crinoid columnals

Mississippian crinoid columnals within a quartz geode from the Mississippian Harrodsburg Limestone in Indiana. Organic material deposited on the sea bottom often became a focus for the accumulation of silica dioxide (SiO2) through a complicated geochemical process. As more and more SiO2 came out of solution at the organic focus, the geode "shell" of quartz formed and grew in size. In most cases, the original focus of the geode growth - a fossil - is obscured; but, occasionally the fossil itself, or a pseudomorph, can be seen.