I found some information about the marimba on the Internet.

http://www.marimba.org/
  "Marimba, which was improved in South America was brought to the
United States eventually, and they started to make marimba around 1910.
Deagan of Chicago changed the wooden pipe to the metal pipe.  Numerous
other improvements were made since then including the rearrangement of
the keytop to resemble the piano.  Modern Marimba is now treated not
only as an orchestra instrument but also as a solo instrument thanks
to the louder sound achieved by the [metal] pipe."

http://www.hereintown.net/~glatta/marimba/marimba.html
  "The marimba is officially a member of the xylophone family with full
length resonators below each bar.  On top-of-the-line instruments the
resonators are made of aluminum or some other metal and are carefully
tuned to bring out the fundamental note of each bar.  On more primitive
instruments the resonators may be simply hollowed-out gourds..."

http://k.faculty.umkc.edu/kaptainl/blood.html
   Dr. Kaptain writes that the marimba is generally acknowledged as
the state instrument of Chiapas, Mexico.  The marimbas of Mexico are
distinguished by the vibrating membranes at the bottom of each
resonator.

http://www.eecs.umich.edu/~pavr/harbury/archive/1999/19990901.html
  On 1 September 1999 the Guatemalan Congress adopted the marimba as
the national symbol of Guatemala.

http://www.deguate.com/infocentros/guatemala/simbolos/marimba.htm
  A passage (in Spanish) is quoted from the 1987 book by J. Eduardo
Tanchez, "La Musica en Guatemala, Algunos Musicos y Compositores [Music
in Guatemala, Some Musicians and Composers]", including speculation of
its possible "re-invention" in Guatemala.  The article concludes with
a quote by New Mexico author Erna Ferguson (I think it's from her book,
"Dancing Gods", of the late 1920s), "The marimba could not _be_ from
Guatemala, but it certainly _is_ Guatemalan."

  In a second article at this web page, historian Celso Lara concludes,
"The marimba evolved in the 17th and 18th centuries, but it is from
1880 to 1920 that it acquired the current characteristics, when the
artisans converted the [diatonic] 'simple marimba' to double [chromatic].
From Quetzaltenango it was taken to Mexico and Costa Rica."

Robbie Rhodes