Supplying of masonry materials in the construction of the crypt of Santa María la Real de la Almudena, Madrid, Spain, 1883-1911 The importance of the petrological and petrophysical studies of stony materials used on architecture is progressively gaining importance, both for the construction of new buildings, and for the restoration and rehabilitation of historical buildings. This work presents an example of the petrophysical studies that were canied out for the selection of the most suitable lithologies for the construction of the crypt of Santa María la Real de la Almudena Cathedral (Madrid) in the second half of the 19th century. It is also shown how, due to different causes, the type of material s used throughout the construction of the Crypt was changed. These changes affected the construction, both delaying it and resulting in a building made up with several different materials. This confirms a tendency that is very often observed on historical building s where construction was developed during a long period of time. It is in these cases where it is especially important to carry out a detaiJed documentary analysis of the factors causing this variability. The importance of this analysis mainly lies on two points. On one hand, it can reveal important historical details concerning the interpretation of the building. On the other hand, it gives us information about the quarries from where the original materials were extracted, allowing us to obtain stones of exactly the same characteristics as those once employed in order to make substitutions of architectonic elements. In that way, this work shows how the combination of both documentary and geological data can be an important approach in M. Gómez Heras R. Fort González order to understand many subjects dealing with the history of architecture. INTRODUCTION The origin of materials used on a building and how it affects its construction is an interesting subject in the study of any building. The origin and transformation processes of the material s used offers us historical, socio-economical and industrial data on one of the oldest human activities: the quarrying and use of stone resources. The determination of original quarries for a building allows for optimal restoration when material has been lost or needs to be substituted. That is an important fact because there are not only changes between stone types but there are also changes between different quarries of the same lithology. These changes could cause incompatibilities between new and original materials. The present work mainly has two aims. First, it reveals the process of selection and the supplying history of the stone in the Crypt of the Cathedral of the Almudena (Cathedral of Madrid). Second, the study of the documentation is completed with some petrological data which allow us locate the quarries used in this building. The Crypt has been selected because it was built with uncommon stony material s in the city of Madrid. The traditional architecture in Madrid has been made Proceedings of the First International Congress on Construction History, Madrid, 20th-24th January 2003, ed. S. Huerta, Madrid: I. Juan de Herrera, SEdHC, ETSAM, A. E. Benvenuto, COAM, F. Dragados, 2003.