The islands of Eigg and Muck lie to the south east of Rum. Both islands are largely formed of lavas, which represent the remnants of what was once a much larger lava field, erupted from a slightly older volcano than Rum. The ridge of An Sgùrr dominates the skyline of Eigg. It provides an intriguing glimpse into the final phases of the volcanic activity in this part of Scotland. Great thicknesses of basalt lava had already been spewed out, creating a raw and inhospitable landscape. Thick layers of conglomerates containing water-rounded pebbles lie in a valley on top of these lavas, marking the location of an ancient water course. As volcanic activity in the area came to an end, a nearby volcano erupted sticky lava of unusual composition which flowed slowly along this ancient river bed. Over time this lava, which is known as pitchstone, cooled and developed impressive organ-pipe-like columns many tens of metres high.

The pioneering Victorian geologist Hugh Miller visited Eigg in 1844. He made some amazing discoveries during his short stay on the island. The rocks at sea level have yielded the fossilised remains of some long extinct animals such as marine turtles, crocodiles and most prized of all, plesiosaurs, more commonly known as the sea dragon. These layers of rock, which are of Jurassic age, accumulated on the floor of a sub-tropical lagoon that pre-dated the Rum volcano.