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Series and Stages of the Silurian System Charles H. Holland The Subcommision on Silurian Stratigraphy of the IUGS has recently completed an eight-year programme resulting in the formal adoption of four standard Series and seven standard Stages for the Silurian System. This review (reprinted from the June 1985 issue of Episodes: International Geoscience Newsmagazine devoted almost entirely to the work of the International Commission on Stratigraphy) summarises the background to the chronostratigraphical subdivisions now established as units of global applicability. With one exception, the illustrations in the original article are omitted. The author, Prof. C. H. Holland, was Chairman of the Subcommission on Silurian Stratigraphy of the International Union of Geological Sciences from 1976-1984. The statement recording the formal ratification of the Series and Stage divisions of the Silurian System, given as an 'Appendix' to Prof. Holland's paper, is taken from an article by Dr. M. G. Bassett in the same issue of Episodes. The top of the Silurian System, that is to say the base of the Devonian, is taken at a point coincident with the base of the Monograptus uniformis Biozone is a boundary stratotype section in the Barrandian area of the Prague Basin, Czechoslovakia. Decisions concerning this boundary were approved by the Commission on Stratigraphy as long ago as 1972, and very full details of the whole subject are given in Martinsson (1977). The base of the Silurian System (the Ordovician/ Silurian boundary) is now agreed at a point coincident with the base of the Parakidograptus acuminatus Biozone, in a boundary stratotype at Charles Lapworth's classic locality of Dob's Linn in the Southern Uplands of Scotland (Cocks 1985). Since the International Geological Congress at Sydney in 1976, the Subcommission on Silurian Stratigraphy has been engaged upon a programme to settle the internal chronostratigraphical divisions of the Silurian System. The work was completed, as far as intended, in time for submissions (supported by substantial majority votes from Titular Members of the Subcommission) to be made at the Moscow Congress in August 1984 (see Holland 1984). These decisions have now been ratified by the Notes Commission on Stratigraphy (see Appendix). Much progress was made at an historic field meeting and other formal gatherings held in Britain in 1979. Decisions concerning the second and third of the four Series, into which it had been decided the Silurian System should be divided, were later ratified by the Commission on Stratigraphy. The Wenlock and Ludlow Series were each divided into two stages, respectively the Sheinwoodian and Homerian and the Gorstian and Ludfordian. The boundary stratotypes for the bases of these divisions were taken in the type Wenlock and Ludlow areas of the Welsh Borderland (see Holland 1982, for a summary). The Llandovery Series and its Stages In the case of the first (lowest) Series of the Silurian System, members of the Subcommission present at the British meeting in 1979 were not impressed with the type Llandovery area in southern Wales, for well documented boundary stratotype sections were not then available and graptolites appeared to be scarce. A working group was set up to investigate the Llandovery area in modern terms and, in particular, to examine the 'northern area' where forestry roads provide a veritable network of sections and where graptolites are more readily available. The base of the Llandovery Series, as it is now agreed it shall be called, together with the base of its Fig. 1 Members ot the bubcomnussion on Silurian Stratigraphy and of the Ordovician-Silurian Boundary Working Group at Ludlow, Welsh Borderland, April 1979.