It became part of the estates of the Breadalbane family and in the early 18th century they began to exploit the rich potential of the Neoproterozoic slate beds.

This document frequently used maps as a source of information and it has been possible to speculate about their modern equivalents based on assumptions about voyages made by early travellers 300–400 years prior to its creation.

Reasons include the island's association with St Brendan, its location on an inshore trade route from Antrim to the north and its suitability for a substantial settlement.

[14][Note 3] It has also been suggested that Seil may be the Innisibsolian referred to in the Chronicle of the Kings of Alba, which records a victory of the Scots over a Viking force during the time of Donald II in the 9th century.

Seil is cut by numerous NW-SE aligned basalt and microgabbro dykes which form a part of the ‘Mull Swarm’ which is of early Palaeogene age.

Raised marine deposits of sand and gravel occur widely around the margins of the island, a legacy of late Quaternary changes in relative sea-level.

[29] Seil then became part of the Netherlorn estates of the Breadalbane family (a branch of Clan Campbell) until the 20th century when their land was sold off as smaller farms and individual houses.

[31] From the late 17th century the Dukes of Argyll began to lease land on a competitive basis rather than as a means of strengthening the welfare of their senior clansmen.

[26] In 1745 Campbell created the Easdale Marble and Slate Company in order to place extractions from the area on a more commercial basis.

Peak production was reached in the 1860s at 9 million slates per annum, with export destinations including England, Nova Scotia, the West Indies, the US, Norway and New Zealand.

In the early morning of 22 November a severe gale from the south-west wind and an exceptionally high tide flooded the quarries on Easdale and at Eilean-a-beithich "a large rocky buttress which supported a sea wall gave way under the excessive pressure of water".

Changes in demand - clay tiles were rapidly replacing slate as the roofing material of choice - led to commercial production ceasing by 1911.

[41] Seil is associated with the 6th century saint Brendan of Clonfert who established a monastery on the Garvellachs and at a later date a cell on the site of Kilbrandon Church to which he gave his name.

[4][42] The modern church, located between Balvicar and Cuan, has five stained glass windows by Douglas Strachan illustrating scenes from the Sea of Galilee.

The central window depicts a ship in distress and "the portrayal in glowing colour of the storm's sweep, the crew's terror, and Christ's self-possession, has a lively drama unique in Highland stained glass".

[43] The well-known pub Tigh an Truish (Gaelic for 'house of the trousers') at Clachan Seil is said to have obtained its name in the wake of the 1745 Jacobite Rebellion.

[48] The Firth of Lorn is the seaway used by vessels going to and from Oban and Fort William from points south and the seas around Seil contain the sites of various shipwrecks.

[49] On 15 August 1900 the 310 t (305 long tons) iron steamship Apollo ran aground on Bono Reef 2.4 km (1.5 mi) south west of Seil.

[50] In February 1933 the Clyde puffer Hafton en route from Toboronochy on Luing to Mull sprung a leak and foundered about 14 km (9 mi) into the journey.

[51] A wreck of unknown provenance has been recorded .5 km (0.3 mi) east of Rubha Garbh Airde at the northern end of Seil.