We show that the simplest assumptions for the dynamics of particle production allow us to understand the fluxes of hadrons and photons at mountain altitudes as well as the structure of individual events. The analysis requires a heavy nuclear component of primary cosmic rays above the ``knee" in the spectrum with average mass number $
= 7.3 \pm 0.9$.
Comment: Revtex, 11 pages, 5 postscript figures ("\special" command used to embed figures at end of tex file). Compressed postscript version also available at http://phenom.physics.wisc.edu/pub/preprints/1996/madph-96-932.ps.Z or ftp://phenom.physics.wisc.edu/pub/preprints/1996/madph-96-932.ps.Z