The Silver Inca Mask is the twenty-third treasure that can be found in Uncharted: Drake's Fortune.
The Silver Inca Mask is found in Chapter 6 - Unlocking the Past. When you first enter this area there will be a fight with some pirates. After you defeat the pirates, head up the stairs and you'll see two very large trees with white bark. There is one large tree growing right by the stairs, and the other one is growing on the mossy green ledge. The treasure is behind the tree growing on the mossy green ledge.