An ocean grows and then shrinks in size, continents join creating Caledonia:

During the Palaeozoic Era the part of the Earth’s crust which included the British Isles and therefore Mull, slowly drifted northwards. During Cambrian times the Iapetus Ocean was gradually opening. At this stage all of Scotland was quite separate from England and Wales, divided by the widening ocean. The start of the Cambrian is characterised by the change in the chemistry of the seas, soft-bodied organisms were now able to produce hard exo-skeletons and new life forms evolved. During the Ordovician the ocean widened further, but during the Silurian the Iapetus Ocean  gradually closed bringing together Scotland’ with ‘England and Wales’. At this time the ‘British Isles’ lay in warm tropical latitudes south of the equator. As the Iapetus Ocean closed, uniting ‘Scotland’ with ‘England and Wales’ it squeezed vast thicknesses of marine sediments into a fold mountain chain which became the Southern Uplands. (No sedimentary formations of  Lower Palaeozoic age are found on Mull but are found further east on the mainland.) In the Highlands of Scotland, including Mull, sediments of Moine and Dalradian age were buried very deeply as the Iapetus Ocean closed and they were metamorphosed into schists and gneisses. (The top part of the Dalradian is known to be of Cambrian to Ordovician age but it is not possible to determine the exact age of the Dalradian rocks on Mull.) The closure of the Iapetus Ocean was the end of the tectonic cycle known as the Caledonian orogeny, and the mountains produced were the Caledonides, or Caledonian mountain chain. By the time that the Caledonian mountain chain had formed Britain’s crust had drifted northwards to desert latitudes within the southern hemisphere and the newly joined land mass was known as the ‘Old Red Sandstone’ continent. This was the beginning of  Devonian times.

Rocks of  Devonian age:

During the Devonian the ‘Old Red Sandstone’ continent was subjected to typical desert conditions. The Dunollie Boulder beds near Oban represent a conglomerate of massive proportions deposited during desert flash floods when rare desert storms swept debris from the denuding mountains down wadis and out onto the desert plains. Volcanic activity took place in what is now the Western Highlands, producing great thicknesses of acid lava in Glen Coe, near Oban, and also on Mull where very shattered lavas are preserved in the Loch Don anticline. The greatest extent of Devonian age rock is the Ross of Mull Granite. This is made from magma intruded deep within the huge Caledonian mountain chain, due to crustal melting of material buried at the base of the new continent. (This crustal material may well have been Lewisian gneiss.) As the melt was rich in silica an acid magma formed. Bodies of this acid magma, known as plutons, slowly rose up within the crust. As they rose they engulfed huge chunks of the country rock they passed through, - a process known as stoping. Over a long period of time the magma cooled producing a coarse grained granitic rock.

The Ross of Mull Granite:

The granite has been dated at 414 MY (plus or minus 3 MY). It is a small pluton or boss with probable dimensions of 12 x 6 miles, with half its outcrop covered by the sea. (It has a total area of about 20 square miles.)

Rocks:

The rock is a pink, or red, biotite granite with a little muscovite and with a coarse even grained texture. It is an alkali granite with more than 2/3rds of the feldspar content potassium rich orthoclase feldspar and less than 1/3rd plagioclase feldspar (the orthoclase is pink or red in colour, the plagioclase white).

Ross of Mull Granite, The Sound of Iona and the Isle of Iona.  There is approx. 2,400 million year time gap between the rocks on both sides of the Sound of Iona.

Grey, glassy quartz forms the next most abundant mineral, with biotite mica (black in colour) forming the third most abundant. Accessory interstitial minerals include:-
Pyrites, muscovite, hornblende, allanite, magnetite, epidote, apatite, titanite and zircon.

The cooling history of the Ross of Mull granite was not simple. The first magma fraction to crystallise was richer in plagioclase feldspar and biotite and this forms the pale pink facies seen today at Camus Tuath. Depleted now in calcium and sodium the next fraction crystallised as potassium rich orthoclase (alkali) feldspars, quartz and biotite, forming the red facies. The last fraction of the magma, rich in residual volatiles, crystallised as aplites and pegmatites. 

Moine Psammite Xenolith in Ross of Mull Granite.