In 1767, Pallas was invited by Catherine II of Russia to become a professor at the St Petersburg Academy of Sciences and, between 1768 and 1774, he led an expedition to central Russian provinces, Povolzhye, Urals, West Siberia, Altay, and Transbaikal, collecting natural history specimens for the academy. He explored the Caspian Sea, the Ural and Altai Mountains and the upper Amur River, reaching as far eastward as Lake Baikal. The regular reports which Pallas sent to St Petersburg were collected and published as "Reise durch verschiedene Provinzen des Russischen Reichs" (Journey through various provinces of the Russian Empire) (3 vols., 1771–1776). They covered a wide range of topics, including geology and mineralogy, reports on the native peoples and their religions, and descriptions of new plants and animals. In 1776, Pallas was elected a foreign member of the Royal Swedish Academy of Sciences.