In 1948 and 1949, the author of the article was a participant in the search and exploration of volcanic deposits of nugget sulfur on the islands of the Great Kuril Ridge - Iturup and Kunashiri. The materials obtained during this work were the basis for the proposed article. Unfortunately, some questions of geology of the studied sulfur deposits are insufficiently covered in the article, since a number of materials are located outside Leningrad. At the same time, we managed to take into account some new ideas of Japanese geologists on the issues raised. Some of them were borrowed from unpublished translations by G. M. Vlasov. The author also used the reports on work in the Kurils by R. Ostroumov. To both one and the other the author expresses his gratitude.