As regards the relation between dunes and the relief of the ground, there is a gradual rise in the general level of the country from north to south of about 1 in 1000, so that the southern ends of most of the Egyptian lines of dunes lie at higher altitudes than the northern ends; but I am fairly certain that this slight general inclination has nothing to do with dune-formation. The straightest and cleanest-cut lines of dunes are found in the flattest and most open parts of the desert, over which the wind can sweep without obstruction. When a line of dunes encounters in its southward progression a sudden fall in the ground- level, as for instance at the north end of Kharga Oasis and to the east of Pottery Hill, it usually breaks up; sometimes, as in Kharga, to resume its course as dunes in more open formation, and at other times, as near Pottery Hill, to spread out into a rolling plain of drifted sand. It is not uncommon for the south end of a line of dunes to split up, even on level ground, and the same can be seen, though less often, at the northern ends; but this is probably merely because the dunes are lower near the ends of the lines, and the width of the individual dunes is consequently less. The great lines of dunes are probably extending southwards at a very slow rate, under the action of the prevailing wind. In this connection it is interesting to note that Arabs always speak of the south end of a line of dunes as its head, and of the north end as its tail; just the opposite of what one would instinctively call them from merely looking at the map, but correct if the dunes are known to have a southward progression. Mr. Beadnell found the average rate of progression of isolated small dunes in Kharga to be 15 metres per annum; but he remarks that large dunes move more slowly than small ones, owing to the greater mass of material to be transported. It is interesting to make a rough guess at the antiquity of the dunes from the length of certain of the longer lines and an estimated rate of progression. The Abu Moharik belt, for instance, has a total length of about 350 kilometres; hence, if its tail has remained stationary and its head has advanced southwards at an average rate of 10 metres a year, some 35,000 years must have been occupied in its formation.