420 PROFESSOR GEIKIE ON THE 
beyond doubt. Owing to the protrusion of a remarkable pink porphyritic mass, 
the bedded rocks can only be seen at intervals along the shore. On the north 
side of Hamna Voe, but still more strikingly on the singularly indented cliffs — 
towards the north-western headland, the diabase-porphyrite appears, with an 
overlying mass of red and grey flaggy 
sandstones, the whole being cut | 
through by the pink porphyry, and 
traversed by many small faults. Again, 
at the north end of the island, on the 
promontory to the east of the Bardie, 
a dark, dirty green slaggy and amyg- 
daloidal diabase-porphyrite forms the 
base of the cliff, and passes under a 
_ series of sandstones, tuffs, and fine 
‘Dcie lavas oveedd by sondetones, (Fouls in the distence) felspathic conglomerates. The detritus 
of which some of these strata have 
been formed, consists mainly of a pink felsitic rock, like that which covers so 
much of the island.* 
Above the stratified beds and associated lava-streams lies an enormous mass 
of a very different character. This apparently intrusive rock consists of a 
compact pink or pale salmon-coloured “ porphyry,” or, in the nomenclature of 
Renarv and DELA VALLEE Poussin, “ porphyroid,” generally decomposing with 
a dull, meagre surface, and then assuming the character of what was termed in 
the Wernerian nomenclature, a claystone. Here and there, as on the slope to 
the east of the Bardie, it assumes a curious concretionary structure. In several 
sections which I have examined under the microscope, the rock presents a 
remarkable absence of any crystalline structure, but on the contrary, shows 
many wavy lines and streaks, formed by layers of finer ferruginous felsitic 
matter,—an arrangement strongly resembling that possessed by many true 
tuffs. In some of the sections a spherulitic character appears, the concretions: 
or spherules having an internal fibrous divergent structure. Many cavities 
occur more or less completely filled with secondary quartz. Occasional crystals 
of orthoclase and octohedra of magnetite may be detected; but as a rule the 
rock has been very much altered. It resembles some of the materials which, 
in the Pentland Hills and elsewhere, fill up voleanic orifices of the age of the 
Lower Old Red Sandstone ; and in connection with these rocks will be referred 
to in a succeeding portion of this memoir. If it. has had a similar origin we 
may be able to reconcile its tuff-like petrographical aspect with the fact that 
U 
























* Jamuson (“Mineralogy of Scottish Isles,” ii. 207) speaks of the “ wacken” (felstone) lying upon 
a kind of breccia at the north end of the island, and traversed by veins of greenstone and basalt. 

