Interesting phrase - "spheroidal disjuction". In humid climates basalt will, of course, break down more readily than granite, and you often see spheroidal weathering. Many lavas are noted by their platey character, defined by crudely developed joints parallel to their surface exposure. Combine this with the perpendicular columnar jointing and you generate rectangular blocks that can be easily attacked by chemical weathering. However, the best spheroidally weathered basalts that I have seen are associated with lavas pouring over wet ground. The steam produced will stream upward through the flow, often rapid chilling and associated jointing having glassy rinds that are readily susceptible to chemical breakdown. This is probably more than you want to know!
Victor Camp, Geologist SDSU – California;
Isabelle SacramentoGrilo, Geologist ChevronTexaco - California isacrame@geology.sdsu.edu