The principal buildings are the collegiate church of Santa Maria de la Seo, the Dominican monastery, and the church of San Ignazio, built over the cavern (cueva santa) where Ignatius de Loyola spent most of the year 1522 in penitentiary exercises and the composition of his Exercitia spiritualia.
Seo de Urgel, near the headwaters of the Segre, is a fortified city which has been an episcopal see since 840, and has had a close historical connexion with Andorra (q.v.).
Selenious acid, H 2 SeO 3, is obtained in the crystalline form when a solution of selenium dioxide in water is concentrated over sulphuric acid.
The acid crystallizes in hexagonal prisms and melts at 58° C. It dissolves in water and yields a hydrate of composition H 2 SeO 4 H 2 O.
The only roads are bridle-paths, and one municipal road by the Balira valley, connecting Andorra with the high road to Seo de Urgel and Manresa; but in 1904 France and Spain agreed to build a railway from Ax to Ripoll, which would greatly facilitate traffic.