Late Precambrian and early Paleozoic sedimentary, volcanic, and plutonic rocks of the Avalon Peninsula, Newfoundland, represent the most easterly flank of the Appalachian folded belt. The oldest rocks, which form the Harbour Main Group, are predominantly subaerial basaltic to rhyolitic volcanic rocks and intercalated tuff and sedimentary beds. These volcanics, which are cut by Holyrood granitic rocks 574 ± 11 m.y. old, are overlain unconformably by about 7,000 ft of siltstone, slate, and graywacke of the Conception and equivalent Connecting Point Groups. In the west, Bull Arm volcanic rocks low in the Musgravetown Group overlie Connecting Point rocks. North-trending horsts were formed south and east of Conception Bay and also in the northwest part of the Avalon Pen nsula in later Precambrian time, and contributed detritus to the intervening shallow basin and probably to the eastern flank of the Avalon Peninsula. In the easternmost part of the Avalon Peninsula, the Cabot Group is about 15,000 ft thick and is believed to be lithostratigraphically equivalent to the arkosic Hodgewater Group, which was deposited in shallow water in the central basin. The latter group changes facies westward across the regional NNE structural trend, and forms the more clastic beds of the Musgravetown Group; these clastic sediments were deposited on Bull Arm volcanic rocks in a dominantly deltaic environment. The arkosic rocks in and west of the Central Avalon basin are overlain with discordance by thin beds of white quartzite of the Random Formation, which is overlain by Lower Cambrian shale and limestone containing fossils of "Atlantic" affinity. On the eastern horst, however, Lower Cambrian beds lacking white quartzite lie with angular unconformity on rocks of the Harbour Main and Conception Groups, and lie nonconformably on the Holyrood Granite.