Here's how he marked it on his map:
Based on the topographic maps from the late 1800s, there was a hill, Hickey Hill, around that spot.
Over the years, the land in that area was farmed in various spots, but largely undeveloped until it was bought in the 1920s to create the US. National Arboretum.
Here's how the area from thia 1890s map corresponds to the current map of the US. National Arb.
More details when I get them!