Augen Gneiss and Moine Sediments of Ross-shire. 339 
Separated from the N.N.E. end of it by a space of 14 miles, most 
of which is occupied by Old Red Sandstone of later age than the 
granite, is the much more extensive plutonic mass of Carn Chuinneag, 
12 miles long and 4 or 5 miles broad, which includes a considerable 
variety of orthogneisses. On the south-west side of the Inchbae mass 
a group of smaller detached outcrops of augen gneiss extend several 
miles in a south-west direction as far as Loch Luichart, and are 
represented also on the west side of a big fault, striking N.N.E. on 
the west side of this loch. 
That the augen gneiss was once a porphyritie granite is proved in 
several ways. In some places the large phenocrysts of orthoclase 
preserve their original idiomorphic outlines and are surrounded by 
a granitic matrix, in which the original grains of quartz have not 
suffered granulitization. The surrounding hornfelses are invaded by 
tongues and veins of granite which have undergone very little 
deformation. At the junctions in some places, as at Kildermorie 
Lodge on the south-east side of Carn Chuinneag, the margin of the 
granite is filled with immense numbers of angular inclusions of baked 
sediment (garnetiferous biotite hornfels, ete.). Moreover, as already 
stated, there is an aureole in which certain sediments, elsewhere 
represented by the Moine schists, have the composition and structure 
of hornfelses derived from impure sandstones and  arenaceous 
shales. 
Within the intrusions various types of plutonic rock may be found. 
Towards the north-west margin of the Carn Chuinneag mass there are 
some outcrops of dark-green basic rocks, which have been proved to 
be gabbros (without olivine) and augite diorites. Others are quartz 
diorites, and there are also more acid types which correspond to 
tonalites and hornblende granites. 
In some places the basic rocks, like the porphyritic granite, are 
nearly free trom foliation, and show their original structures fairly 
well preserved; elsewhere all these rocks have been converted by 
movement after consolidation into amphibolites, hornblende schists 
and hornblende gneisses. The basic rocks occur in relatively small 
amount, and are entirely in the form of inclusions, large or small, 
surrounded and veined by the granite. The field evidence proves that 
they are earlier than the granite, and it is probable that they 
originated by the differentiation of an acid magma in much the same 
way as the basic rocks of Garabal Hill and of so many other intrusions 
of the Newer Granite Series in the Scottish Highlands and the 
Southern Uplands. 
Towards the centre of the Carn Chuinneag mass there are several 
areas of segirine riebeckite gneiss, a rock extremely rich in alkali 
felspars, with quartz and soda-pyroxenes and amphiboles. We may 
mention also the remarkable occurrence of masses of magnetite and 
cassiterite (containing sometimes nearly 16 per cent. of tinstone) on 
the north-west shoulder of Carn Chuinneag; these are accompanied by 
a peculiar dark biotite gneiss with large rounded garnets and much 
albite. ‘This mineral paragenesis is almost unique, and the origin of 
this ore-deposit is a problem very difficult of solution. The finer- 
grained granite gneiss of Carn Chuinneag, in which the egirine 
