Formation of Saida clays (Middle Jurassic of Western Algeria) : biostratigraphy (ammonites) and mineralogy.- The study section is located in the Saïda region (NW Algeria), around the locality of "Hot Waters" (Hammam Rabbi). It shows the succession of several limestone and sandstone beds belonging to the lower member of the Saida clay Formation of the lower to middle Callovian age. Its lower limit is marked by a discontinuity corresponding to a reddish surface with stromatolitic "cakes". This constitutes a fossiliferous level at the base of the section. The upper limit of the formation is marked by sandstone and coarser beds. The facies are monotonous, essentially clayey to marly, revealing indurated levels of limestone. The X-ray diffractometry analysis of the samples shows the presence of calcite, dolomite, quartz, kaolinite, illite, chlorite and illite-vermiculite mixed layers and reveals the absence of smectite. The results obtained from a faunal study carried out on ammonites make it possible to develop a local biostratigraphic scale which is correlated to the standard Mediterranean scale.