506 PROFESSOR GEIKIE ON THE 
indeed be discriminated except by actual fracture and inspection, the whole 
mass appearing as one and indivisible. The upper part is too hard to be 
worked with profit, and is therefore thrown aside, but the workmen cannot fix 
any line below which the stone becomes valuable, except by the ease with 
which it yields to their tools. 
A few hand-specimens, selected from various parts of the upper harder 
band, give the following characters :—Finely crystalline base of a dirty, blackish 
green colour, and a tolerably homogeneous but dull texture, showing many ill- 
defined greenish white points, apparently of decayed felspar, with minute 
facettes, some of which are pyrite ; fracture, splintery ; hardness, 3 to 4. 
The variety from Inchcolm was brought to my notice by one of my students 
in the University, Mr Ernest Apy, who landing on the island, and being struck 
with the external aspect of the rock, took specimens, and sliced them for the 
microscope. It occurs in beds, under which lie some hardened sandstone, 
limestone, and shale. It seems to be an intrusive mass, as are those of the 
adjacent islands and the neighbouring coast of Fife. It is rather coarsely 
crystalline-granular. Even to the naked eye its honey-yellow grains of olivine, 
dark glancing crystals of augite, occasional plates of glistening brown biotite, 
and serpentinous interstitial matter, are quite apparent. But under the micro- 
scope it becomes an object of surpassing beauty. 
2. Microscopic Characters.—The lower portion of the Blackburn rock con- 
sists chiefly of serpentine. This mineral occurs in (1) irregular patches, the 
edges of which are sometimes sharply defined against the other ingredients, — 
while elsewhere they seem entangled among the latter ; (2) in more definite — 
forms, which are almost certainly those of former olivine ; (3) in tufts and 
fibrous streaks ; and (4) in veins of true chrysotile. Between the abundant — 
portions of serpentine traces of a pale mineral full of serpentinous veins, which 
behaves like olivine in polarised light, are probably the last recognisable 
remnants of that mineral, which at first appears to have constituted the main 
part of the rock. Large, much-flawed crystals of a pale brown to claret — 
coloured mineral are scattered through the serpentine, and are enclosed in the — 
less altered olivine. These answer to augite in general behaviour with polar- 
ised light, but I have been unable to obtain any satisfactory cleavage angles. 
Numerous needles and fine prisms of apatite are crowded into some parts of 
the rock, Abundant iron black particles of titaniferous iron or magnetite 
occur, and a little pyrites. A few prisms of triclinic felspar also occur. There — 
is no distinct ground-mass separate from the general base of serpentine. 
The upper portion of the Blackburn rock presents a marked contrast in its 
minute structure. It contains among its constituents a feeble quantity of a dis- 
tinct glass which occurs occasionally in large interspaces, and then shows a dusty _ 
character, resolvable with a high power into exceedingly minute dark globules. 
