Mencken mentions a poem by that name.
Source: The Collins English Dictionary © 2000 HarperCollins Publishers:

thanatopsis [ˌθænə'tɒpsıs]
noun a meditation on death, as in a poem
[ETYMOLOGY: 19th Century: from Greek thanatos death + opsis a view]
For the William Cullen Bryant poem "Thanatopsis" see :
http://www.msu.edu/~cloudsar/thanatop.htm

I share some but not all of his views. Won't be long until
I check them out. Or not as I suspect is more probable.