Velocity of Elastic Waves in Granite and Norite

The velocities of elastic waves were investigated in granite at Rockport, Massachusetts, and norite at Sudbury, Ontario. The waves measured were generated by dynamite explosions, and recorded by portable seismographs at distances up to 6260 feet in granite and 10,320 feet in norite. The observed velocities in km/sec. with probable errors indicated by ± were: $$\begin{array}{cccc}& \mathrm{Longitudinal}& \mathrm{Transverse}& \mathrm{Rayleigh}\\ \mathrm{Rockport\; granite}& \text{5.14±0.005}& \text{2.70±0.02}& \text{\hspace{0.17em} 2.19±0.007}\\ \mathrm{Sudbury\; norite}& \text{6.22±0.003}& \text{\hspace{0.17em} 3.49±0.006}& \text{2.79±0.01}\end{array}$$ From these velocities, and the densities of specimens taken from the shooting locations, the following elastic constants may be computed. They are shown below, for comparison, with those computed by a similar procedure in an earlier report for Quincy, Massachusetts, granite. A quartz‐rich zone in the norite was found to transmit the waves with a slower speed. The results are compared with statical measurements of Zisman on rocks from the same locations, a part of the same program of geophysical research at Harvard. Compressibility, measured directly, under hydrostatic pressures, on specimens exposed to the liquid transmitting the pressure is found to be the only statically determined constant which yields seismically effective values. The geological significance of granite velocities and elastic constants is discussed. It is concluded that a longitudinal‐wave speed of 5.5 km/sec. cannot be regarded as unique for granite, or even necessarily suggestive of granite, as has been repeatedly proposed; and that there is as yet no seismological justification for the identification of any ``layer'' of the earth's crust as either sedimentary or granitic.