I found this reference while reading letters on the Mark Twain Project:
Davis, Chester L. 1967. “Letter to Paine from John P. Vollmer.”
Twainian 26 (March-April): 3–4.

This is said to contain a description of the trip from Aspinwall to
Panama City, across the Isthmus of Panama.  I'm unable to find any
copies of the journal cited and am hoping someone might be able to send
me a copy of Vollmer's description of this trip.

Thanks...