First one has to determine if there ever was a treasure as described in the narration? When one considers all of the discrepancies and inaccuracies in the narration then it starts to become extremely doubtful, if not completely impossible. We know it was impossible for the alleged treasure to have been mined as detailed in the narration so if it came from somewhere else then where? Second, was the description of the treasure in the narration in actual weights as assumed or was it in British pounds, the later being the only possible way in which it could have been mined. If the later is true then suddenly we'd be speaking of a much smaller amount, one that would hardly be worth the investment and tome to recover.
Then there comes the possibility of an actual trust, such as a blind bank trust, the fact that Morriss was being made an executor of a trust with a ten-year term attached to it certainly making that a possibility. So cave? A pit? I seriously doubt that either were employed given that such a transferring of funds could have taken place at all without a lot of advanced planning. Neither a cave or a pit would be a secure location so it is extremely doubtful that either would have been risked.