Delaware Block, 36 W. Randolph.  Wheelock and Thomas, 1874 (extended later).  (Photo by Thomas Leslie) 

Chicago’s soil was a hurdle to tall construction regardless of building type. Chicago rested up on a hundred-foot-thick layer of waterlogged clay that frustrated attempts to build on bedrock below. . . .

This spurred development of a type of floating foundation unique to Chicago, but it also meant that a structure’s weight had to be carefully considered. A structure that eschewed masonry in favor or lighter iron or steel could, theoretically be built higher on the same soil.

Taken from Chicago Skyscrapers, 1871-1934 (University of Illinois Press, June 2013).