A 5-km section of basalt lavas exposed at Tr¿llaskagi on the western flank of the Eyjafj¿rdur structural dome in northern Iceland has been subjected to K--Ar age and paleomagnetic analysis from which detailed results are tabulated (34 dated lavas, 455 paleomagnetic directions). The upper 4 km of the sequence was erupted between about 11 and 9 m.y. ago. The rate of growth of the lava pile changed about 9.5 m.y. ago. Growth rates in the lower part were of the order of 1 km/m.y., whereas growth rates in the upper part may have been as high as 4 km/m.y. A 1.8 km-thick normal polarity interval in the middle part of the section is correlated with marine magnetic anomaly 5. A good match is observed between the polarity sequence of the Tr¿llaskagi section and the marine magnetic polarity pattern above and below anamaly 5. Regression analysis of the K-Ar data gives an age of 10.5 m.y. for the older boundary of anomaly 5 but a questionable o.3-m.y. age for its younger limit. Structural characteristics of the lava pile indicate that eruption of the sequence was associated with a now extinct rift axis in the western part of northern Iceland. Low-lattice virtual poles are more prevalent in this survey than in two comparable recent surveys on younger Icelandic lavas and much more common than may be expected from a Fisherian distribution. There is no evidence to confirm that such transitional poles are predominantly due to nondipole fields. No significant difference was found in overall strength or stability between normal and reverse geomagnetic fields, as recorded in over 1000 lava flows in Iceland.