A study of the primary cosmic ray composition in the energy range 5ṡ1014-5ṡ1015 eV is performed through the analysis of the deep underground muons and of the e.m. component of Extensive Air Showers detected in coincidence by the MACRO and EAS-TOP experiments at the Gran Sasso Laboratory. We conclude that: a) pure proton and Iron primary beams are excluded by the experimental data; b) a fraction of nuclei heavier than Helium is necessary to explain the experimental data both below and above the knee; c) a mixed composition, obtained a priori from extrapolations of the spectra directly measured below 100 TeV, yields a reasonable account of the observations up to the knee of the primary spectrum.
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We gratefully acknowledge the support of the director and staff of the Gran Sasso National Laboratory, and the invaluable assistance of the technical staff of the institutions participating in the experiments. This work was supported in part by the Italian National Institute for Nuclear Physics (INFN), the Italian National Research Council (CNR), the US Department of Energy, and the US National Science Foundation.