Indeed, even as they brought the snorting horses in, and started to fasten the loads on their backs, the rumble had increased to a steady roar, so that it seemed to Roger he could actually feel the ground vibrate under the pounding of those myriad of hoofs, as the heavy animals galloped toward the river. Whenever the lightning flashed, and this was more frequently than before, with peals of thunder following, both boys found themselves compelled to shoot quick glances of both curiosity and alarm toward the advancing peril. And what they saw was a sight never to be forgotten. It was like the border of a troubled sea, that tremendous line of moving animal life, heaving and tossing, and coming steadily on like a dust cloud impelled by the wind. As yet they could not distinguish the units comprising this immense whole; but it was easy to imagine them, for both boys had shot buffaloes before now, and knew what they were like, though they had never looked upon a herd of more than half a dozen at a time. "Oh! there must be millions in that lot!" cried Roger, when a particularly vivid flash came, that showed them the whole level stretch covered with the advancing horde as far as their eyes could reach. "Too many for us to stay here, and try to divide!" Dick answered. "How lucky that you noticed where the trees grew along the river," said Roger; "because that will be our best chance, don't you think, Dick?" "Yes, and the sooner we're off the better," was the other's answer. He knew that their horses must still be tired from the long journey of the day before, and, laden as they were, might not be able to run as swiftly as under other conditions. Just how fast that avalanche of shaggy forms could advance he had no means of knowing. If further frightened by the flash of lightning, and the crash of thunder, a stampede of the herd would mean that the bison would come on the full gallop, madly seeking to find shelter from the howling blast. "Have we got everything, do you think?" asked Roger, ready to mount his horse, which could hardly be held in hand, such was its terror at the approach of this thundering mass of heavy beasts; it was as though the intelligent animal understood the danger that threatened, and wanted to be on the move. "No matter, we cannot afford to waste another minute hunting for anything now. Our lives are worth more than any other thing we possess. Get in your saddle, quick, Roger; and be careful not to let the horse throw you, or all is lost!" The warning came not any too soon, for the fretful animal was jumping and tugging at the bridle, trying the best it could to break away, so that it might dash off, and keep a space between that approaching peril and its fleet heels. But Roger was a pretty fair horseman, and succeeded in mounting, in spite of all the prancing of the steed. Then away they went, helter-skelter, allowing the horses to have their heads.