chap. in. Concretions in Pumiceous Tuff. 55 



in the map ' Crater of an old volcano,' has no claims to 

 this appellation, which I could discover, except in being 

 surmounted by a circular, very shallow, saucer-like 

 summit, nearly half a mile in diameter. This hollow 

 has been nearly filled up with many successive sheets of 

 ashes and scoriae, of different colours, and slightly con- 

 solidated. Each successive saucer-shaped layer crops 

 out all round the margin, forming so many rings of 

 various colours, and giving to the hill a fantastic 

 appearance. The outer ring is broad, and of a white 

 colour ; hence it resembles a course round which horses 

 have been exercised, and has received the name of the 

 Devil's Riding School, by which it is most generally 

 known. These successive layers of ashes must have 

 fallen over the whole surrounding country, but they 

 have all been blown away except in this one hollow, 

 in which probably moisture accumulated, either during 

 an extraordinary year when rain fell, or during the 

 storms often accompanying volcanic eruptions. One of 

 the layers of a pinkish colour, and chiefly derived 

 from small, decomposed fragments of pumice, is remark- 

 able, from containing numerous concretions. These 

 are generally spherical, from half-an-inch to three 

 inches in diameter ; but they are occasionally cylin- 

 drical, like those of iron-pyrites in the chalk of Europe. 

 They consist of a very tough, compact, pale-brown 

 stone, with a smooth and even fracture. They are 

 divided into concentric layers by thin white partitions, 

 resembling the external superficies ; six or eight of 

 such layers are distinctly defined near the outside : but 

 those towards the inside generally become indistinct, 

 and blend into a homogeneous mass. I presume that 

 these concentric layers were formed by the shrinking of 

 the concretion, as it became compact. The interior 

 part is generally fissured by minute cracks or septaria 



