Wide angle recordings at offsets between 20 and 40 km in the Ruby Mountains consistently show 5 strong reflections between 4 and 11 s with enough moveout to estimate velocities to the base of the crust. The uppermost ''layer'' with a temperature corrected velocity of 6.2 km/s and thickness of 9 km corresponds to quartzofeldspathic rocks such as metasedimentary rocks, migmatites and deformed granites and is underlain by a 7-km-thick ''layer'' with velocities of 6.4 km/s which corresponds to quartzofeldspathic material interlayered with amphibolites. The lower crust consists of a heterogeneous, 9 km-thick zone with velocities of 6.7 to 6.8 km/s correspondng to mafic rocks interlayered with smaller amounts of quartzofeldspathic rock. The lowermost crust is marked by the ''X'' reflection overlying a 3-km-thick ''layer'' with velocities between 7.4--7.8 km/s corresponding to anomalous material, possibly layered cumulates. The suhorizontal, layered structure of the crust is caused primarily by ductile extension, which may well be superposed on material added to the crust by underplating. At most, approximately one third of the present crust could have been added by underplating in the Cenozoic.