The relief may not project at all from the original surface of the material, as in the sunken reliefs of the Egyptians, and may be nearly flat, as in the Panathenaic procession of the Parthenon. It is circular in shape, measuring a mile in circumference, and is ft.
The Bass Rock is an intrusive mass of phonolitic trachyte or orthophyre. No nepheline has been detected in the rock, but analcite is present in small quantity together with abundant orthoclase and green soda-augite. It bears a close resemblance to the eruptive masses of North Berwick Law and Traprain Law, but is non-porphyritic.
It is regarded by Sir A. Geikie as a plug filling an old volcanic vent, from which lava emanated during the Calciferous Sandstone period. It used to be grazed by sheep, of which the mutton was thought to be unusually good, but its principal denizens are sea-birds, chiefly solan geese, which haunt the rock in vast numbers. A lighthouse with a six-flash lantern of 39, candle power was opened in For a considerable distance E. St Baldred, whose name has been given to several of the cliffs on the shore of the mainland, occupied a hermitage on the Bass, where he died in In the 14th century the island became the property of the Lauders, called afterwards Lauders of the Bass, from whom it was purchased in by government, and a castle with dungeons was erected on it, in which many Covenanters were imprisoned.
At the Revolution four young Jacobites captured the Rock, and having been reinforced by a few others, held it for King James from June to April , only surrendering when threatened by starvation.
Dismantled of its fortifications in , the Bass passed into the ownership of Sir Hew Dalrymple, to whose family it belongs.
