What region of africa is the kalahari desert in?

It takes up the following regions:

all of Botswana, the eastern third of Namibia, and the northernmost part of Northern Cape province in South Africa.
 
And in the southwest it merges with the Namib, the coastal desert of Namibia.

Early in the story when tom finds an indian skull in the woods which he kicks in temper. the devil then makes his appearance and, as tom surveys him, he notices that the devil is neither "negro" nor "indian." in fact, he has a "swarthy" complexion and is "begrimed with soot."