From much of the coast along the Firth of Forth in southeast Scotland, and from coastal hills such as Arthur’s Seat in Edinburgh, the impressive piece of rock called the Bass Rock forms a prominent landmark. This steep island is the neck of a Lower Carboniferous volcano, rising 107m above sea level. Scuba divers, on the north side of the island, have shown the sea bed to be around 40m in depth, so the neck would be 150m high if we could see it all.
The rock is made of phonolitic trachyte, that is, an alkali igneous rock with less silica content than a ‘normal’ trachyte, so the alkali feldspar is accompanied by one of the silica-deficient feldspathoid minerals, such as analcime. Unfortunately, this interesting mineral assemblage is too fine-grained to see easily, except in thin sections under a microscope. In winter, Bass Rock is a dark brown as might be expected, but, in summer, it turns white from both the seabirds that crowd every available surface and their accumulated guano.