Limonite Concretions Which Occur in a Pleistocene conglomerate near Iowa City, Iowa, are divided into two groups according to origin. The concretions of the first group, for which the term parogenetic is introduced, were incorporated in the conglomerate as clastic units, having been formed in a pre-existing rock. The parogenetic concretions are of two types, those with a limonite shell which is filled with argillaceous calcium carbonate and those whose shell is nearly or entirely empty. The second group consists of concretions which originated when gravel was cemented to form the conglomerate, and hence are syngenetic. The latter concretions are irregular masses of glacial till around which ferric hydroxide has cemented the materials of the conglomerate into a shell or crust.