Abstract

Dr C. S. Exley has sent the following written contribution. Tammemagi & Smith {Journal 1975 vol. 131, 415–27) make two points which call for comment. The first relates to the collection and presentation of some of their data.

When discussing sampling techniques (p. 418) the authors explain first that while they took fresh specimens of granite wherever possible, some weathered material was included in their collection, and secondly that the rocks were classified into (relatively) 'fresh' and 'weathered' categories. It transpires that they used a 'kaolinized' category too, but this was not mentioned.

Of the 56 granite samples listed in Supplementary Publication No SUP 18009, only 27 have been classified in this fashion and of the 50 used in the compilation of their table 1 and fig. 2 only 22 have been so classified; indeed the authors state that 'intermediate' and borrowed specimens were not classified at all. Since it is almost always possible to separate 'fresh' from 'altered' granites from SW England on a fairly arbitrary basis this is surprising but not necessarily important. What does seem to be important is that a rather significant conclusion, based on whether or not the rocks were altered, was reached by the authors to explain some of the results of their U and Th analyses.

The concentrations of these elements vary considerably and in table 1 standard deviations are given as 4·3 ppm for U and 5·7 ppm for Th. The scatter becomes very apparent in the histograms presented as fig.

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