The drowsy chirp of crickets, and shrill voices of katydids in the lush grass near by, told of the summer night. Many times had Frank listened to this same chorus as he lay in his blanket on the open prairie, playing the part of night-wrangler to the herd of saddle horses belonging to the round-up party of cow-punchers.
He could hear some lurking rabbit slinking through the hazel bushes over at one side. Somewhere off on the level, where the sage grew so heavily, there must have been a prairie dog village; for the sound of the peculiar barking of these queer little animals frequently floated to his ears as the breeze changed.
The two horses were still feeding at the time Frank dropped off into a sound and refreshing sleep, but doubtless they would soon lie down. Bob was already breathing heavily, which would indicate that he had passed beyond the open door to slumber-land.
The minutes passed, and several hours must have gone.
Frank was dreaming of the excitement attending some of the many dashing gallops he had lately enjoyed in company with his chum, looking up stray cattle, helping to brand mavericks, watching the cowmen mill stampeding herds, or chasing fleet-footed antelopes just to give the horses a run.
He was suddenly aroused by a strange sound that seemed to cause the very earth under him to tremble. The trample of a thousand hoofs would make such a noise; if one of those old-time mighty herds of bison could have come back to earth again; or a stampede of an immense herd of long-horns might cause a similar vibration.
But Frank Haywood knew that neither of these explanations could be the true one, even as he thus sat upright on his blanket to listen. The ominous, growling, grumbling noise was more in the nature of approaching thunder, just as though one of those furious summer storms, tropical in their nature, and often encountered in this country where plains and mountains sharply meet, had crept upon them as they calmly slept.
And yet, strange to say, neither of the two boys jumped quickly to their feet in wild dismay, seeking to prepare for the rain that might soon burst upon them. On the contrary they continued to sit there, straining their ears to catch the rumbling reverberations that kept coming, with little respites between.
"Say, now, what d'ye think of that, Bob?" asked Frank, when silence again held sway for a brief period. "Nary a cloud as big as your hand in the sky; and yet all that grumbling oozing out of old Thunder Mountain! Looks like we might have the biggest job of our lives finding out the secret of that pile of rocks. There she starts in again, harder than ever. Listen, Bob, for all you're worth!"