Don't encourage him! By Robert I assume you mean Robert the Clark.

True, fossils are vastly more likely in sediments than in a hunk of say, intrusive gabbro. That does not imply that because a rock is sedimentary, it is at all likely to contain fossils.

But boy that little stack looks sedimentary. At least repeated ash falls or something like that, and even if pyroclastic, it still raises the question of why relatively flat-lying fine layers of ash in Home Plate, but not elsewhere. I really am eager for an APXS or even TES -- is this just more basalt, just in layered ash instead of solid rock? Or do we have sulfates and all that jazz here too?