In 1904 the late Dr. Arthur Vaughan, in collaboration with the writer, described1 the Rhaetic rocks exposed in constructing the South Wales direct branch of the Great Western Railway between Stoke Gifford and Chipping Sodbury. During the last few years, extension of the big quarry in Carboniferous Limestone at Chipping Sodbury has exposed a fine section of Rhaetic, the description of which and of the field relations of the associated rocks is the object of the present paper.

This paper gives an account of an investigation of the Carboniferous glacial deposits that crop out along the valley of the Arroyo de las Cabeceras, which is situated in one of the western front ranges of the Precordillera of San Juan, Western Argentina. The field work was carried out by one of us (J. Keidel) and the detailed examination of the fossils collected by the other (H. J. Harrington).

The olivine-rich rocks have acquired a particular significance in petrogenetic discussions as a result of the growth ofthe realization of the difficulties involved in postulating the existence of anhydrous melts of corresponding composition.The belief that such rocks have originated through the local accumulation of olivine crystals, without significant remelting (Bowen, 2, p. 108; 3, pp. 133–174)1 has received added support from the experimental determination ofthe melting relationships of the olivines. The facts “seem susceptible of no other interpretation than that, when such masses become rich in olivine, they do so through the accumulation of crystals of that mineral, which remain as such and do not become a part of the liquid by remelting or re-solution” (4, pp. 394–5).

Sule Stack is composed of an orthogneiss traversed by shatter belts and small pegmatite veins composed of microcline and quartz. The chief constituent minerals of the gneiss are oligoclase and orthoclase felspars, hornblende altering towards chlorite, augite, and apatite. In addition some sections show magnetite and a few quartz. Comparisons are made between the gneiss of Sule Stack and that of the neighbouring island of Sule Skerry.