and a solid wall of blank rock is encountered."

By "blank rock" Tom meant rock that did not contain a promising
or paying amount of metal in the ore.

"What it means?" Montez asked. "No; I can't say that I do."

"The wall of blank rock, found at the end of a vein of gold, Don
Luis, often, if not usually, means that the vein has run out,
and that it is useless to dig further."

"I did not know that," murmured the Mexican, in a tone of merely
polite astonishment. "Then you believe that _El Sombrero_ will
not turn out much more profitable ore?"

"I didn't say that," Tom continued. "But I will admit that finding
the wall of blank rock ahead made me a bit nervous. Some great
mines have been started, Don Luis, as you must be aware. For
a few weeks they have panned out ore of the highest value. Much
capital has been put into such mines, and for a time men have
thought they owned a new Golconda. Then--suddenly--the blank
wall, and no more gold has ever come out of that mine. In other
words, it was but a pocket of rich gold that had been struck, and
nothing more. Hundreds of men have ruined themselves by investing
in such mines."

"I see," murmured Don Luis, thoughtfully.

"You did not know this before?" Tom asked, in some amazement.