The phenomenon of which this paper treats occurs in a basalt dyke exposed in a small quarry by the roadside north-east of Dippin, near the twelfth milestone from Brodick. This dyke runs from north-west to south-east, and is intrusive into the great teschenite (or crinanite) sill of Dippin. It is probably the one referred to in the Geological Survey Memoir on North Arran, South Bute, and the Cumbraes (1903), p. 119. It averages 12 feet in thickness, although it is extremely irregular, owing perhaps to the difficulty experienced in penetrating the tough, coarse, massive rock of the sill. The contacts are not plane, but about against the crinanite in a very intricate manner, sometimes along vertical or horizontal joints, sometimes along irregular surfaces unconnected with jointing. Both contacts show thin films of tachylyte, which, as the rock is quarried, are left adhering to the irregular surfaces of the sill rock. Another dyke-like mass, 1 foot thick, is seen in the same quarry, and is doubtless an offshoot from the larger dyke.

Quenstedt described three distinct varieties of Ammonites Davœi, with nine illustrations. They all belonged to the 7 zone of Lower Lias:—

1. Ammonites Davœi, whose tubercles are somewhat clavate and with unsymmetrical prorsiradiate ribs.

2. A. Davœi enodis is continental. It is a small specimen without tubercles, with fine symmetrical and prorsiradiate ribs.

3. A. Davœi nodosissimus is also continental. This variety shows nine large bullate tubercles on each whorl, with close, fine, and symmetrically disposed flexiradiate ribs.

In continuation of the preceding paper (Geol. Mag., April, p. 156) which, as a preliminary to the present one, outlined the new classification of the crystalline rock formations and the nomenclature of the pietre verdi of the Piémontese Alps, I now propose to briefly describe the principal pietre verdi areas with which I became familiar during a long stay on repeated occasions in Turin. This city, apart from its rich collections of the rocks and minerals of the Piémontese Alps, is a most central and convenient starting-point for examining the different valleys debouching into the plain of the Po from the magnificent crescent formed by the Maritime, Cottian, Grajan, and Penniue Alps, which, as seen from Turin, afford by far the most extensive and fascinating Alpine panorama in Italy.

A Lacroix in his researches on volcanic rocks and their inclusions has shown that tridymite occurs in two distinct forms, each produced under special conditions. “Il est important de constater dans un même échantillon l'existence de tridymite produite par deux modes de genèse distincts; qui lui ont imprimé deux formes différentes. L'une d'elles est caracterisée par des cristaux épais, A macles binaires, elle a été formée par fusion; l'autre se présentant en lamelles extrêmement minces, empilées a été produite par action pneumatolytique. Cette variété de tridymite constitue le dernier des minéraux drusiques formés, et elle résulte très probablement de l'attaque A haute température d'un résidue de matière vitreuse très siliceuse dont des restes peuvent être parfois encore constatés directement” (1, p. 337). Both forms occur in Icelandic rocks.

In the Geological Magazine for April, 1913, I drew attention to certain features of the country forming the Yunnan–Tibet border. Further travels there in 1913 and through the Burmese hinterland in 1914 enable me now to supplement and extend those notes.