We describe the measurement of the depth of maximum , X _ { \mathrm { max } } , of the longitudinal development of air showers induced by cosmic rays .
Almost four thousand events above 10 ^ { 18 } eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis .
The average shower maximum was found to evolve with energy at a rate of ( 106 ^ { +35 } _ { -21 } ) g/cm ^ { 2 } /decade below 10 ^ { 18.24 \pm 0.05 } eV and ( 24 \pm 3 ) g/cm ^ { 2 } /decade above this energy .
The measured shower-to-shower fluctuations decrease from about 55 to 0pt26 .
The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed .