Now it remained to put their theory to the proof—­a task of no small difficulty.  Indeed, it took them three days of hard, continual work.  It will be remembered that the floor of the cave was cemented over, and first of all this cement, which proved to be of excellent quality, being largely composed of powdered granite, must be broken up.  By the help of a steel crowbar, which they had brought with them in the waggon, at length that part of their task was completed, revealing the rock beneath.  By this time Benita was confident that, whatever might lie below, it was not the treasure, since it was evident that the poor, dying Portuguese would not have had the time or the strength to cement it over.  When she told the others so, however, Meyer, convinced that he was on the right tack, answered that doubtless it was done by the Makalanga after the Portuguese days, as it was well known that they retained a knowledge of the building arts of their forefathers until quite a recent period, when the Matabele began to kill them out.

When at length the cement was cleared away and the area swept, they discovered—­for there ran the line of it—­that here a great stone was set into the floor; it must have weighed several tons.  As it was set in cement, however, to lift it, even if they had the strength to work the necessary levers, proved quite impossible.  There remained only one thing to be done—­to cut a way through.  When they had worked at this task for several hours, and only succeeded in making a hole six inches deep, Mr. Clifford, whose old bones ached and whose hands were very sore, suggested that perhaps they might break it up with gunpowder.  Accordingly, a pound flask of that explosive was poured into the hole, which they closed over with wet clay and a heavy rock, leaving a quill through which ran an extemporized fuse of cotton wick.  All being prepared, their fuse was lit, and they left the cave and waited.

Five minutes afterwards the dull sound of an explosion reached their ears, but more than an hour went by before the smoke and fumes would allow them to enter the place, and then it was to find that the results did not equal their expectations.  To begin with, the slab was only cracked—­not shattered, since the strength of the powder had been expended upwards, not downwards, as would have happened in the case of dynamite, of which they had none.  Moreover, either the heavy stone which they had placed upon it, striking the roof of the cave, or the concussion of the air, had brought down many tons of rock, and caused wide and dangerous-looking cracks.  Also, though she said nothing of it, it seemed to Benita that the great white statue on the cross was leaning a little further forward than it used to do.  So the net result of the experiment was that they were obliged to drag away great fragments of the fallen roof that lay upon the stone, which remained almost as solid and obdurate as before.