The area lying between the Lower Zambezi and its northern tributary the Lower Shire forms a relatively small salient between the better-known parts of Portuguese East Africa lying to the north and to the south, and a study of its geology in relation to that of the larger territory has yielded interesting results. The area described includes the southern end of the Nyasaland Protectorate and that part of Portuguese East Africa lying between it and the Zambezi.

A Study of stream action very soon shows that things are by no means so simple as they appear at first sight. Stream erosion is the basis of earth sculpture in temperate regions, except where direct differential weathering carves some of the details; yet its mechanism and the evolution of its topographical effects do not seem to be understood with any certainty. Even the classic discussions by G.K. Gilbert in his Geology of the Henry Mountains, and by W.M. Davis in his collected Geographical Essays, are, to the present writer at least, unconvincing at those points where the deductive structure is carried over, so to speak, to fit the work of actual streams, particularly with regard to the development of the curves of their longitudinal profiles. The parts given to this subject in the geological and geographical text-books are too short to allow a fully reasoned argument, but all appear to adopted the conclusions of Gilbert and Davis; though the line adopted in the present paper is to some extent approached by Chamberlin and Salisbury.

As is well known, the chronological order of deposition of the sediments of that broad band of schists which extends across Scotland from Banffshire and Aberdeenshire in the north-east to Argyllshire in the south-west is a most difficult problem to determine. The noteworthy development that has been made during recent years, leading to a more intimate understanding of the structure of the Southern Highlands, has made this problem much more vital. The question as to the top and bottom of these beds has been much discussed in recent years, and it can not be denied that views have been widely divergent on this matter. When a foreign geologist desires to call attention (through this paper) to certain conditions of fact, it may be pointed out that he is carrying on similar studies of another section—the Norwegian—of this same mountain chain, where one meets similar problems as in the Scottish Highlands.

During the summer of 1929 members of the International Summer School of Geology and Natural Resources, Princeton University, had the opportunity of examining with Mr. E. B. Bailey geological sections in the South-west highlands of Scotland and assisting in the determinations of tops and bottoms of beds in certain critical areas. The subdivision of the series into formations and the distribution of these formations were already for the most part known, but some important details of the structural interpretation were in doubt. In 1924, Thorolf Vogt, after visiting the area, stated that he had determined the order of succession in parts of the folded series; but these determinations by Vogt and their implications had not been verified or accepted by Mr. Bailey by the early part of 1929. Detailed observations made by the Princeton party in 1929, on strata which admitted of top and bottom determinations on the basis of cross-bedding, channelling and texture gradations, confirmed and extended the findings of Vogt; and Mr. Bailey is now in a position to restate the structural interpretation of the South-west Highlands making use of satisfactorily determined sequences in some of the units where formerly the sequence was in doubt or had been incorrectly inferred.

1. The Loch Awe Nappe succession was correctly stated in 1922.

2. The Iltay Nappe succession was correctly stated in 1922.

3. The Ballappel Foundation succession was wrongly stated in 1922.

4. The recumbent-fold structure of the Ballachulish district is due to rotation of middle limbs by drag from above or from below.

5. Graded bedding is as important as current bedding in marking a facies group among sandstones. It often indicates accumulation in rather deep water, to which sandy material has penetrated, perhaps as a result of seaquakes.