We were able to stretch a string in a straight line from the lowest point we reached, through the interstices of the blocks of stone, and up to the entrance-hole, and this measurement gave 50 feet.  Considering the inclination of the upper ice-floor, and the sharpness of the angle between the wall of ice and the line of our descent to this lowest point, I believe that 50 feet will fairly represent the height of the ice-wall from this point to the foot of the slope from the upper wall; so that 72 feet will be the whole depth of ice, from the top of the third ladder to the point where our further progress downwards was arrested.  The correctness of this calculation depends upon the honesty of Mignot, who had charge of the farther end of the string, and was proud of the wonders of his cave.  A dishonest man might easily, under the circumstances, have pulled up a few feet more of string than was necessary, but 50 feet seemed in no way an improbable result of the measurement.

[Illustration:  SECTION OF THE LOWER GLACIERE OF THE PRE DE S. LIVRES.]

The ice was as solid and firm as can well be conceived.  The horizontal bands would seem to prove conclusively that it was no coating of greater or less thickness on the face of a vertical wall of rock, an idea which might suggest itself to anyone who had not seen it, and I think it probable that the amount of ice represented in the section of the cave is not an exaggeration.  We were unable to measure the whole length of the wall in the lower cave, from the large number of blocks of stone which had fallen at one end, and lay against its face.  Probably, from the nature of the case, it was not so long as the 72 feet of wall above; but we measured 50 feet, and could see it still passing on to the right hand as we faced it.  In trying to penetrate farther along the face, I found a wing of the brown fly we had seen in considerable abundance on the ice in La Genolliere, frozen into the remains of a column.

There was so very much to be observed on all sides, and the measurements took up so much time, owing to the peculiar difficulties which attended them, that I did not examine with sufficient care the curious floor of ice through which we cut our way to the lower cavern.  Neither did I notice the roof of the cavern thus reached, which may be very different from the shape of the upper surface of the floor composing it.  If the ice-wall goes straight up, and the roof is formed of the ice-floor alone, then it is a very remarkable feature indeed.  But, more probably, the lower wall leans over more and more towards the top, and so forms as it were a part of the roof.  It is possible that, as the wall has grown, each successive annual layer has projected farther and farther, till at last some year very favourable to the increase of ice has carried the projection for that year nearly to the opposite stones, and then an unfavourable year or two would form the foot of the upper wall.