DURING the course of the past six years it has fallen to my lot to take part in, or to conduct, a series of expeditions in various countries on the fringes of Arabia. These include the Sudan Red Sea littoral, Transjordania, Syria, Mesopotamia, Persia, the Persian Gulf coasts, and lastly, in the winter 1925-26, 'Oman and the south-eastern Arabian coast as far as the province of Dhofar. Systematic exploration of the interior of Arabia, or indeed of its fringes, is greatly hampered by formid? able natural barriers and by the still more serious obstacles caused by the independent spirit of its inhabitants. For the latter reason I must admit partial failure of my last expedition, but I was able to make a number of new observations which throw some light on old difficulties, although perhaps I have raised more new problems than I have disposed of old. I was unsuccessful in penetrating into the heart of the 'Oman ranges, the great Jabal al Akhdhar (or the Green Mountain), but I traversed a considerable extent of hitherto unexplored country farther to the north, crossing the mountain ranges six times in all. The expedition consisted of Mr. K. Washington Gray and myself as geologists ; Mr. Joseph Fernandez, an Indian botanist and collector; and Mr. A. F. Williamson, better known as Haji Abdulla Williamson, whose intimate knowledge of all things Arab, acquired during a thirty years' sojourn with the tribes, made him invaluable as our political agent. We were accompanied during part of our journey by Capt. G. J. Eccles, the officer commanding the Muscat State Levies, and during part by Mr. B. S. Thomas, the Financial Minister to the Sultan. Without the cordial assistance of these gentlemen our traverses into the interior would not have been possible. The preliminary arrangements for the expedition were made by Sir Arnold Wilson; thanks are due to him and to Sir John Cadman, and the Directors of the D'Arcy Exploration Co., Ltd., for permission to make public the geographical and geological results of the expedition. 'Oman has been the subject of a series of papers to this Society during recent years, notably by Col. S. B. Miles (1, 2 of the Bibliography at end of paper) and by Sir Percy Cox (3, 4); and Capt. Eccles has already lectured to the Central Asian Society on the more human aspect of our journey. I propose, therefore, to confine my attention more to the physical geography and geological tectonics of the country, adding as appendices some disconnected notes dealing with Bahrein and Qatar, and reports on flint implements, birds and insects collected. The botanical collection was presented to the Bombay Natural History Society, by whom it is being examined.