The extended array at the Tonto Forest Seismological Observatory in central Arizona has been used to measure dT/dΔ of direct P waves from 212 earthquakes in the distance range between 30 and 100°, and these data have been inverted to obtain a velocity model for the lower mantle. Travel times calculated for this model are in good agreement with empirical travel times. The dT/dΔ data from different azimuths and from different focal depths are all in reasonably good agreement with the exception of anomalously large values from earthquakes on the mid-Atlantic ridge. The effect of the core on the measured values of dT/dΔ at distances greater than 90° is shown to be significant, and a correction is made for this effect. A curve fit to the dT/dΔ data contains anomalous regions near the epicentral distances of 34.5, 40.5, 49.5, 59.5, 70.5, and 81.5° which may correspond to increased velocity gradients near the depths of 830, 1000, 1230, 1540, 1910, and 2370 km. PcP times were used to estimate a core radius of 3481 km.

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