The correct answer to this open question is the following.
What Horace Greeley meant when he stated "fortune lies upon the surface of the earth as plentiful as the mud in our streets," was to the prominent situation lived during the California Golden Rush.
Many people started to arrive in the state of California when the rumor spread that there was gold everywhere. Or as Greeley said: "fortune lies upon the surface of the earth as plentiful as the mud in our streets." He tried to say that there was plenty of gold for everybody. People from every corner of the country and even abroad moved to California seeking fortune.