Too much cast down to speak, we stumbled down the dark tunnel and made our way back to the camp.

One incident occurred, however, before we left the gorge, which is of importance in view of what came afterwards.

We had gathered in a little group at the bottom of the chasm, some forty feet beneath the mouth of the cave, when a huge rock rolled suddenly downwards—­and shot past us with tremendous force.  It was the narrowest escape for one or all of us.  We could not ourselves see whence the rock had come, but our half-breed servants, who were still at the opening of the cave, said that it had flown past them, and must therefore have fallen from the summit.  Looking upwards, we could see no sign of movement above us amidst the green jungle which topped the cliff.  There could be little doubt, however, that the stone was aimed at us, so the incident surely pointed to humanity—­and malevolent humanity—­upon the plateau.

We withdrew hurriedly from the chasm, our minds full of this new development and its bearing upon our plans.  The situation was difficult enough before, but if the obstructions of Nature were increased by the deliberate opposition of man, then our case was indeed a hopeless one.  And yet, as we looked up at that beautiful fringe of verdure only a few hundreds of feet above our heads, there was not one of us who could conceive the idea of returning to London until we had explored it to its depths.

On discussing the situation, we determined that our best course was to continue to coast round the plateau in the hope of finding some other means of reaching the top.  The line of cliffs, which had decreased considerably in height, had already begun to trend from west to north, and if we could take this as representing the arc of a circle, the whole circumference could not be very great.  At the worst, then, we should be back in a few days at our starting-point.

We made a march that day which totaled some two-and-twenty miles, without any change in our prospects.  I may mention that our aneroid shows us that in the continual incline which we have ascended since we abandoned our canoes we have risen to no less than three thousand feet above sea-level.  Hence there is a considerable change both in the temperature and in the vegetation.  We have shaken off some of that horrible insect life which is the bane of tropical travel.  A few palms still survive, and many tree-ferns, but the Amazonian trees have been all left behind.  It was pleasant to see the convolvulus, the passion-flower, and the begonia, all reminding me of home, here among these inhospitable rocks.  There was a red begonia just the same color as one that is kept in a pot in the window of a certain villa in Streatham—­but I am drifting into private reminiscence.

That night—­I am still speaking of the first day of our circumnavigation of the plateau—­a great experience awaited us, and one which for ever set at rest any doubt which we could have had as to the wonders so near us.