basial

I further draw my conclusions from the fact, that all the hills on the country are much about the same height -- two or three hundred feet above the basial surface of the land; and I could only see the top of the hill like a hazy brown spot, contrasted in relief against the clear blue sky.

What Led to the Discovery of the Source of the Nile John Hanning Speke 1845

The N'yanza, as we now see, is a large expansive sheet of water, flush with the basial surface of the country, and lies between the Mountains of the Moon (on its western side), having, according to Dr Krapf, snowy Kænia on its eastern flank.

What Led to the Discovery of the Source of the Nile John Hanning Speke 1845