A stratigraphic analysis of the Queenston (Ordovician) and Medina (Silurian) Formations of southern Ontario, based upon detailed measurement of seventeen sections, shows that these formations constitute a regressive sequence, with the source of terrigenous sediment lying to the southeast in New York State or Pennsylvania. The Whirlpool and Manitoulin Members of the Medina Formation are a marine wedge, which thickens to the northwest, within the predominantly regressive sediments.

The shales of the Queenston and Medina Formations are red or green, silty, calcarous and predominantly illitic in composition. X-ray studies, together with eight whole rocks and two <2µ fraction analyses for major elements indicate that the shales are composed of three major fractions: (i) a silty fraction composed mainly of quartz and heavy minerals, (ii) a carbonate fraction composed mainly of calcite (Queenston) or dolomite (Medina) and (iii) a fine grained

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quartz-illitic clay fraction. The percentages of the first two fractions vary antipathetically while the third fraction remains relatively constant, and as a result CaO, MgO, MnO and CO_2, which are strongly intercorrelated, are negatively correlated with SiO₂ and TiO₂. Boron, Cr, Mn, Ga, V, Ti, Ni, Zr, Sr and Ba have been determined for fifteen whole rock samples and fifty-two fine fractions, using a precise emission spectographic technique in which samples are burnt with a Pd-graphite internal standard in an argon-oxygen atmosphere. Positive trace element correlations are found between Cr and V, Cr and Zr, V and Zr, V and Ni, V and Ti, Zr and Ni and negative correlations between Cr and Mn. Probably B, Cr, Ga, V and Ba are found predominantly in the fine fraction, Zr and Ti are predominantly associated with the silty fraction and Mn and Sr are predominantly in the carbonate fraction.

For those shales low in iron, analysis for boron was possible and indicates that the Queenston was probably deposited in water of about 18 p.p.t. salinity while all the Medina shales (with the possible exception of those from the Thorold Member at De Cew Falls) were deposited in sea water of normal salinities.

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