The source of materials for base course for a section of the Trans - Caprivi Highway between Katima Mulilo and Ngoma in Namibia had to be changed during construction at short notice. The only material available was a weathered basalt indicated to be non-durable by means of laboratory testing and of dubious service history in neighbouring countries. Weathered basic rocks are notorious for increasing in plasticity in service with consequent premature distress or even failure of the pavement. It is therefore good practice to build in a safety factor by modifying them down to non-plastic, especially in wet areas and for other than light duty. It is the purpose of this paper to report on an unusual problem in which the plasticity index was increased and not decreased by the addition of road lime. Keywords: Basalt, durability, lime, stabilization, plasticity.