S U M M A R Y:The time and duration of igneous activity in the separate component areas of the British Tertiary Igneous Province (BTIP) are investigated using radiometric dates that pass internal consistency tests, used in conjunction with palaeomagnetic polarities and the available stratigraphic information.Reliable results are available only for the currently subaerial parts of the Province. It is found that activity occurred within the approximate interval 63-52 Ma, with most activity at about 59 Ma. Later activity was predominantly of acid magmas though basic rocks preponderate in the province as a whole. The types and span of igneous activity in the separate areas followed no common pattern but areas of more complex geology tend to have had a longer span of activity.Magnetic polarities are predominantly reversed, with all the lavas having this polarity. This predominance is probably largely, but not entirely, due to reversed polarity intervals being longer than normal ones at this time, and with much activity occurring in a single reversed interval. The sequence of polarities found within the BTIP cannot be fully reconciled with the polarity timescales of either Harland et at. (1982) or Berggren et al. (1985).

Evidence is presented from Guernsey, Iceland, Ireland, and Scotland that basic and acid magmas have commonly come together. Examples of this are found in surface extrusions and tuffs and in intrusions of various sizes formed at different depths. The basic magma has commonly been chilled against the acid, and the relationships indicate that the acid component was highly mobile; these two facts are thought to be intimately related, in that the mobility of the acid magma is due to the transfer of heat from the basic magma. Consequences discussed include the question of the relative age of contiguous intrusions where basic rock is veined by acid: the age-sequence may be the reverse of that normally postulated. The possibility is also considered that basic magma may occasionally be necessary for the uprise of acid magma to high crustal levels.

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Synopsis There have been a number of recent papers on ‘giant dykes’, i.e. dykes exceeding 100 km in length. This paper extends the known length of the Tertiary (Palaeogene) Cleveland Dyke (CD) by about 64 km, giving a total proven length of c. 260 km. The results of a detailed investigation using ground magnetic surveys have discovered 26 additional exposures in southern Scotland, allowing magnetic, petrographic and geochemical measurements and magnetic modelling to be carried out, resulting in one of the most detailed studies of a ‘giant’ dyke. The structure of the CD complex in the Southern Uplands of SW Scotland has been influenced by pre-existing structures, in particular faults and granitic intrusions. It is chemically heterogeneous without systematic trend along its length. This heterogeneity, plus segmentation along strike, numerous offsets, in particular one of 5 km along the Water of Ken, en-échelon structure, narrow width at many localities (2.3–0.8 m) with lack of baking of adjacent country rocks, all argue strongly against lateral intrusion from the Mull intrusive complex, as has been proposed. Instead, aeromagnetic surveys and wide-angle seismic and gravity surveys make it probable that the dyke relates to a regional magma reservoir beneath the Southern Uplands.