THE HIGHLAND BORDER ROCKS OF THE ABERPOYLE DISTRICT. 
195 
which are characteristic of albite diabases elsewhere,* namely, augite, albite, and 
ilmenite (see Plate III, fig. -3). Occasionally biotite occurs in considerable amount, 
and the rocks are rich also in accessory apatite. Olivine appears to be absent. 
The pyroxene has the purple tint usual in titanaugites ; it is sometimes perfectly 
fresh, sometimes partially or completely altered to chlorite and carbonates — in one 
instance only, in the fine-grained margin of the largest intrusion, it is entirely 
replaced by fibrous green hornblende ; in the coarser parts of the rock it is anhedral, 
in the finer parts it tends to show crystal outlines. The albite which occurs in 
narrow laths penetrating the augite in subophitic fashion is more often in broad 
rectangular crystals, and is the most abundant constituent of the rock. The crystals 
have the “ spongy” centre, with the associated chlorite, epidote, and prehnite usually 
found where lime-rich plagioclase has undergone albitisation ; the marginal portions 
are compact, and grade sometimes into interstitial water-clear albite. In the weathered 
rocks, which are rich in chlorite and carbonates, the albite is usually unaltered. 
The ilmenite is in large skeletal growths, occasionally in graphic intergrowth with 
augite, and always more or less altered to leucoxene and sphene. Biotite, which is 
for the most part chloritised, occurs in association with the ilmenite in the same 
fashion as in the Carboniferous teschenites,f and also as isolated crystals enclosed in 
the felspars ; it contains sometimes minute inclusions with pleochroic halos. 
Highly felspathic acid veins and segregations, the pink colour of which is in 
striking contrast with the dark bluish-grey of the mass of the rock, are conspicuous 
features of the largest intrusion. Included fragments of black cherty shale have 
been converted into spotted adinoles. 
As regards the age of these intrusions, they are of course younger than the black 
shale and chert into which they are intruded ; how much younger the field evidence 
does not enable us to say. But their petrographical characters show clearly their 
affinities with the spilites, an association which, as Flett and Dewey J have pointed 
out, is invariable both in Europe and America. 
Petrography of the Serpentine- Gabbro Complex . — The igneous rocks of this 
complex present many puzzling features. They are for the most part in a highly 
altered condition, and their study is further complicated by a great development of 
vein rocks associated with the faults which bound the complex on either side. It 
is clear, however, that the original intrusion included both serpentine and gabbro. 
At Maol Ruadh and at the Upper Dounans Limestone Quarry occur good 
examples of dunite serpentine rich in chrome spinels (Plate III, fig. 4). In the latter 
the olivine is replaced partly by serpentine, partly by tremolite. Sections of the 
Maol Ruadh rock are of especial interest, because along narrow lines of shearing 
the serpentinised olivine is traversed by anthophyllite in lamellar crystals and 
tufted fibrous aggregates. The mesh structure in both of these rocks is well seen, 
* H. Dewey and J. S. Flett, Geol. Mag., dec. v, vol. viii, p. 206, 1911. 
f Trans. Edin. Geol. Soc., vol. ix, p. 126, 1910. J Loc. cit., p. 206. 
