 The "solid bodies within solids" that attracted Steno's interest included not only fossils, as we would define them today, but minerals, crystals, encrustations, veins, and even entire rock layers or strata.He published his geologic studies in "De solido intra solidum naturaliter contento dissertationis prodromus", or "Preliminary discourse to a dissertation on a solid body naturally contained within a solid" in 1669.This book was his last scientific work of note.