This, perhaps the most complex crystalline area of the Piémontese Alps, is also eminently sui generis, for it forms a unit of rocks outside, and different from the great calc-schist horizon with pietre verdi of secondary composition. As previously stated, it is the continuation of the mica-schist and minute gneiss belt which skirts the Po Valley from the Rocciacorba group to Avigliana, Monte Musiné, and Lanzo, and, extending from Lanzo to Ivrea, Biella, and Val Sesia, reaches beyond the latter into Lombardy to Lakes Orta and Maggiore. Within the Piémontese Alps, viz. from Lanzo to Val Sesia, it is bounded on the north by the calc-schist and pietre verdi fringe which skirts the Monte Rosa gneiss massif, and on the south by the Pliocene and Pleistocene deposits of the Po Valley, its superficial area being roughly 90 by 15 kilometres or 1,350 square kilometres, equal to over 500 square miles. The lower hills of the southern margin vary from 600 to 900 metres in altitude, which in the centre of the area increases to 1,000 and at the northern margin to 2,500 metres. In its general direction south-west to north-east, the area is intersected more or less at right angles by the following affluents of the Po: Stura di Lanzo, Malone, Orco, Chiusella – Dora Baltea, Elvo–Cervo, and Sessera–Sesia. The valley floors vary at the lower limit of the crystalline area from 300 to 500, and in their upper parts from 600 to 1,000 metres in altitude.