From the Chicago Architecture Foundation site:
"The exterior is an experiment in using historical decorative elements to inform a contemporary office building in the late 19th century. The facade contains decorative elements reminiscent of late Roman, Venetian, Moorish and medieval-European architecture.
Horizontal bands across the Adams and LaSalle facades draw the eye side to side to minimize the perceived height of the building. These two load-bearing walls are supported by a “floating raft foundation,” a crisscross pattern of iron rails embedded in a concrete foundation that support the immense width of the building."