And at length, when despair had absolutely seized us, and van Reenen lagged behind me and I lagged behind Du Toit, I saw him again light a match and search the sand, and this time there came a hoarse cry. He had found the spoor; and revived, and with the terrible fear and anxiety lifted from us, we threw ourselves down and rested, knowing that we were saved. By daybreak our terrible thirst had conquered fatigue and we moved on again, and within an hour our rifle-shots were answered, and our “boy,” Gert Louw, was on his way to meet us with blessed, Heaven-sent nectar in the shape of a canvas bag of muddy water. The fire had been lit, but not on the big dune. Now, we had only been forty-eight hours away from camp and a bare thirty-six without water, yet we were nearly dead, and without a shadow of a doubt should have been dead of thirst before another sunset without water. Yet I have read of men living four or five days under like circumstances. The experience taught us several things. In future no broad, pleasant _straat_ ever tempted us in the wrong direction, we carried more water, even for short trips, and took compass bearings more accurately. Moreover we set the “boys” to work collecting dry wood, and made them stack it on the summit of the big dune, where a fire would have been seen from the Salt Pan, and where their laziness had prevented them from taking it before. CHAPTER XVI FORMATION OF THE DUNES AND PANS--“KOBO-KOBO” PAN--RAIN IN THE DESERT--SCORPIONS--AAR PAN--MIRAGE AND “SAND-DEVILS”--KOICHIE KA--THE PIT AND THE PUFF-ADDER. The long, parallel, wave-like formation of the sand-dunes of the Kalahari has been explained by many scientists as the result of the action of the prevailing wind, but this theory scarcely holds water. For though the majority of the dunes trend between west-north-west and east-south-east, this uniformity is broken in the vicinity of each of the numerous pans, around which the dunes are often formed in concentric rings, whilst the dried-up river-beds are usually flanked for some distance with dunes running parallel to their devious courses. So prominent is this concentric formation near many of the almost circular pans, that it appears probable that volcanic activity may have been the disturbing factor which formed both “pan” and dunes. Certain it is that no “prevailing wind” such as still blows, and still moves sand, would have formed either “waves” or rings as they lie to-day; and though science may be able to refute my theory, it is that the former were formed by successive and severe earth tremors (or earth waves) passing in the same direction whilst the sand was loose and without the binding vegetation much of it bears to-day, whilst the upheaval of volcanic magma similar to that of the Kimberley mine had in many instances formed the latter.