In this manner, in early days, were formed those vast and prodigious layers of coal, which an ever--increasing consumption must utterly use up in about three centuries more, if people do not find some more economic light than gas, and some cheaper motive power than steam. All these reflections, the memories of my school studies, came to my mind while I gazed upon these mighty accumulations of coal, whose riches, however, are scarcely likely to be ever utilized. The working of these mines could only be carried out at an expense that would never yield a profit. The matter, however, is scarcely worthy consideration, when coal is scattered over the whole surface of the globe, within a few yards of the upper crust. As I looked at these untouched strata, therefore, I knew they would remain as long as the world lasts. While we still continued our journey, I alone forgot the length of the road, by giving myself up wholly to these geological considerations. The temperature continued to be very much the same as while we were traveling amid the lava and the schists. On the other hand my sense of smell was much affected by a very powerful odor. I immediately knew that the gallery was filled to overflowing with that dangerous gas the miners call fire damp, the explosion of which has caused such fearful and terrible accidents, making a hundred widows and hundreds of orphans in a single hour. Happily, we were able to illumine our progress by means of the Ruhmkorff apparatus. If we had been so rash and imprudent as to explore this gallery, torch in hand, a terrible explosion would have put an end to our travels, simply because no travelers would be left. Our excursion through this wondrous coal mine in the very bowels of the earth lasted until evening. My uncle was scarcely able to conceal his impatience and dissatisfaction at the road continuing still to advance in a horizontal direction. The darkness, dense and opaque a few yards in advance and in the rear, rendered it impossible to make out what was the length of the gallery. For myself, I began to believe that it was simply interminable, and would go on in the same manner for months. Suddenly, at six o'clock, we stood in front of a wall. To the right, to the left above, below, nowhere was there any passage. We had reached a spot where the rocks said in unmistakable accents--No Thoroughfare. I stood stupefied. The guide simply folded his arms. My uncle was silent. "Well, well, so much the better," cried my uncle, at last, "I now know what we are about. We are decidedly not upon the road followed by Saknussemm. All we have to do is to go back. Let us take one night's good rest, and before three days are over, I promise you we shall have regained the point where the galleries divided." "Yes, we may, if our strength lasts as long," I cried, in a lamentable voice. "And why not?" "Tomorrow, among us three, there will not be a drop of water. It is just gone."