The geological feature which strikes the traveller in Iceland perhaps most forcibly is the complete contrast afforded by the well-ordered, level, or gently dipping basalt sheets with almost negligible tuff intercalations which constitute the Pre-glacial Kainozoic Plateau and the Late-glacial and Recent volcanic formations, and the enormous, chaotic, rarely-bedded accumulations of the Palagonite Formation in which lava flows are subordinate in amount; and he is at once presented with the question: Why did igneous action which produced lava sheets in Pre-glacial times result in dominant tuffs and breccias in the period which followed, and then revert to the production of lavas in later times ? This question has not yet been fully answered.

1. The igneous complex shows a remarkably complete series of types from ordinary biotite granite, through contaminated granite, intrusion gneisses, lit-par-lit aplitic intrusions, to aplites and muscovite pegmatites. 2. Pale-coloured reaction zones occur between xenoliths and the granitic masses. The evidence points to an exchange of material between xenoliths and the surrounding magma. 3. All the rocks are mylonized and it is suggested that the micro-structure can only be explained by movement during the final stages of crystallization. The folding is believed to have taken place in late Mesozoic times, as the area forms part of the Indo-Malayan Mountain System.

In a series of memoirs by Burckhardt (1), Groeber (2), and Gerth (3), the stratigraphical sequence and structure of the Cordillera of South Mendoza has been lucidly described. On the basis of these memoirs, of my own field observations (12), and of the study of the large collection of rocks made by Dr. Gerth, I shall attempt in this paper to give a picture of the magmatic evolution of this part of the Cordillera of South America (35 to 37 degrees south latitude).

The Geological Survey has recently formed a very extensive collection of the dyke-rocks of Galloway. This collection was made mainly by Mr. Robert Eckford, and, to a less extent, by Mr. Wm. Manson. One or more specimens were taken of practically every dyke shown on the original 6-inch field-maps. Since there are over 500 collected dykes in Wigtownshire alone, the size and importance of this collection is apparent. A dyke-suite of this magnitude, so thoroughly collected without the personal bias of the petrographer, supplies unique material for study. By the kindness of Sir John S. Flett, F.R.S., Director of H.M. Geological Survey, I have been permitted to investigate that part of the collection coming from Wigtownshire. This Wigtownshire portion consists of considerably over 1,000 rock-specimens, nearly 200 of which have been sliced. In this present paper I propose to give an account of the distribution and petrography of one group—the mica-lamprophyres—of the Wigtownshire dyke-rocks.