What does fortune lies upon the surface of the earth as plentiful as the mud in our streets mean?

"Fortune lies upon the surface of the earth as plentiful as the mud in our streets" means that the gold that will transform the status of the nation is in much abundance and has become the subject matter as compared to mud in the streets.

Explanation:

The above statement was made by Horace Greeley during the gold rush. America, at that time experienced a transformation when gold was discovered.

By early 1849, it became a gold fever epidemic. There were discussions of gold almost in every corner of the nation. Young men saw that parting away with their wives that would be worth the hardship.

All these led to the statement of Horace Greeley the of New York Tribune:

"Fortune lies upon the surface of the earth as plentiful as the mud in our streets. We look for an addition within the next four years equal to at least One Thousand Million of Dollars to the gold in circulation."