ends on the rounded domes which are to 

 be observed, and which everywhere slip under the waves. The 

 columns in a cliff from 15 to 20 feet high show the slightly wavy, 

 starch-like arrangement so often to be met with among the 

 plateau-basalts. 



The rock presents a tolerably uniform texture throughout, though 

 in some parts it is blacker, more resinous, and less charged with 

 porphyritic enclosures than in the general body of the rock. Large 

 fresh felspars are generally scattered through it. To the naked eye 

 it reproduces every feature of the pitchstone of the Scuir of Eigg. 



A microscopic examination completes our recognition of the 

 identity of these two rocks. Mr. Harker has examined a thin 

 slice prepared from the Hysgeir pitchstone, and remarks regarding it 

 that ' the large felspars are not the only porphyritic element. The 

 microscope shows the presence also of smaller imperfect crystals of 

 augite, very faint green in the slice, and small grains of magnetite. 

 The felspars have been deeply corroded by the enveloping magma, 

 and irregular included patches of the groundmass occupy nearly 

 half the bulk of some of the crystals. This latter feature is seen 

 especially in some of the larger crystals, which seem to be sanidine. 

 They are, for the most part, apparently simple crystals, but in 

 places there is a scarcely defined lamellar twinning, or, again, small 

 patches not extinguishing with the rest; so that we are probably 

 dealing with some perthitic intergrowth on a minute scale. 1 



'Rather smaller felspar-crystals are rounded by corrosion, but 

 lack the inclusions of groundmass ; these have albite- and some- 

 times pericline-lamellation, and may be referred to oligoclase- 

 andesine. The groundmass of the rock is a brown glass with 

 perlitic cracks, enclosing very numerous microlites of felspar about 

 •001 inch in length [6619]. The rock is probably to be regarded 

 as a dacite rather than a rhyolite, and thus agrees with Mr. Barker- 

 North's analysis of the Eigg pitchstone.' 2 There is no trace of 

 any conglomerate in situ like that under the Scuir of Eigg, nor of 

 any other rock, aqueous or igneous. As the pitchstone everywhere 

 slips under the sea, its geological relations are entirely concealed. 



The great variety of materials met with in the form of boulders 

 on Hysgeir is a testimony to the transport of erratics from the 

 neighbouring islands and the mainland during the Glacial Period. 

 The most abundant rock in these boulders is Torridon Sandstone, 

 derived, no doubt, from the hills of Rum ; but there occur also 

 various kinds of schist, gneisses, quartzites, granites, porphyries, 

 probably from the west of Inverness-shire, as well as pieces of white 

 sandstone, probably Jurassic, which may have come from Eigg. 



That the pitchstone of Hysgeir is a continuation of that of the 

 Scuir may be regarded as highly probable. If not a continuation, 

 it must be another stream of the same kind, and doubtless of the 

 same date. If it be regarded as probably a westward prolongation of 



1 Comp. Prof. Judd's remarks on the Scuir of Eigg rock, Quart. Journ. G-eoL 

 Soc. vol. xlvi. (1890) p. 380. 

 a Ibid. p. 379. 



