As long ago as 1912 (pp. 128 -9) the late Sir John
Flett made reference to the occurrence in the Moine series of the
Northern Highlands of Scotland of a peculiar suite of basic rocks
in which high proportions of ferromagnesian minerals, including
hornblende, biotite, pyroxene and olivine, are associated with
alkali feldspars (orthoclase and sodic oligoclase) and quartz.
In concluding his description of a particular member of the
suite (connected with the scyelite of Carn Cas nan Gabhar), he
directed attention to the abnormal character of the rock in the
following words: "The rock is far from being a normal diorite;
neither is it a syenite. The abundance of ferromagnesian
minerals in a mesostasis rich in silica and alkalies is a feature
which recalls the lamprophyres ".
Read encountered a large number of these curious basic
rock masses during his regional geological investigation of the
Northern Highlands (1925, pp. 45 51; 1926, pp. 154-166; 1931,
pp. 165 -172) and he gave them the name Hybrid rocks of Ach'uaine
Type after the district of Ach'uaine, 2.5 miles north-north-east
of Bonar Bridge, where they are typically developed. According
to Read these rocks constitute a group of minor intrusions of
widely varied composition ranging from ultrabasic to acid; he
attributed the various members of the series to the mixing of a
granitic magma either with an ultrabasic magma or with solid
ultrabasic rock". (1925, p. 45; 1926, p. 155). He dated the
"hybrid" rocks (i.e. the time of hybridisation of the earlier
ultrabasic intrusions by the later granitic intrusions) as later
than the regional metamorphism of the Moines, but earlier than
the deposition of the local Old Red Sandstone. The granite
intrusions of the region are of two main types: (a) Migdale
granite; (b) Lairg -Rogart granodiorite (1926, pp. 9 and 148).
Later, after studying both the Loch Choire and Strath Halladale
Complexes and the rocks of Ach'uaine Type associated with them,
Read entertained the view that The acid end of the series is
possibly derived from the granitic injections of Loch Choire or
Strath Halladale, so that these hybrids may be a special facies
of injection-rocks ". (Read, 1931, p. 165).
MacGregor and Kennedy (1932, pp. 105-119) drew attention
to the close resemblance between the Lairg-Rogart granodiorite
and its associated rocks of Ach'uaine Type and the Morvern-Strontian 'Granites' and its associated Appinitic suite. They
established the time -sequence as follows (p. 119):
(1) Regional injection of acid pegmatitic magma.
(2) Intrusion of ultrabasic to acid igneous rocks
(3) Intrusion of Morvern-Strontian 'Granite' of Caledonian,
probably Lower Old Red Sandstone, age.
In his summary of the regional geology of the Northern
Highlands, J. Phemister (1936, pp. 60-1), following MacGregor and
Kennedy, grouped the rocks of Ach'uaine Type with the rocks of
the Appinitic suite and dated them as members of the Newer
Igneous Rocks of Caledonian age. Phemister also stressed the
important facts that the rocks of Ach'uaine and Appinitic types
are definitely associated, in their distribution, with the
'Newer Granite' stocks and that they appear to be absent from
central and southern Rossshire where the 'Newer Granites' are
In the course of the present investigation the writer
has made a detailed study of the rocks of Ach'uaine and Appinitic
types in the Rogart area. In addition to the masses of Ach'uaine
type recorded by Read (1925, pp. 45-51) from this area, he has
mapped many more newly discovered occurrences of such rocks.
Besides mapping the basic bodies on the six-inch scale, he has
also made large -scale drawings of typical exposures. He has,
moreover, visited the type localities of the so- called 'Hybrids'
in the Ach'uaine region itself near Bonar Bridge under the
guidance of miss J. Watson, and has examined the rocks of the
Bettyhill area, the "Hornblendic Complex" of which has already
been described in great detail by Y.C. Cheng (1942, p. 67; 1944,
In a description of an agmatite formed from a hornblende-biotite-pyroxene-schist in the Rogart area, the writer (Ma,
1948,pp.8-14; and pp.153-9 of this thesis) has already shown that
hornblendite developed as a basic front in the schist as a result
of introduction and fixation of cafemic constituents displaced
from parts of the rock body which were undergoing granitisation.
The subsequent granitisation of the initially basified
rocks gave rise to the series: hornblendite ---> appinitic
rocks ---> dioritic migmatite ---> granodiorite. Some of these
rock types resemble the 'Hybrids of Ach'uaine Type' so closely
as to provide a clue towards the solution of the problem of the
origin of this curious suite of rocks.