Larger quantities of deposit may be conveniently collected by means of the dredge, which can be worked in any depth and brings up large stones, concretionary nodules or fossils, of the existence of which a sounding-tube could give no indication.
They occur (a) in crystalline igneous and metamorphic rocks as an original constituent, (b) in veins associated with igneous rocks, and (c) in sedimentary rocks either as organic fragments or in secondary concretionary forms.
In whatever form they were originally deposited they often suffer complete or partial solution and are redeposited as concretionary lumps and nodules, often called coprolites.
The di-calcium salt, Ca2H2(P04)2, occurs in a concretionary form in the ureters and cloaca of the sturgeon, and also in guano.
As small concretionary nodules, it occurs disseminated through sandstone at Kommern in the Eifel.