Abstract: It is proposed in this paper that the sedimentary dark siliceous rock is of enclosed gulf facies or lagoon facies according to the characteristics of rocks,minerals and paleontology.This denies the possibility of regarding them as of swamp or deep sea facies.Dark siliceous rock is essentially a kind of biological skeleton boundstone.This kind of rock serves not only as favourable source rock and reservoir but also as favourable bed for metal to be enriched there to form various kinds of ore deposit.Further study on the formational condition of this kind of rock, its sedimentary environment and changes in the landscape can be of much help in reconstructing the global tectonic of both the past and the present.