Mariano Eduardo de Rivero y Ustáriz (1798–1857) was a Peruvian scientist, geologist, mineralogist, chemist, archaeologist, politician, and diplomat. After schooling in Arequipa, he was sent in 1810 at age 12 to London to study mathematics, physics, and languages. In 1817 he traveled to France to the École royale des mines de Paris to study mineralogy and chemistry. In France he met Joseph Louis Proust, Gay-Lussac, and Alexander von Humboldt. The latter became his mentor and, during the course of his travels in Europe, Rivero discovered a new iron-oxalate that he named humboldtine in the older man’s honor. In 1822 Rivero was hired on Humboldt’s recommendation by the Colombian government to direct the first mining school in Bogotá, assisted by several other young European scientists. They included the French mineralogist Jean-Baptiste Boussingault (1802–87), who had studied at the school of mines at Saint-Etienne and later moved to Alsace to work in asphalt mining. Rivero and Boussingault traveled through the northern part of South America in support of the scientific explorations of General Simón Bolívar. Presented here is “Memoria sobre diferentes masas de hierro encontradas en la cordillera oriental de los andes” (Report on the different masses of iron, found in the Eastern Cordillera of the Andes), published in 1823, one of a number of articles on mineralogy on which the two men collaborated. It presents an analysis of the different types of iron ore found in the Eastern Cordillera of the Andes Mountains near Santa Rosa, Colombia. The work is included in a compilation of Rivero’s work called Colección de memorias cientificas, agrícolas e industriales publicadas en distintas épocas (Collection of scientific, agricultural, and industrial papers published at different times), which appeared in 1857.