granite, porphyry, trap of any kind, or limestone, the result is the same. Sandstone is, perhaps, the least friable amongst the several varieties of rock. Let any one scramble up the sides of a hill formed of granite, porphyry, or limestone, and he will find everything give way under his feet. If the side be pretty steep he will experience the not very agreeable sensation of treading upon a heap of loose cinders. The stones have no cohesion; they are burnt, broken, gritty, and most unpleasant to tread on. I was often reminded on such occasions of the ascent of the slaggy cone of Vesuvius.[1] Again, let any one try to collect specimens of granite, porphyry, or other varieties of trap-rock, and he will find it often next to impossible to get one with a fresh fracture. He strikes with his hammer an apparently solid block, and he finds it break into small pieces, each surface of which (except that of the original exterior) is coated with dust which has somehow penetrated into little joints and fissures quite invisible to the eye. When my son and I were engaged in collecting specimens from the ridge of Samrat Fiddân, we had frequent instances of this kind; and it was with much difficulty we could obtain specimens with fresh surfaces.
What, it may be asked, are the agents of disintegration in a region where frost and snow are unknown, and where the rainfall is small and fitful? The answer, as it seems to me, is to be found when we consider the extraordinary diurnal ranges of temperature to which this region is subjected. During the day the powerful rays of a nearly vertical sun in a cloudless sky raise the temperature of the rocks to a high degree. They are sometimes almost too hot to be touched by the bare feet of the Arab, or the bare hand of the European. Thus they are baked and scorched to an extraordinary degree; all moisture is driven out, and the material expands with the high temperature. Afterwards night ensues, and when the sun sets the rapid radiation of the heat into the atmosphere speedily reduces the temperature, which after midnight falls very low. A temperature of 85° Fahr. to 90° Fahr. in the shade, and 120° in the sun, falls during the night as low as 45°-50° Fahr. Expansion and contraction are consequently continually operating upon the outer portions of the rocks,
- ↑ Similar phenomena have been observed by Mr. A. B. Wynne in the districts of Kutch, and in the Trans-Indus salt region, where the atmospheric conditions, and some parts of the geology, are not unlike those of the district above referred to. See "Geology of Kutch," Mem. Geol. Survey, India, vol. ix, p. 22, and "Geol. of Trans-Indus Salt Region," ibid., vol. xi, p. 17.
