The hat is one I've seem in pictures of engineers and survey parties in the field during that general period . The boots are consistent with the same group. It would follow that perhaps Capt. Morrell's branch was Engineers, and going out on a limb, Mounted Engineers. Regretfully, on enlargement, I could not see any collar insignia that would support my theory. Anyway, I just love the boots. They a bit like motorcycle courier boots as used by the Brits.
http://theworldwar.pastperfectonline.co ... 7457483671
Cheers

Tom