Mackinlay Cantor wrote a poem in which there is a recurring line of:
Boomlay, Boomlay Boom. What is the name
of this poem? If he is not the author, who is?

Dear gerray,

Mackinlay Cantor is indeed not the author of the poem you have in
mind. Actually, it was written by Vachel Lindsay (10 Nov 1879 - 5 Dec
1931). The name of the poem is "The Congo: A Study of the Negro Race",
and Lindsay wrote it in 1913.

You can learn more about Vachel Lindsay's life and work from these websites:

Modern American Poetry: Vachel Lindsay
http://www.english.uiuc.edu/maps/poets/g_l/lindsay/lindsay.htm

iPoet.com: Vachel Lindsay
http://www.ipoet.com/archive/rhythmic/Lindsay/Vachel.html

PoemHunter: All Poems by Vachel Lindsay
http://www.poemhunter.com/vachel-lindsay/poems/poet-6634/page-1/ 


You can read the full text of the poem (which is, by the way, in the
public domain) on these sites:

University of Toronto: The Congo - A Study of the Negro Race
http://eir.library.utoronto.ca/rpo/display/poem1303.html

PoemHunter: The Congo - A Study of the Negro Race
http://www.poemhunter.com/p/m/poem.asp?poet=6634&poem=27629

Oldpoetry: The Congo - A Study of the Negro Race
http://oldpoetry.com/poetry/4400


Search terms used:
"boomlay boom"
://www.google.de/search?hl=de&ie=UTF-8&newwindow=1&q=%22boomlay+boom%22&btnG=Suche
"Vachel Lindsay"
://www.google.de/search?q=%22Vachel+Lindsay%22&hl=de&lr=&ie=UTF-8&newwindow=1&start=0&sa=N
"Vachel Lindsay" congo 1913
://www.google.de/search?hl=de&ie=UTF-8&newwindow=1&q=%22Vachel+Lindsay%22+congo+1913&btnG=Suche


Hope this answers your question!
Regards,
Scriptor