Which preposition to use with concretions

of Occurrences 18%

The rock is not one mass of stone, but a concretion of pebbles and earth, so firm that it does not appear to have mouldered.

as Occurrences 1%

That the author of the Religio Medici, mounted upon the airy stilts of abstraction, conversant about notional and conjectural essences; in whose categories of Being the possible took the upper hand of the actual; should have overlooked the impertinent individualities of such poor concretions as mankind, is not much to be admired.

Which preposition to use with  concretions