Negative buildings

The other evening I was reading some of Walt Whitman's prose works from the 1860s, and that period of time in US history seemed vividly alive in my imagination, both oddly familiar and completely alien. In the night I dreamt I had a thick book that described architectural projects undertaken then. I was looking at etchings of monuments. Architects had designed eight or ten different lofty neoclassical buildings with columns and domes, and workers had carved them out of solid stone in the Rocky Mountains, but as negative spaces, leaving building-shaped holes like molds into which some substance could have been poured or sprayed. Tourists would enter them through broad tunnels underneath, and look up into the dimness at the vaults of the domes; there were small skylights cut into the top to let light in.