Turning now to the work of the GEOLOGICAL SURVEY in the district, the maps and descriptions in the present volume are the result of surveys carried out by me in the three seasons 1905-1908, or about twenty-two months’ work in all. The survey was commenced primarily with the view of enabling mining concessions to be accurately marked out. How little possibility of this existed so recently as 1902 may be gathered from the fact that although a ministerial order of that year had defined the administrative frontier between Egypt and the Sudan as being a line joining certain important mountains and wells, which were named, yet it was impossible to lay this frontier down correctly on a map because the geographical positions even of these important features were uncertain to many kilometres. The main interest in the field methods used, which will be described in detail in a subsequent chapter, lies in the fact that many of them are wholly or in part new, having been devised as the work proceeded to meet the special exigencies of the case. The costly nature of camel-transport, and the relatively small value of the country, precluded the employment of the ordinary sequence of survey operations, and it was necessary to carry on reconnaissance, precise triangulation, detailed topographical mapping, and geological surveying, all at once, and to move rapidly so as to cover a large area in a moderate time. Starting from a measured base near Gebel Muelih, which had been previously connected by triangulation with the Nile Valley, a network of large triangles was thrown over the country. The essential feature of the triangulation was the employment of observations at relatively few occupied main summits, to fix large numbers of points by intersection; in some cases over a hundred triangulation points were sighted from a single occupied station. The triangulation was continued so as to join to a second base line near Gebel Um Harba (_Ck_), and was also connected on to main points in the trigonometrical survey of the Sudan, thus linking up a continuous chain of triangles from Alexandria to Berber. The total number of main (occupied) stations was sixty-four, while the intersected points numbered 450. In addition to these, about 1,200 minor points were fixed by subsidiary triangulation from short local bases. Levels were taken trigonometrically for all points fixed by triangulation, using the actual sea-level as the datum. The altitudes of nearly all camps were likewise found trigonometrically, and between successive camps aneroid readings, adjusted to the initial and terminal points of the day’s march, were employed to supplement trigonometrical determinations made on the journey. For the control of the maps, latitudes and azimuths were taken at intervals, the former by equal altitudes of three or more stars, the latter by elongations of close circumpolar stars.