The ophiolitic rocks of the "Préalpes Romandes" can be divided into three main groups. In the first one we find sill-like intrusions out-cropping in the Fluhmad-Zweisimmen region. Rocks of the second group are of submarine volcanic origin and are found as pillow lavas and pillow breccia near Saanen (Les Fenils) and near the pass of Jaun. In this locality the fragments of pillows are associated with fragments of older rocks, for instance gneissic granite. The third group is made of rocks of secondary detritic origin (breccias of Trom and Lac Lioson).

The petrology of the ophiolitic rocks has been studied at hand of numerous thin sections and chemical analysis. They exhibit a typical spilitic trend with high soda values and little lime. Their mineralogical composition is rather uniform : mainly albite and chlorite with some augite in the intrusive rocks; accessory minerals as hematite, titanite and calcite are abundant. However these rocks can be classified into different types using criteria of texture. They exhibit a main fabric of albite lathes and a ground mass, mostly of chloritic composition. Some types are porphyritic with albite phenocrysts. The textures are named after the various patterns of the main fabric (intersertal, intersertal divergent, arborescent, spherulitic, variolitic).

The author is of the opinion that the spilitic composition is primary. The shales forming the wall-rock of the sill have undergone a sodic metasomatism near the intrusive contact with development of albite crystals.

The age of formation of the ophiolite has been ascertained by K/A age determination on augite of one of the sill; a value of 76 m. y. has been found in good agreement with the stratigraphie position of this formation. (Upper Campanian to Upper Maestrichtian.) A lead alpha determination on the zircon of the gneissic granite found in blocks in the Jaunpass breccia has given an age of 342 m. y. indicating that this rock is not related to the ophiolites but belong to the hercynian magmatic cycle.

The close association of the Zweisimmen-Flühmad sills and of the Fenils pillow lava with radiolarian black shales of the Nappe de la Brèche suggests that the ophiolites have been intruded in the Flysch sediment of this unit and that in some places the magma has flowed on the bottom of the Flysch sea.