At first the walking was not difficult, the trees standing far apart, and the soil beneath being covered with moss and long ferns; but after a time the woods grew denser, and it was only with considerable effort that they were able to move along. Every step seemed to make it worse. Long, slender trees shot up, not large enough to prevent their progress, but sufficient to form an impediment; and working their way among these proved very difficult, and very fatiguing. What was worse, the ground began to grow rougher at every step. Soon they emerged from the thick underbrush, and came to where the trees were larger, and farther apart from one another. But here the ground was spongy; and in every direction lay fallen trunks of trees, some prostrated but lately, others lying where they had lain for years, in every stage of decay. These they had to clamber over, or crawl under; and so it was that, in order to make any progress, they were compelled to use their hands as well as their feet. The soil beneath was hidden under a covering of moss; but it was swampy and spongy, and the soft earth was interlaced with long, fibrous roots, in which their feet caught at every other step. This moss overspread everything. It grew over the fallen trees; it enfolded in its green mantle the huge granite bowlders that from time to time upreared themselves beneath the trees. At length they came to a slight rise in the ground, where the soil was dryer. Here the doctor paused, and the boys all flung themselves down on the ground, breathing heavily. They had walked for two hours now, without stopping, and began to feel exhausted. The doctor also showed signs of exhaustion. He said nothing, however, but scanned very narrowly the scene around him. He found but little before his eyes that was in any way different from what he had been looking at for the last two hours. He could see a few paces before him, but no farther. There the trees arose, and the prospect terminated in the dim aisles of the forest, with their shadowy vistas. All around--before, behind, and on either side--it was the same. There was the dense forest, just such a forest as they had traversed--it spread all around them. The only place where a view could be had for any distance was overhead. But even here it was no better. For the boys, as they flung themselves down, saw that during their journey a change had taken place overhead. The blue sky was no longer to be seen. In its place there came close down a dull, gray, indistinct haze, which descended even to where they lay, hiding the tree tops from view. It was the fog.