Bailey, et al. 1924. Tertiary and Post-Tertiary geology of Mull, Loch Aline, and Oban (a description of parts of sheets 43, 44, 51, and 52 of the geological map). HMSO [for Geological Survey]
Chapter 11 Various dolerites and gabbros
Introduction
Such dolerite and gabbro-masses of Mull as are lettered eD on sheets 43, 44, and 51 are covered by the present chapter, with the exception of the Loch Uisg Gabbro (chapter 20) and two small intrusions outside the loch Bà Felsite at Coille na Sròine and Sròn nam Boc (Chapter 33). It will not be possible to refer to every occurrence, or to give a connected account of what is, after all, a heterogenous assemblage. The sequel is divided under two headings Field-Relations and Petrology, and, throughout, geographical classification is adopted for the sake of ready reference. All except two of the districts selected fall within Sheet 44.
There is good reason to believe that some of the intrusions considered here are of the same age as some of the lavas dealt with in preceding chapters. This is particularly true in regard to the small- and big-felspar dolerites shown in
Field-relations
Dolerite-plugs, 'S Aìrde Beinn and Loch Frisa (Sheet 51)
One of the most interesting features of the north end of Mull is the volcanic plug which forms the hill of 'S Aìrde Beinn, about 3 miles west-south-west of Tobermory. Good illustrated descriptions have already been given by Prof. Judd (p. 46) and Sir Archibald Geikie. From the hills south of Loch Meadhoin, the plug is seen as a large knob of rock protruding with more or less vertical walls out of the surrounding almost flat lava-flows. What immediately strikes the eye from this point of view is the fact that the knob contains a central depression, the site of a picturesque lochan. There is little wonder that this hollow is often wrongly interpreted as the crater of an extinct volcano. If one approaches the hill from the northeast, the intrusive nature of the mass is well demonstrated by the fact that the edges of several flows of basalt are seen to terminate abruptly against the vertical wall of the plug.
Let us now examine this intrusive mass in detail. It is found to run north-north-west, with a maximun length and breadth of 950 and 430 yds. respectively. Except for a short distance at the south-eastern corner, where the overflow-stream from the loch emerges, it has an almost vertical wall of complex nature. On examination, the material which forms the outer edge of this wall is seen to consist of a fine-grained hard splintery rock, which in places shows good amygdaloidal structure. Under the microscope, this outer portion proves to be the same type of basalt as that of the surrounding lava-flows, only in an altered condition due to its having been baked by the intrusion of the volcanic plug. No abrupt junction can be seen between this altered basalt and the dolerite of the plug; in fact, the one passes almost insensibly into the other. Apparently the temperature of the molten dolerite in the plug was sufficient to melt up the edge of the adjoining basalt-lava, with a consequent local mingling of material from the two sources. The dolerite, followed in from its edge towards the loch, is seen to become gradually coarser in grain for a certain distance, and then to retain an almost uniform texture throughout.
The origin of the central loch now demands our attention. No rock except dolerite is exposed near its margin, and this dolerite is not different in composition from that 20 yds. or so away from the edge of the loch; nor are any fragments of ashy material exposed along the shore. The only visible reason for the presence of the loch is the occurrence of a north-north-west line of fracture, which can be traced for miles across the country, and is well seen, in a cleft, on the northern face of the hill. At the south-east end, the stream from the loch emerges through an opening along the same line of fracture, so that in all probability the hollow of the loch has been eroded in a belt of shattered dolerite determined by faulting.
A mile and a half north-north-west from 'S Aìrde Beinn, and on the south side of the road leading from Tobermory to Glengorm Castle, a small plug forms a prominent crag known as Cnoc a' Chrocaire. It is only about 100 yds. in diameter, and is cut by a thin dyke and an irregular sheet of fine-grained dolerite.
In the other direction, a mile and a half south-south-east of 'S Airde Beinn, there are two more small plugs of dolerite outcropping above Loch Frisa. The more southerly is interesting as showing good almost horizontal columns. It is a medium to coarse-grained dolerite. G.V.W.
Dolerite-plugs, Dùn Mòr, Ulva Ferry (Sheet 43)
Two small dolerite-plugs occur a mile east of Ulva Ferry. The larger of them constitutes Dùn Mòr, a very conspicuous landmark contrasting strongly with the terraced lavas of the neighbourhood. The dolerite of Dùn Mòr is fairly coarse. That it is a plug is clear from its appearance as a whole, but no satisfactory junctions are exposed in confirmation of this interpretation. In fact, on its western side, a part of the dolerite can be seen at one point (under a little 2-ft. basalt-sill, which cuts it) resting with an unchilled base upon basalt-slag, just as if it were a lava. A combination of intrusive and superficial characteristics is, of course, a common feature in vent-intrusions.
E.B.B.
Dolerites and gabbros of Ben More (Sheet 44)
On Ben More, a few small masses of early dolerite, or gabbro, are lettered eD on the one-inch Map. They are generally of more or less sheet-like form, but are much thicker and more laccolithic than the later sheets of the neighbourhood lettered D on the Map and described later on in Chapter 26.
Gabbro of Beinn nan Lus (Sheet 44)
There is little to be said about the two small outcrops of gabbro mapped on Beinn nan Lus, beyond the fact that the rock is of early date as compared with the Beinn a' Ghràig Granophyre (Chapter 27) with which it is in contact.
Beinn na Duatharach and other gabbros and dolerites within North-West Caldera (Plate 5 and Sheet 44)
There are several groups of gabbro and dolerite outcrops within the North-West Caldera outlined on
Dolerites and gabbros, Coire Mòr, Maol Uachdarach and Beinn Talaidh (Sheet 44)
A considerable mass of coarse dolerite or fine gabbro, cut to ribbons by Late Basic Cone-Sheets (Chapter 28) reaches in a south-east direction from Coire na Lice Duibhe along the top of Coire Mòr into Beinn a' Mheadhoin. It is of finer texture than the neighbouring Corra-bheinn Gabbro.
What may be part of the same mass of dolerite reappears along the summit of Maol Uachdarach where it is locally broken up to yield agglomerate. Its exposures show it cut by Late Basic Cone-Sheets running east and west, and by ring-dykes (Chapter 29) running north and south. The dolerite gives rise to little crags and thus greatly assists in the mapping of the ring-dykes. J.E.R.
On the one-inch Map, Sheet 44, a marginal line is inserted including within its scope the summit of Beinn Talaidh, and bounding, an area where lenticles of dolerite are found between the prevalent cone-sheets of the mountain. The fullest exposures are on Maol nam Fiadh, where the dolerite, though sometimes strikingly vesicular, shows a chilled contact against neighbouring porphyritic lavas (
An outcrop of gabbro, two-thirds of a mile north-west of Beinn Talaidh summit, is a black and white gabbro like much of the Beinn Bheag mass (Chapter 22), of which very likely it is an isolated part.
Separated from Beinn Talaidh by the pass connecting Glen Forsa with Glen More, one may note two strips of dolerite on the slopes of Beinn Bhearnach, one mile west-south-west of the summit of Sgùrr Dearg. The more north-easterly band is a dark rock that may fairly be classed as fine gabbro. It is interesting because in clear exposures it may be seen breaking up to yield neck-agglomerate.
Small-Felspar Dolerites between Allt nan Clàr and Sgùrr Dearg (Sheet 44)
A group of dolerites characterized by small felspar-phenocrysts will now be considered
A large outcrop occurs in Allt nan Clàr. It is intensely cut up by cone-sheets as shown on Sheet 44, and it is of coarse crystallization so that it is not easy to separate it from the Beinn Bheag Gabbro (
More interesting are smaller outcrops on the north-east slope of Beinn Bheag. The small-felspar dolerite occurs here as massive sills intruded into fine Tertiary sediments (p. 66). The dolerite and sediment are broken up as they approach vent-agglomerate exposed on their north-east side.
A large mass of small-felspar dolerite runs roughly parallel with Abhuinn an t-Stratha Bhain. It is chiefly noteworthy as having yielded blocks constituting a very prominent median moraine in Glen Forsa.
Several small occurrences may be noted in the gneiss north of Sgùrr Dearg. They are so baked and crushed that it is certain they are of an earlier date than the neck-agglomerate of the Sgùrr Dearg vents (Chapter 16).
Big-felspar gabbros and dolerites between Sgùrr Dearg and Loch Spelve (Sheet 44)
Continuing the same line as the small-felspar dolerites just described, comes an equally definite suite of big-felspar gabbro and dolerite intrusions
The more conspicuous masses of big-felspar gabbro are closely associated with the Moine gneiss of Sgùrr Dearg (
Mesozoic sediments sometimes intervene between the gneiss and the gabbro even where these two occur close together; a tongue of the gabbro—or dolerite—clearly chills against Tertiary lavas on Beinn Bheag (
The evidence for a relatively early date of intrusion is afforded by the crushing and alteration shown by the outcrops at Sgùrr Dearg, and also by the fact that the rocks of these outcrops are clearly older than the vent-agglomerates of the neighbourhood. This last point will be dealt with in detail in Chapter 16. Here it may be added that, along with the Sgùrr Dearg agglomerates, the big-felspar intrusions are cut into lenticles by early Basic Cone-Sheets (bI. of one-inch Map).
In its more important exposures, the Sgùrr Dearg Gabbro behaves as a sill intruded into the Mesozoic rocks (
If it is correct to refer the Sgùrr Dearg Big-Felspar Suite of gabbros and dolerites to the period that produced the big-felspar basalt-lavas of the Coire Mòr Syncline (p. 121), then of course it follows that the non-porphyritic dolerite forming the main part of the An Eiligeir Plug must also correspond in date to lavas that are still preserved to us in Mull. E.B.B.
Petrology
Dolerite plug of 'S Aìrde Beinn
The rock forming this mass (S18064)
This rock, in its felspathic nature and in the mode of occurrence of the augite, has points that connect it genetically with some of the pillow-lavas of the Central Region (Chapter 9). Specimens illustrating its metamorphic action on the basalt-lavas with which it is in contact are easy to obtain (S18065)
In another specimen, amygdaloidal basalt (S19268)
Dolerites and gabbros of Ben More
An interesting rock from a point 530 yds. north-west of Ben More summit is a coarse olivine-dolerite (S17145)
Gabbro of Beinn nan Lug
The more westerly mass (S17983)
The more easterly outcrop (S14702)
Beinn na Duatharach Gabbro of the North-West Caldera
(Anal. II.;
This mass is represented by the analysed rock (S14846)
The rock locally shows signs of crushing, disruption (S14565)
Contact-alteration, probably by the Glen Cannel granophyre, has affected the rock subsequently to its crushing to a variable degree. Serpentine, that formerly was present both as pseudomorphs after olivine, and as interstitial growths between the felspars, has, in the lower grade of metamorphism, yielded epidote, chlorite, and hornblende (S14707)
The chemical relationship of the Gabbro to the Porphyritic Central Lavas of Mull and the Cuillin Gabbro of. Skye is discussed in Chapter 1. (p. 24).
Bìth-bheinn dolerite within the North-West Caldera
A mass on the northern face of Bìth-bheinn (S17131)
Dolerites West of Clachaig within the North-West Caldera
A small mass (S15568)
A larger mass, west of Clachaig (S17125)
Small-felspar dolerites of (Figure 22)
This 'small-felspar' group of rocks is chiefly remarkable for reproducing, in some cases, the petrographical peculiarities of the porphyritic pillow-lavas of the central region (p. 150). As exemplified by the interior of an intrusion to the north-west of Sgùrr Dearg (S16472)
Coarser representatives of the same group are genuine ophitic dolerites. An example from Allt nan Clàr (S14356)
A small mass on Cruach Choireadail (S17199)
Big-felspar dolerites and gabbros of (Figure 22)
The distribution of these rocks is shown in