A MARKED transition in form and function accompanied the rapid population growth in several North American cities during the middle decades of the nineteenth century. Although commerce continued to dominate the urban economy, new types and organizational forms of manufacturing increasingly assumed important roles.1 Such developments generally altered or even displaced some traditional crafts in the manufacturing and occupational structures.2 At the same time, unprecedented numbers of German and Irish immigrants arrived in the cities. Despite innovations in intracity transportation, the alterations in scale, function, and population composition occurred without commensurate changes in the mode of urban movement, and the big cities remained essentially pedestrian at mid century.3 This combination of events resulted in urban areas with a geography that was transitional between the early mercantile city and the later turn-of-the-century industrial metropolis.4 In characteristics such as population size and composition, emergence of large-scale and producer-goods' manufacturing, and embryonic phases of commuting, the mid-century city displayed industrial form. In characteristics such as dominance of commerce and pedestrian movement, persistence of some craft activities, and general heterogeneity of land use, the American city was still mercantile. The geography of the mid-century transitional city is not clearly understood. Conventional descriptions of a congested, heterogeneous urban landscape frequently rest on impressionistic surveys of contemporary data and confusing, often imprecise, sketches of change. Indeed, interpretations of the transformation of the mercantile city into the industrial metropolis understate the patterns and processes of the transition that, by encompassing decades of change, comprises a period of urban experience capable of influencing the subsequent industrial one.5 The early industrialization of some sectors of production, by involving large numbers of people and sizeable parcels of land, contributed to the evolution of a mid-nineteenth-century urban structure. In this paper, the location of all industrial activities in 1860 for the