Morphology, distribution, associated 工ocks, phase relationship, mineral com position as well as texture and structure studies indicate that in the studied region the crustal movement was rather stable and sea water rather deep and quiet in Middle Silurian. The characters of the sedimentary environment are also reflected in such trace elements as U, Zn, V, Mo, Cu, Co, Ni, Mn, Ba, Sr,P and organic substances (carbon and asphaltene) in the silicious limestones. The assemblage of these elements are usually precipitated in a relatively closed, oxygen-lacking reducing environment. The silicious limestones were formed mainly by chemical precipitation under the function of biota.