Weathered Lava/basalt Fossil - Scallop

[Wrangellian]

But at least our chronostrat divisions are supposed to be based on something tangible, like the end of the last Ice Age/etc, but that date sounds more arbitrary for defining 'fossil' than for defining the (st)age. Is it because the Holocene is synonymous with 'Recent', and recent shells/etc are not fossils? And as soon as they define the Anthropocene or any new stage, then the Holocene fossils will become true fossils?

[Tom Dill]

I have a very similar looking specimen from Beverly Beach north of Newport Oregon. My Pecten is in a dense, dark matrix, but it is a concretion and definitely not basalt. It is neat that you have both halves!

As an aside, volcanic rocks often contain fossils. They are usually in volcaniclastic rocks, the broad category of fragmented volcanic rocks. Most of the fossils in volcanic rocks I am aware of are in mudflow deposits, ash fall tuffs, and volcanic sediments re-deposited by rivers. But a significant number are from ash flow tuffs (often carbonized wood, but a few vertebrates as well) and from lava flows (mostly carbonized or molds of plant materials). Many truly great fossil deposits occur in volcaniclastic rocks or are associated with volcanism (Ashfall State Park of Nebraska, Florissant National Monument of Colorado, and Hagerman Fossil Beds of Idaho, to name just a few).

[Wrangellian]

There is a spot on the SE coast of this Island which apparently has volcanic rocks (of which kind I don't recall exactly) that had reached a beach during the Paleogene and encased gastropods which are now cavities (molds) in the rock.

I would call these true fossils even though the shell itself is gone. See: http://en.wikipedia.org/wiki/Ichnofossil (scroll down to "Confusion.."