The next great immigration that we hear about is that of the Southern Dravidian people, who crossed the Lower Indus to Guzerat, and in course of time had settled themselves in the southern angle of India, in the Madras Presidency. They were a great building race of people. Another immigration took place in the first or second century B.C., and continued for some centuries after the Christian era. These people occupied nearly the western half of India, and erected buildings from Mysore in the south to Delhi in the north. This architecture is known as the Chalukya and Jaina styles. The fourth great immigration was that of the Mohammedans from the eleventh to the fourteenth centuries. The four principal styles of Indian architecture are the Buddhist, the Dravidian, the Northern Hindoo, and the Chalukyan or Jaina. In addition to the edict-pillars as illustrations of Buddhist architecture, many solid mounds of masonry, called topes, dagobas, or stupas, are found in some parts of the Punjab and north of India. These are relic-mounds, erected over the supposed relics of Buddha and of Buddhist priests, and are sometimes erected alone to the honour of Buddha. One of the most important is the Sanchi Tope in Bhopal, Central India (Fig. 325). Mr. Ferguson, in his “Study of Indian Architecture,” describes this remarkable monument as follows: “It was built probably (the tope) B.C. 500, the stone railing B.C. 250, and the gateways A.D. 19 to 37. The principal part of the building consists of a dome 106 feet in diameter and 42 feet in height. The fence by which this tope is surrounded is extremely curious. It consists of stone posts 8 feet 8 inches in height, and a little more than 2 feet apart, surmounted by a plain architrave, and between every two uprights three horizontal cross-pieces of stone are inserted. Still more curious are the four stone torans or gateways, one of which—the eastern—is shown at Fig. 325. It consists of two square pillars covered with sculptures, and with bold elephant capitals, rising to a height of 18 feet 4 inches. Above these are four lintels slightly curved upwards in the centre, and ending in Ionic scrolls; they are supported by continuations of the columns, and three uprights are inserted between the lintels. All this construction is covered over with elaborate sculpture, and surmounted by emblems. The total height is 33 feet 6 inches.” Sir G. Birdwood says: “The symbols are the _trisula_, the _wheel_, and the _lion_, representing the Buddhistic triad, Buddha, the law, and the congregation. The ground plan of the stupas or topes, with the return railings and the projecting doorways or entrances, form a gigantic swastika (‘auspicious'’), the mystic cross (fylfot) of the Buddhists.” Ferguson says the Buddhist dagoba is a direct descendant of the sepulchral tumulus of the Turanian races, like those found in Etruria, Lydia, and among the Scyths of the Northern Steppes.