The central part of the Bucksport, Maine, topographic map area is underlain by a medium-textured, porphyritic, biotite granite, termed here the Mt. Waldo batholith. The age of the granite has not been determined but is probably either Devonian or Carboniferous. The pluton is roughly circular in plan, with a diameter of approximately 10 miles. Planar flow structures in the granite tending to curve parallel to the margins of the intrusion about the northern end stand steep to vertical. Within the intrusion, the planar flow structure averages about north in strike and vertical in dip. Linear structure is not developed. On the north, northeast, and south, the older metamorphic wall rocks, probably Paleozoic, are crosscut by the granite; elsewhere, the structure of the wall rocks tends to conform to the outlines of the intrusion. Close to the intrusion, the axes of the minor folds in the wall rocks are steeply inclined or vertical; farther from the margins, the pitch of minor folds decreases. The overlying rocks were perhaps pushed bodily upward by the intrusion.
Intrusives older than the granites of the Mt. Waldo batholith are both granitic and more basic. Both groups are of undetermined age. The earlier gneissic granites are foliated and locally show linear structure consisting of biotite streaking on the foliation planes. This type outcrops in small masses. Pegmatite and aplite dikes are present, some older and others younger than the batholith granites. Diabase dikes are the latest intrusions in the area.