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 mineralogic 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. Olenellus 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 at the carbonate sequence suggest that most of the carbonate rocks are of shallow-water deposition, and that the rate of deposition therefore approximately paced the rate of subsidence or transgression.

Diagenesis has affected greatly the character of the succession. Pore space is virtually absent from the sandstone, having been reduced principally by pressure solution and reprecipitation. Diagenesis in the carbonate rocks progressed as (1) recrystallization, (2) dolomitization, (3) silicification, (4) calcitization, and (5) dolomitization.

Recognition of this paragenetic order solves several problems in the petrologic and field relations of the limestone, dolostone, and chert.