This is formed by a rocklike accumulation of calcareous skeletons
.... and limetsone - were formed by accumulation of calcareous particles 
in the... [DOC] [TLE] Silurian : Overview - Department of Paleobiology - Smithsonian [PAR] ... and he noticed that they appeared to be distinct from both the older (lower) 
rocks that formed the Cambrian and the younger (higher) rocks of the Devonian.