The error occurred in this way: The trail that the boys had been assiduously following was so faintly marked that the wonder was they did not go astray sooner. In many places, there was little choice as to the route, because it was so broken and crossed that one was as distinct as the other. Nevertheless, Frank pressed on with scarcely any hesitation, until he again reached a depression where the soft ground failed to show the slightest impression of shoe or hoof.

"My gracious!" he exclaimed, stopping short and looking at his companion; "how far can we have gone wrong?"

"We can find out only by returning," replied Roswell, wheeling about and leading the way back.

They walked more hurriedly than before, as a person naturally does who feels that time is precious, and he has wasted a good deal of it.

The search might have been continued for a long time but for a surprising and unexpected aid that came to them. They had halted at one of the broken places, in doubt whither to turn, and searching for some sign to guide them, when Roswell called out:

"That beats anything I ever saw!"

As he spoke, he stooped and picked up something from the ground. Inspecting it for a moment, he held it up for Frank to see. It was a large nugget of pure gold.

"These mountains must be full of the metal," said Frank, "when we find it lying loose like that."

"Not so fast," remarked his companion, who had taken the nugget again, and was turning it over and examining it minutely. "Do you remember that?"

On one of the faces of the gold something had been scratched with the point of a knife. While the work was inartistic, it was easy to make out the letters "F. M."

"I think I remember that," said Frank; "it is one of the nuggets I found yesterday, and marked it with my initials. Those folks must have dropped it."

There could be no doubt of it. What amazing carelessness for a couple of men to drop a chunk of gold worth several hundred dollars and not miss it!

It must have been that the mouth of the canvas bag containing the nuggets had become opened in some way to the extent of allowing a single one to fall out.

"I wonder how many more have been lost," mused Frank, as he put the specimen in his pocket.

At any rate, it served to show the right course to follow, and the boys pressed on, looking more for nuggets than for their enemies. The mishap must have been discovered by the men in time to prevent its repetition, for nothing of the kind again met the eyes of the youths, who once more gave their attention to hunting for the lawless men that had despoiled them of so much property.

The trail steadily ascended, so broken and rough that it was a source of constant wonderment how the burro was able to keep his feet. He must have had some experience in mountain climbing before, in order to play the chamois so well.

The boys fancied they could feel the change of temperature on account of the increased elevation. They knew they were a good many feet above the starting-point, though at no time were they able to obtain a satisfactory view of the country they were leaving behind. They seemed to be continually passing in and out among the rocks and bowlders, which circumscribed their field of vision. Considerable pine and hemlock grew on all sides, but as yet they encountered no snow. There was plenty of it farther up and beyond, and it would not take them long to reach the region where eternal winter reigned.

A short way along the new course, and they paused before another break; but although the ground was dry and hard, it was easy to follow the course o............