Five types of replacement silica are recognized in the Lower Mississippian Virden Mem-ber carbonates on the northeastern flank of Williston basin:microcrystalline quartz,chalcedonic quartz,anhedral megaquartz,euhedral megaquartz,and stringy megaquartz.Silica tends to replace various bioclasts,and all except the stringy megaquartz also occur as non-replacive void-filling cement or as silica forming chert nodules and silicified limestone.Although crinoids,brachiopods,corals,bryozoans,molluscs,trilobites,forams,and ostracodes are present in the sediments studied,only the first three show evidence of silicification.Crinoids are commonly replaced by microcrystalline quartz whereas brachiopods typically by spherules of length slow chalcedony.Coalesced spherules,often in concentric rings(beekite rings),may form sheet-like masses on the surface of corals and brachiopods.Although bryozoans are common in the Virden Member,none showed any evidence of silicification.The difference in the susceptibility to silicification may be related to the shell microstructure,biological group,size of organism,skeletal mineralogy,and organic content of the bioclasts.Biogenic silica de-rived from the dissolution of siliceous sponge spicules is considered to be the most likely silica source for silicification.Most silica is believed to be released during early diagenesis before the sediments were deeply buried.The Virden Member carbonate may have experienced two episodes of replacement,the first affecting the bioclasts,the second producing silicified limestone and chert nodules. Five types of replacement silica are recognized in the Lower Mississippian Virden Mem-ber carbonates on the northeastern flank of Williston basin: microcrystalline quartz, chalcedonic quartz, anhedral megaquartz, euhedral megaquartz, and stringy megaquartz. Silica to replace various bioclasts, and all except the stringy megaquartz also occur as non-replacive void-filling cement or as silica forming chert nodules and silicified limestone. Although crinoids, brachiopods, corals, bryozoans, molluscs, trilobites, forams, and ostracodes are present in the sediments studied, only the first three show evidence of silicification. Crinoids are commonly replaced by microcrystalline quartz and brachiopods. bryozoans are common in the Virden Member, none showed any evidence of silicification.The difference in the susceptib ILITY to silicification may be related to the shell microstructure, biological group, size of organism, skeletal mineralogy, and organic content of the bioclasts. Biogenic silica de-rived from the dissolution of siliceous sponge spicules is considered to be the most likely silica source for silicification.Most silica is believed to be released during early diagenesis before the sediments were deeply buried. The Virden Member carbonate may have experienced two episodes of replacement, the first affecting the bioclasts, the second productive silicified limestone and chert nodules.