The Catskills are sedimentary rock, almost entirely conglomerates, sandstone and shale in alternating bands, and virtually all Devonian (~450 MYa). The sandstones and shales are fossil-poor, although the occasional shell can be spotted in the conglomerates, and imprints of soft-bodied creatures and worm casts have been found in the shales. You'll look a long time before spotting such a thing, though.
Since the conglomerates formed in a river delta in the Devonian Sea, there is all sorts of erratic material in them. I've seen a couple of nice nodules of jasper that I surely wasn't about to chip out. (Too much work, apart from being unlawful!)
The talus includes a large quantity of glacial till in which you could find all sorts of stuff that doesn't belong: granites and schists from the Canadian Shield, labradorite from the Adirondacks, rose quartz, pyrite, who knows what! It's all mixed together without rhyme nor reason because it's the junk that the glaciers left behind when they retreated.
There is reported to be a tiny outcropping of coal, no more than a few feet thick, on the north side of Slide near the summit. It's the same stratum as the deep Pennsylvania coal beds, uplifted in the orogeny that formed the Catskill plateau. Everywhere else, the glaciers scoured that stratum clean away.
Because the sandstone is so readily quarried and so uniform-grained, Catskill bluestone became widely used in the 18th and 19th centuries, and was sent as paving stone the world over.
If you really want to learn this stuff, get some of Robert Titus's books
, or join one of his hikes
with Mountain Top Historical Society - he leads a few each year.