One of Pliny's authorities is Marcus Terentius Varro. In the geographical books, Varro is supplemented by the topographical commentaries of Agrippa, which were completed by the emperor Augustus; for his zoology, he relies largely on Aristotle and on Juba, the scholarly Mauretanian king, "studiorum claritate memorabilior quam regno" (v. 16). Juba is one of his principal guides in botany; Theophrastus is also named in his Indices, and Pliny had translated Theophrastus's Greek into Latin. Another work by Theophrastus, "On Stones" was cited as a source on ores and minerals. Pliny strove to use all the Greek histories available to him, such as Herodotus and Thucydides, as well as the "Bibliotheca Historica" of Diodorus Siculus.