The blues is a genre of African American folk music that is the basis for much of modern American popular music. Blues can be seen as part of a continuum of musical styles like country, jazz, ragtime, and gospel; though each genre evolved into distinct forms, their origins were often indistinct. Early forms of the blues evolved in and around the Mississippi Delta in the late 19th and early 20th centuries. The earliest blues music was primarily call and response vocal music, without harmony or accompaniment and without any formal musical structure. Slaves and their descendants created the blues by adapting the field shouts and hollers, turning them into passionate solo songs. When mixed with the Christian spiritual songs of African American churches and revival meetings, blues became the basis of gospel music. Modern gospel began in African American churches in the 1920s, in the form of worshipers proclaiming their faith in an improvised, often musical manner (testifying). Composers like Thomas A. Dorsey composed gospel works that used elements of blues and jazz in traditional hymns and spiritual songs.