Just read your query for John Geltz born 1842 possibly in Whittenberg, Germany and settled in Illinois. Found a few similar points to a John Geltz I am researching.
I am also looking for a John Geltz, brother to my great-great grandfather, Isaac Geltz. John was born in York Co, PA in 1842 to Gottleib and Louisa Geltz. Gottlieb Geltz was from Wurttemberg, which was a country on it's own, but is now part of Germany (since 1918).John Geltz was living in 1868 when his father died in York Co., but after that I haven't been able to locate him; he may have moved west somewhere. His sister, Caroline and her husband, Andrew Amsbaugh had moved west to Illinois then Indiana after 1868. Just a stab in the dark, but thought I'd share it with you just in case you thought this might be the same John Geltz.