The development of rapid and efficient methods of chemical analysis of major and minor elements in rocks has given rise to the production of large sets of data constituting a comprehensive 'picture' of the chemical composition of a rock unit or a rock group in a given area.The complex structure of this multidimensional picture is globally analyzed by 'correspondence analysis' as an interpretative method of pattern recognition. Significative trends of variation are determined and expressed in geological terms. Density distribution of representative points in the factor space is evaluated in search of homogeneous substructures inside the global one. The resulting 'natural clustering' is compared to an a priori partitioning of the structure on the basis of petrographic criteria.The whole analysis is performed by a computer program that can accommodate some 600 samples measured on more than 20 variables. The analysis of a chemical picture of the Monteregian petrographic province is given to illustrate the method.