Therearethreevarieties，spiral，univalve，andbivalve。
Ineverysalt-paninthecountrythereisaspringofwaterononeside。
IcanremembernoexceptiontothisruleThewaterofthesespringsisbrackish，andcontainsthenitrateofsodaInoneinstancetherearetwosprings，andonemoresaltishthantheother。
Ifthissupplycamefrombedsofrocksaltthewaterwouldnotbedrinkable，asitgenerallyis，andinsomeinstances，wherethesaltcontainedinthepaninwhichthesespringsappearhasbeenremovedbyhumanagency，nofreshdepositoccursItisthereforeprobablethatthesedepositsofsaltaretheremainsoftheveryslightlybrackishlakesofantiquity，largeportionsofwhichmusthavebeendriedoutinthegeneraldesiccation。
WeseeaninstanceinLakeNgami，which，whenlow，becomesbrackish，andthisviewseemssupportedbythefactthatthelargestquantitiesofsalthavebeenfoundinthedeepesthollowsorlowestvalleys，whichhavenooutletoroutgoinggorge；andafountain，aboutthirtymilessouthoftheBamangwatothetemperatureofwhichisupwardof100Degwhilestronglyimpregnatedwithpuresalt，beingonaflatpartofthecountry，isaccompaniedbynodeposit。