Abstrack The Cambrian-Ordovician sedimentary succession of northwest Scotland is interpreted to be the deposits of a gradually transgressing sea. The basal unconformity of the Cambrian-Ordovician oversteps Lewisian metamorphic and Torridonian sedimentary rocks, and is perceived as the erosional front of the transgression. A revised composite thickness of more than 4,000 ft is offered for the Eriboll Sandstone, the An t-Sron Formation, and the Durness Carbonate, which successively overlie the unconformity. The progression of the miner- alogic and textural parameters of the succession toward greater maturity appears to reflect an increasing isolation from detrital sources rather than increasing water depth. The Eriboll Sandstone includes a “Lower” cross-bedded member, composed of subarkose and orthoquartzite, and the Pipe Rock Member, composed of orthoquartzite. Numerous tubelike trace fossils normal to the bedding in the Pipe Rock Member are the result of the burrowing of suspension-feeding organisms. The Fucoid Beds Member and the Serpulite Grit Member are convenient subdivisions of the An t-Sron Formation. O lenellus from the Fucoid Beds Member provides perhaps the only stratigraphic index of the succession. The discovery of abnormal quantities of potash in the “Fucoid Beds” in 1962 raises the problem of genesis. The thick succession of carbonate rocks which composes most of the Cambrian-Ordovician sequence is mainly biochemical, but possibly includes some chemical carbonate deposits of a shelf or platform environment. The oolitic and algal stromatolitic units interspersed throughout much of the carbonate sequence suggest that most of the carbonate rocks are of shallow-water deposition, and that the rate of depositio