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The Rb-Sr whole-rock method has been used to test stratigraphic correlations in the Penobscot Bay area, eastern Maine. The Castine Volcanics, Thorofare Andesite-Vinalhaven Rhyolite, and Cranberry Islands Volcanics are consanguineous and were probably formed 390 ± 5 m.y. ago; this Devonian age is supported by new paleontologic evidence. Calcite-bearing rocks from these three formations usually yield spurious results and are considered unsuitable for Rb-Sr work; careful petrographic and x-ray studies are necessary for selection of suitable samples.

Rb-Sr ages of Cambrian-Ordovician (or possibly older) rocks were determined for the Ellsworth Schist that unconformably underlies the Devonian Castine Volcanics and Middle Silurian Ames Knob Formation on the east side of Penobscot Bay. The part of the Penobscot Formation on the west side of Penobscot Bay, informally called Benner Hill by C. V. Guidotti, yielded an age of 460 ± 10 m.y., in good agreement with new fossil evidence for a Middle to Late Ordovician age. Silurian or older ages were obtained for the Penobscot, Appleton Ridge, and Battie Formations; however, the overlap in error limits leaves their relative ages in doubt.

Retrograded high-grade rocks in a horst in Islesboro Township have been dated as Precambrian; a muscovite pegmatite cutting these rocks yields a late Precambrian mineral date. These are the first Precambrian rocks reported from Maine. The major faults mapped by D. B. Stewart on either side of Islesboro Township occurred before Middle Devonian time, according to dates determined for the stratigraphic units and for the plutons that cut and seal the faults.

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