This paper focuses on the role played by the Geological Survey of Portugal in the emergence and consolidation of geology as a government science in the nineteenth century, within a general policy of control over territory. The period under consideration covers the directorates of its first leaders, Pereira da Costa (1809-1888) and the military engineers Carlos Ribeiro (1813-1882), and Nery Delgado (1835-1908). When the Geological Survey was created in 1857 as part of the Directorate of Geodesic, Chorographic, Hydrographical Works of the Kingdom (Direcção Geral dos Trabalhos Geodesicos, Chorographicos, Hydrographicos do Reino) established at the Ministry of Public Works, Trade and Industry (Ministério das Obras Públicas, Comércio e Indústria), Portugal lacked a geological culture and a tradition in geological research. The Portuguese Geological Survey was to be marked by the idiosyncrasies of the local culture and of the organization of the State. In this paper, the emergence of geology as a government science is analysed, by taking into consideration the structure, organization, and a general overview of contributions to the Survey. A particular emphasis will be given to its subordinate staff - the field assistants - not only because their contributions to the geological knowledge of the country have hitherto been largely ignored, but also because they materialized the only possible way for common people to become involved with geology in nineteenth-century Portugal. Generally, with only a basic education, field assistants gradually acquired enough expertise to make decisions in the field, and a few of them even became what Nery Delgado termed 'practical geologists'. The letters in which they reported their work in addition to vivid accounts of less glamorous aspects of fieldwork and its organization provide an image of the country and of its culture by portraying the reactions of civil society to the practice of geology.