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Synopsis Basic dykes showing cross-cutting relations to one another, and different tectonic and metamorphic histories, but having similar chemical composition, are interpreted as having been emplaced during the Inverian and Laxfordian orogenies respectively.

Letters to die Editors LEWISIAN CHRONOLOGY SIRS, The discussion by Park(1970) of aspects of Lewisian chronology centres around an assessment of the general review and interpretative papers of Bowes (1968d, b, 1969) which dealt mainly with the Lewisian of the Northwest Highlands of Scotland. However, neither much of the original evidence on which these syntheses are based, nor other significant information relating to the Lewisian of the Northwest Highlands and the Outer Hebrides (e.g. Bennison and Wright 1969, pp. 41-6; Hopgood 1964) are taken into account. In addition, a considerable amount of information now published, or being published, was not available to Dr Park. This is particularly the case for the central region of the Lewisian of the Northwest Highlands (cf. Bowes 1969, figs. 1, 2; Park 1970, fig. 1) which includes the type areas for the Scourian and Inverian orogenies (' cycles' of Bennison and Wright 1969) and for the Scourie dyke. The evidence presented by Bowes and Khoury (1965) that basic minor intrusions were emplaced during both the Inverian and Laxfordian orogenies is rejected by Park (1970, p. 388) together with the relevant chronology of the central belt (Bowes 19686, table 1; 1969, table 1) as if this were the sole evidence available (cf. Park 1970, table 1, note 2). Without the detailed evidence of Khoury (1965, 1968a, b) and Barooah (1967), hypothetical correlations are made of events separated by Bowes (19686, 1969) and a * simplified' chronology is presented by Park (1970, table 2). But the field evidence for the district between Kylesku and Scourie (Bowes 1969, fig. 2), based on systematic mapping, is clear. One set of basic dykes was emplaced during the Inverian orogeny about 2190 m.y. ago (cf. Park 1970, p. 381). This dyke set is deformed by the pre-Torridonian fold belts which are generally accepted as representing Laxfordian deformation (F of Park 1970, table 1). Another set of basic dykes intrude the deformed dykes (cf Bowes and Khoury 1965) and cross-cut the fold belts. Their emplacement during the Laxfordian orogeny is shown by their deformation by late Laxfordian movements. * Correlation ' of these different suites on the assumption that there is only one suite of basic dykes in the central region (' the so-called Scourie dyke suite'-Park 1970, p. 387), the extension of this assumption to include basic intrusions in the southern region as members of the ' main Scourie dyke suite ' (Park 1970, p. 388), and the related assumption that a strong foliation formed immediately preceding basic dyke injection in a sequence is the dominant Inverian foliation (Park 1970, p. 388) explains much of the confusion and the differences in interpretation of Park (1970) and Bowes (19686, 1969). Evidence against these assumptions is not confined to the central belt of mainland Lewisian. Both Dash (1967; 1969, p. 349)

Synopsis Complex patterns, of folds with horizontal axes perpendicular to the foliation in gneiss, occur on vertical foliation surfaces. Similar complex patterns, of corresponding folds with vertically disposed axes, occur on horizontal surfaces. The vertical and horizontal surfaces represent cross-sections near to or within the steep limb of N.W.-S.E. monoclinal fold belts which cut rocks of a crystalline basement. Heterogeneous flow appears to have been operative in the formation of the complex folds. Local intensely developed small-scale folding along narrow N.W.-S.E. fold belts possibly resulted from their coincidence with pre-existing N.W.-S.E. master joints in the basement.

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