Such phenomena are common to the desert, more particularly where these extensive saline depositions exist, but varying from certain causes. In most cases, this powerfully magnifying and reflecting medium is a vertical stratum; at first dense and opaque, it gradually attenuates with increased temperature, till the maximum of heat, which it can no longer resist, drives it off in an ethereal vapour. This optical deception, well known to the Rajputs, is called _sikot_, or ‘winter castles,’ because chiefly visible in the cold season: hence, possibly, originated the equally illusory and delightful ‘Chateau en Espagne,’ so well known in the west.[1.54] =The Desert.=—From the north bank of the Luni to the south, and the Shaikhavat frontier to the east, the sandy region commences. Bikaner, Jodhpur, Jaisalmer are all sandy plains, increasing in volume as you proceed westward. All this portion of territory is incumbent on a sandstone formation: soundings of all the new wells made from Jodhpur to Ajmer yielded the same result: sand, concrete siliceous deposits, and chalk. Jaisalmer is everywhere encircled by desert; and that portion round the capital might not be improperly termed an oasis, in which wheat, barley, and even rice are produced. The fortress is erected on the extremity of a range of some hundred feet in elevation, which can be traced beyond its southern confines to the ruins of the ancient Chhotan erected upon them, and which tradition has preserved as the capital of a tribe, or prince, termed Hapa, of whom no other trace exists. It is not unlikely that this ridge may be connected with that which runs through the rich province of Jalor; consequently an offset from the base of Abu. Though all these regions collectively bear the term Marusthali, or ‘region of death’ (the emphatic and figurative phrase for the desert), the restrictive definition applies to a part only, that under the dominion of the Rathor race [19]. From Balotra on the Luni, throughout the whole of Dhat and Umrasumra, the western portion of Jaisalmer, and a broad strip between the southern limits of Daudputra and Bikaner, there is real solitude and desolation. But from the Sutlej to the Rann, a space of five hundred miles of longitudinal distance, and varying in breadth from fifty to one hundred miles, numerous oases are found, where the shepherds from the valley of the Indus and the Thal pasture their flocks. The springs of water in these places have various appellations, _tar_, _par_, _rar_, _dar_, all expressive of the element, round which assemble the Rajars, Sodhas, Mangalias, and Sahariyas,[1.55] inhabiting the desert. I will not touch on the salt lakes or natron beds, or the other products of the desert, vegetable or mineral; though the latter might soon be described, being confined to the jasper rock near Jaisalmer, which has been much used in the beautiful arabesques of that fairy fabric, at Agra, the mausoleum of Shah Jahan’s queen.