The thickness of the Greenland Ice Cap was determined by seismic sounding along the trail from Camp Tuto to Camp Century in Greenland and on traverses northwest and southwest from Camp Century. The average velocity of vertically traveling seismic waves at each shot location was estimated using the first-arrival data from reflection records and the 10-m temperature at each location. The results of three long refraction profiles and measurements of temperature, density, and seismic velocities at the Camp Century drill hole were used to check velocity estimates. An empirical formula from Robin satisfactorily related seismic wave velocities to the temperature and density of the firn and ice. A two-layer glacier model having a homogeneous ice layer overlain by a firn layer in which the P-wave velocity increased linearly with depth was used. (Author).