The NW peninsula of Iceland is built mainly by tholeiitic, low K flood basalts that show a regional southeasterly dip, generally of less than 10¿. From the lavas mapped out drilled for paleomagnetic measurements (a total of 1261 flows), two continuous composite sections were constructed, one on the western side of the peninsula 4055 m thick comprising 456 lavas and the other adjacent to the east coast, 3165 m thick comprising 396 lavas, K-Ar age measurements were made on more than 70 lavas distributed throughout the two sections. These two lava sequences area about 90 km apart along strike, and although direct stratigraphic correlation is not possible between them, the evidence indicates that the eastern section is stratigraphically younger than that in the west, with some overlap. The results show that the lava sequences were erupted over an interval extending from about 14 to 8 Ma ago in the middle to late Miocene. Remarkably linear relations are found between the measured age and cumulative stratigraphic thickness for both sections, indicating rather uniform local rates of growth of the lava piles. For the western composite section a growth rate of 1820 m/Ma is found, contrasting with a much lower growth rate of 670 m/Ma for the eastern sequence. The average thickness of individual lavas in both sections is close to 8 m. The average time between eruptions of lavas in the western section is 5000 years, compared with a value of 12,000 years for the eastern section. The regional geology together with our measurements are interpreted in terms of crustal accretion processes by which the NW peninsula of Iceland was constructed by eruption of lavas from a spreading center of northeasterly strike. It is not clear whether the differences in rates of growth of the lava pile in the western and eastern sections are because of variation in eruptive activity has varied along the strike of the spreading center. Paleomagnetic measurements on the lavas provide a detailed history of polarity of the geomagnetic field during the interval 14-8 Ma ago. For the eastern composite section there is excellent and unambiguous correltion of the observed polarity pattern with the marine magnetic anomaly record. From these data we obtain an age of 9.6 Ma for the younger boundary of marine magnetic anomaly 5 and derive a revised polarity time scale for part of the Miocene which gives ages for individual polarity interval boundaries that are more than 5% greater than in recently published polarity time scales. In the revised time scale anomaly 5 time extends from 9.6 to 11.1 Ma ago.