The second fundamental problem of petrogenesis is the following question:—By what processes have there been derived, from the original magma or magmas, all other magmas known as igneous rocks? It is now almost universally admitted that many igneous rocks are genetically connected and produced from an original magma by differentiation, and that in many cases differentiation may be considered as liquation or separating into secondary magmas. But is differentiation (Spaltungen) alone sufficient to explain the formation of all igneous rocks? And by what is a magma stimulated to differentiation? These are two questions which must be elucidated. It must be first of all emphasized that not every magma is subject to differentiation. My standpoint is that the principal factors producing and regulating differentiation are on one side the process of crystallization (differentiation by crystallization, Krystallizations-differenzierung), and on the other the chemical composition of the magma and its tendency to the formation of eutectics (magmatic differentiation).

The single specimen which forms the subject of this note was sent to me for examination by Mr. Hewitt, of the Albany Museum, Grahamstown, Cape Colony; it was found at Cyphergat and presented to the Museum by Mr. S. R. Gardner. Fossil plants from Cyphergat in Cape Colony have been described by Feistmantel and, more recently, by myself from beds containing the remains of a flora assigned to the Rhætic age. Our knowledge of this flora is based on material obtained from the Molteno Beds at the base of the Stormberg Series at Molteno, Stormberg, Cyphergat, and other places.

During an investigation of the ‘Yellow Sands’ and Magnesian Limestone of Durham and Northumberland, several lenticular and irregular hollows or caverns, containing ‘Yellow Sands’ and light yellow marl, were noticed in the Lower Brecciated Magnesian Limestone at Trow Rocks, about one mile south of the Tyne (see Map, Fig. 1).

Various forms of Macrourous Decapod Crustacea referable to the genus Eryon have attracted my attention from an early date, and in the GeologicalMagazine for 1888 I gave a summary of all the then known species.