I am in sympathy with many of the views expressed by Westoll et al. in their letter on the Silurian-Devonian boundary published in vol. 127 part 3 of the Journal, and in particular with their contention that the claims of a boundary at the base of the M. ultimus Zone were not given sufficient consideration by the Silurian-Devonian Committee of the International Commission on Stratigraphy. Nevertheless, this committee finally voted almost unanimously in favour of a boundary at the base of the M. uniformis Zone and I believe that a world-wide referendum of interested geologists would also show overwhelming support for this boundary. It is not popular in Britain because of the extreme difficulty in recognising the equivalent level in our continental development but it is not to be expected that a chronostratigraphic boundary will be an easily mappable horizon throughout the world. We tolerate in this country a Permian-Triassic boundary which lies somewhere in the middle of our New Red Sandstone and we must now come to accept a Silurian-Devonian boundary which can only be vaguely positioned within hundreds of feet of poorly exposed and sparsely fossiliferous Old Red Sandstone. Stratigraphical boundaries are essentially arbitrary and the most important criterion is international uniformity in usage. This uniformity can only be achieved by the sacrifice of individual and national preferences and I consider that the interests of stratigraphy are best served in this instance by acquiescence in the international decision on the M. uniformis boundary.

My support for the