The Milagros Formation, here named in the Bayaney quadrangle consists of gray feldspathic lapilli tuff and brownish-gray volcanic breccia with some andesite lava, vitric and crystal tuff, volcanic sandstone and siltstone. Contains an unnamed volcanic breccia member in southwest corner of quadrangle. Conformably overlies the Matilde Formation; upper contact is not exposed. Thickness is at least 1,700 m. Age is probably Eocene as it overlies the Matilde of Paleocene and Eocene age.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Contact between rocks of the Rio Culebrinas and underlying Milagros Formations in the eastern San Sebastian and Bayaney quadrangles is conformable and gradational. The Guacio Member of the Rio Culebrinas, always present at the base of the Rio Culebrinas to the southwest, is not present where the Rio Culebrinas rests on the Milagros. The Milagros may be in part equivalent to the lower part of the Rio Culebrinas of the southern area. Early middle Eocene Foraminifera collected from the Matilde Formation which underlies the Milagros, support this interpretation and indicates that the Milagros is probably middle Eocene, same as the Rio Culebrinas. An unconformity between Cretaceous and middle Eocene rocks is recognized in west-central PR. The unconformity may also be present in northwestern PR as Paleocene and possibly lower Eocene rocks are missing there.

Source: GNU records (USGS DDS-6; Reston GNULEX).