public Mantle(IDrillBit drillBit, ILayer nextLayer)
 {
     _drillBit  = drillBit;
     _nextLayer = nextLayer;
 }
 public Core(IDrillBit drillBit)
 {
     _drillBit = drillBit;
 }
 public Crust(IDrillBit drillBit, ILayer nextLayer)
 {
     _drillBit  = drillBit;
     _nextLayer = nextLayer;
 }