The purpose of this paper is to give a brief description of the stratigraphy of the Lower and Middle Ordovician formations of the St. Lawrence Lowlands together with a short summary of the historical geology of the time. The basis for the paper is the literature on the subject, as the author's personal investigation has been restricted to one summer's work in a small section of the Lowlands. For the purpose of this description, the limits of the area are first defined and the region is then subdivided into three parts. There follows a description of the Beekmantown formation in which the problem of the Potsdam and Nepean sandstones is discussed. Afterwards, the historical geology of the epoch is given in resume. The Chazy formations are in turn examined, and the historical geology of the period is supplied. The more complex problem of the Black River, Trenton, and Utica stratigraphy is then analyzed. Changes in facies and thicknesses of contemporaneous formations in different parts of the Lowlands are studied with particular reference to the work of Raymond, Kay, Okulitch, Clark, and Wilson. An attempt to correlate these formations is made, but, owing to the complexity of the problem and the small amount of information available on many areas, the picture is left incomplete. The historical geology of this third marine invasion covering the St. Lawrence Lowlands from Black River to Utica time, is reviewed. The bibliography does not pretend to be exhaustive. It contains works on the stratigraphy of the Lower and Middle Ordovician Period and some works dealing with the palaeontology have also been included as a useful complement to the stratigraphy. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The Saluda Formation, a lithologically distinct but spatially variable unit, is mappable in the subsurface. Lithologic characters such as dolomitic and laminated finegrained carbonates, paucity of fossils and terriginous detritus, birdseye structures, and intraclasts distinguish the formation and aid in interpreting the depositional environment. Similarities of features for modern and ancient carbonate tidal-flats and those of the Saluda suggest a tidal-flat environment of origin for the formation. Lateral and vertical lithologic relationships with contiguous formations record spatially variable but time transgressive subenvironments of deposition for the tidal-flat complex and surrounding sea.

The Middle-Ordovician Simpson group, deposited in a shallow epicontinental sea, is composed of a sequence of interbedded limestone, shale, and smaller amounts of sandstone. By means of electric and radioactivity logs the group is divided into eight zones traceable across Pecos County, except for its absence, over the Fort Stockton uplift area, caused by post-Simpson erosion. Each zone thins persistently eastward toward the Texas Peninsula, which separated the West Texas basin from the Oklahoma basin of deposition. A northwestward increase in sand content of each zone indicates that the source area for the sand was to the northwest. In Pecos County oil is produced from the Simpson at present in only a small north-central area. / text

The Ordovician rocks of the Bail Hill area, Sanquhar, South Scotland : volcanism and sedimentation in the lapetus Ocean. by M. J .McMurtry. The Bail Hill area lies in the "Northern Belt" of the Southern Uplands and contains sedimentary and volcanic rocks of Llandeilo/Caradoc age. The sedimentary succession has been divided into four formations - the Glenflosh, Kiln, Spothfore and Guffock Formations. The Glenflosh and Guffock Formations are mainly arenaceous and were largely deposited by turbidity currents flowing to the southwest. They typically consist of Tae units. The fine-grained sandstones and siltstones of the Kiln Formation were also deposited by turbidity currents. Parallel-laminated units in the lower part of the formation represent "overbank" deposits, whilst lenticular- bedded units in the upper part are interpreted as channel- mouth deposits. Bouma sequences are not common in these lithologies. The Spothfore Formation consists of rudites deposited by a variety of sediment and fluid gravity flows close to a feeder system. The petrography of greywackes within the formations shows no significant variation across strike, in contrast to successions studied to the southwest. The provenance of detritus in the sediments is believed to be the north-westerly Laurentian continent with a significant but variable intrabasinal contribution of sedimentary and volcanic debris. A stratigraphic succession for the area is proposed, although graptolite evidence for the relative ages of the formations is equivocal. A new stratigraphic unit, the Bail Hill Volcanic Group, is proposed and the petrography and field relations of the subdivisions within this group are discussed. Field relations suggest that the Bail Hill volcano was a composite, central-type structure. Early basaltic lavas were succeeded by more differentiated lithologies and pyroclastic activity increased with time. The mineral and whole-rock chemistry indicates the Bail Hill Volcanic Group has differentiated along the sodic alkaline series (alkali basalt - hawaiite - mugearite - trachyte). Gabbroic and dioritic xenoliths in the extrusive rocks are believed to be fragments of a large sub-volcanic intrusive mass that underlay the Bail Hill volcano in Ordovician times. The volcanic pile as a whole has undergone metamorphism to zeolite facies grade. Higher grade assemblages in the xenoliths suggest that they were hydrothermally altered prior to incorporation in the extrusive rocks. It is concluded that the Bail Hill Volcanic Group represents the remnants of a seamount within the Iapetus Ocean, whilst the sedimentary rocks record the transition from the abyssal plain into a Lower Palaeozoic trench. Northwesterly subduction of the lapetus Ocean crust resulted in the accretion of the Bail Hill area on to the facing edge of the northwesterly Laurentian continent.

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Continuing activity in the Upper Ordovician mudrocks of the Montreal area for manufacturing purposes and construction encouraged this study of their weathering behavior in relation to composition and microstructure. / The mudrocks are grouped into four primary lithological divisions based on clay contents. Illite, chlorite and small amounts of mixed-layer clay minerals are present in similar proportions throughout all microlithologies. The distribution of calcium and magnesium in X-ray scanning images indicates that carbonates are present as silt grains rather than as cement. The specific surface of the mudrocks ranges from 2 to 20 m('2)/g for nitrogen and from 15 to 40 m('2)/g for water vapor. / The higher degree of susceptibility to moisture of the more clay-rich facies is attributed to their greater parallelism of microstructure, the presence of mesopores (2-50 nm) and less compacted packing. Changes in apparent cohesion at the menisci of capillary held water in the irregular network of passages appear to occur as cycles of adsorption and desorption proceed, resulting in the opening and closing of cracks and intermittent crack development.

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