This insect was slightly larger than a mosquito, and Ed suggested it might be a cranefly, so I explored that possibility when I got home to my computer. Sure enough, what we have here is a Winter Cranefly, one of the few insects we have that actually emerge from their larvae and mate in winter. I wonder what they find to eat in the frozen environment? Perhaps they don't eat at all, but simply find mates and breed and die.
Ed got back to me to tell me that the rock was quartzite, which is metamorphosed sandstone. Its fractured surface looks glassy, usually white to clear. Marble, he told me, is metamorphosed limestone and, here in the Adirondacks, usually has flat crystalline fracture surfaces .