The results of a study of seismic waves generated by eight nuclear explosions and recorded at 31 locations between the Nevada test site (NTS) and Ordway, Colorado, are discussed. The line of recording stations crosses the eastern part of the Basin and Range province, the Colorado plateaus, and the southern Rocky Mountains, and it extends into the Great Plains. In the eastern Basin and Range province and the western part of the Colorado plateaus (0≤Δ≤385 km), the time-distance curves for Pg and Pn can be expressed, respectively, as T1 = 0.8 + Δ/6.0 and Ts = 5.8 + Δ/7.6. A third phase, tentatively identified as P*, is represented by the equation T2 = 3.8 + Δ/6.5. Using the crustal structure and Pn velocity (7.9 km/sec) found for the NTS region by other authors, we find that the above relations indicate that the thickness of the crust increases from about 25 km at NTS to about 42 km in the western part of the Colorado plateaus. East of this boundaiy the velocity of P in the upper mantle increases to 8.0 km/sec; depth to the M discontinuity in the Colorado plateaus is approximately constant over the range 435<Δ≤645 km. Beyond 850 km, first arrivals in the southern Rocky Mountains and Great Plains provinces indicate an apparent velocity of about 8.4 km/sec. Amplitudes of Pn attenuate according to the equation A = A0Δ−1/2 (Δ - d)−3/2 e - 0.0022Δ over the distance range 150<Δ<850 km (d = 60 km). This relation yields a value of Q, for Pn, of about 520. The amplitudes of Pg attenuate extremely rapidly, and beyond about 130 km this phase cannot be identified with certainty. An extension of the Pg travel-time branch at long distances could be associated with waves reflected beyond the critical angle, from the base of the crust. This phase, called P¯ after Mohorovicic, appears to attenuate as A = A0 e−0.0076ΔΔ−1/2. The value of Q indicated by this equation is about 200.