On the origin of the acid volcanic rocks around Lake Toba, North Sumatra a further contribution to the knowledge of welded rhyolite-tuffs deposited along the Sumatran longitudinal fault-trough system, and a critical examination of data concerning the formation of the great Toba cauldron - Westerveld, Jan 1905-1962 (Noord-Hollandsche Uitg. Maatschappij, 1947)