I don't recall for sure, but the entire uniform set was up for sale (from either AGM or James Mountain) about 18-24 months ago. As indicated by the collar insignia, he was a Reserve officer. The patch on the sleeve would lead me to believe he served in France (shoulder patches were not authorized in the continental US until the 1920s).
Robert Davis Longyear was born in 11 July 1892 in Minnesota, the son of Edmund J. Longyear. He earned a BA from Williams College in 1914 and an MA from the University of Wisconsin in 1915. In 1915-16, he was a graduate student in Geology at Stanford. He was shown as being in the Coast Artillery during WWI, and his name appears on the Official List of the Officer's Reserve Corps of Minnesota (1919 edition) as a 2LT of Coast Artillery - though the uniform clearly shows 1LT insignia on the epaulettes.
Since there were no Coast Artillery Corps units in Minnesota, one wonders why the Official List of the Officers' Reserve Corps of Minnesota (1919-1920) lists two captains, nine 1LTs, and 47 2LTs in the Coast Artillery Section. It may be that Longyear either volunteered or was "voluntold" (based on his geology studies) that his services could be of use to the CAC. No idea the CAC regiment in which he landed.
Longyear took over the E.J. Longyear Company (a worldwide mining company) from his father.
Longyear died 20 May 1970 and is buried in the Lakewood Cemetery in Minneapolis.