To which I have now given you backing that it is not a MYTH of the Church because whether or not you have the particular reference to that Church in question or not, makes no difference.
The present spacious church in Baroque style was begun in 1681.
According to an ancient Spanish tradition, given in the Roman Breviary (for 12 October, Ad. mat., lect. vi), the original shrine was built by St. James the Apostle at the wish of the Blessed Virgin, who appeared to him as he was praying by the banks of the Ebro at Saragossa. There has been
much discussion as the truth of the tradition. Mgr L. Duchesne
denies, as did Baronius, the coming of St. James to Spain, and reproduces arguments founded on the writings of the Twelfth Ecumenical Council, discovered by Loaisa, but rejected as spurious by the
Jesuit academician Fita and many others. Those who defend the tradition adduce the testimony of St. Jerome (PL XXIV, 373) and that of the Mozarabic Office. The
oldest written testimony of devotion to the Blessed Virgin in Saragossa usually quoted is that of Pedro Librana (1155). Fita has published data of two
Christian tombs at Saragossa, dating from Roman days, on which the Assumption of the Blessed Virgin is represented.
http://www.newadvent.org/cathen/12083b.htm
However this may help you in your endeavors