The Scottish slate industry probably started on the island of Easdale on the west coast of Scotland. It is not known precisely when quarrying began, but there is a report of a cargo of slates being sent to St Andrews in 1168. At about the same time, the Norwegians discovered slate on the Island of Belnahua nearby. Reliable records began in 1745 when the Earl of Breadalbane established the Marble and Slate Company and opened slate quarries on Easdale Island. As the demand for slate increased, the company expanded rapidly, establishing several quarries on the adjacent islands of Ellanbeich, Luing and Seil. (Ellanbeich is no longer an island but connected to Seil by slate waste.) For the next century Easdale slate dominated the Scottish slate industry until superseded in 19th century by Ballachulish. It survived many disasters; the introduction of a tax in 1799 on slate transported by sea was particularly onerous for quarries on these remote islands. Natural disasters also threatened the survival of the quarries. In 1879, the night of the Tay Bridge disaster, several of the islands were swept by exceptionally high tides flooding quarries and houses alike. In this case most of the quarries were pumped out and work continued. However a few years later, in 1881 another storm caused severe damage on Easdale and Ellanabeich, breaching the seawall of one of the Ellanbeich quarries which was never restored.
Production reached a peak at the end of the 19th century, producing 10 million slates annually. However in the 20th century, all of the quarries faced the common problems of the industry, namely competition from imported slate and artificial roofing materials. Production ceased completely during the First and Second World Wars and in both cases a few quarries never reopened afterwards. Production finally ended in 1960s when the remaining few quarries closed.
The geology of the Easdale area is composed of coarse-grained quartzite, superimposed by fine-grained slate, giving way gradually to marble and then phyllite. These are all metamorphosed sediments of the Argyll Group, the second youngest group of the Dalradian Supergroup. There is an abrupt change from coarse-grained sediments, which have been metamorphosed to quartzite, to fine-grained muds which have been metamorphosed to slate. This rapid change from shallow to deep water sedimentation is probably due to subsidence of the sea floor. As the basin fills up, there is an increasing supply of turbidites, giving way gradually to carbonate deposits which have become limestone. (Although metamorphosed carbonate rocks are marble, those in Dalradian Supergroup are usually referred to as limestone.)
Easdale slates are similar to Ballachulish in that they contain carbon and iron sulphide minerals. They also have a well-developed crenulation cleavage which is often used to distinguish between Ballachulish and Easdale slates. However, this identification is not conclusive, as crennulation cleavage is also present in Ballachulish although generally not as well-developed as in Easdale. Other properties, such as the concentration of barium, need to be taken into account to distinguish between these slates.