on the railroads in Iowa. Many of the Mexicans recruited to work on the railroads established communities at Fort Madison, Council Bluffs, Des Moines, and Davenport. Additionally, a significant revolution in Mexico in 1910 led more Mexicans to emigrate, and ultimately arrive in Iowa. Mexican immigration dropped off during the great depression as the economy weakened, and remained relatively low even into the latter half of the 20th century. The late 1980s and subsequent periods have brought a resurgence of Mexican immigration as the demand for labor in the food processing industry has increased. African-American Though African-Americans began immigrating to Iowa in more significant numbers through the 1860s after obtaining freedom, some of the earliest immigrants would have been brought in as slaves by settlers from southern states after the Black Hawk Purchase, despite the fact that slavery never officially existed in Iowa. The absence of legally sanctioned slavery in Iowa did not mean that the state was free from discrimination, however. An 1838 Act prevented African-American settlement in Iowa unless he or she could present a "fair certificate" of "actual freedom" under the seal of a judge and give a $500 bond. Though this undoubtedly slowed African-American immigration, a few immigrants nonetheless came in the 1840s; most worked in the mines of Dubuque or in the river towns. Frequently, they came to be free of slavery. The third general assembly passed an act in 1851 similar to that of the 1839 act, but it appears to have rarely been enforced and was ultimately ruled in 1863 to be unconstitutional. Initial African-American settlement in Iowa after the Civil War was in agricultural communities near the southern border, as well as along river towns on the Mississippi and to a lesser extent the Missouri. Polk County was also a destination for immigrants. Those along the river often worked on boats, though many worked on the railroads and in the lead mines of Dubuque. Over time, African Americans migrated from agricultural communities to urban areas, and from the river towns to the coal mines of southern Iowa. This urban shift started around 1870, while the coal mining shift started in 1880. The coal mining shift began when African-Americans were brought from the southern states to the southern Iowa coal mines as strikebreakers, after which they remained employed there. This immigration was augmented by poor economic conditions in the southern states resulting from