  Results of a study of the substructure of the highest transverse momentum
(pT) jets observed by the CDF collaboration are presented. Events containing at
least one jet with pT > 400 GeV/c in a sample corresponding to an integrated
luminosity of 5.95 inverse fb, collected in 1.96 TeV proton-antiproton
collisions at the Fermilab Tevatron collider, are selected. A study of the jet
mass, angularity, and planar-flow distributions is presented, and the
measurements are compared with predictions of perturbative quantum
chromodynamics. A search for boosted top-quark production is also described,
leading to a 95% confidence level upper limit of 38 fb on the production cross
section of top quarks with pT > 400 GeV/c.
