I could not find any info on Theunis A. Merrion, but this may help in your hunt.
This info comes from Historical Register of Officers of the Continental Army & the Library of Congress.
Robert Erskine, New Jersey, Geographer and Surveyor to the army of the United States, July 27, 1777; Died October 2, 1780.
Letters of Delegates to Congress: Volume 5 August 16, 1776 - December 31, 1776
Benjamin Franklin to Robert Erskine .
Robert Erskine, (1735-80), Scottish geographer, hydraulic engineer, and fellow of the Royal Society, had emigrated to New Jersey in 1771 to represent investors interested in developing iron mines in Passaic County. Sympathetic with the American cause, he became a captain in the Bergen County militia in 1775, and in July 1777 was appointed "geographer and surveyor of the roads" in the Continental Army.
Civilian & Military Surname Searcherhttp://www.civilwarthosesurnames.blogspot.com