White limestone in the Troy Formation has been recrystallised (by resubmergence after formation) and has occasionally been dolomitised: ie CALCITE (CaCO3) has been partially replaced by mineral DOLOMITE (CaMg(CO3)2).
The source of Magnesium is, of course, seawater.

A simple test using dilute acid will show that limestone reacts vigorously whereas dolomite reacts hardly at all