Compressional- and shear-wave velocities obtained by logging and laboratory measurements on samples from the base of Hole 504B between 1725 and 1836 m sub-basement are indistinguishable from classic Layer 3 velocities obtained by refraction (V_p = 6.7 km/s, V_s = 3.75 km/s). Point counts of thin sections from this interval show that the rocks consist not of gabbro, but of greenschist facies metadiabase dikes dominated by plagioclase and actinolite. Comparison with thin sections and logs from higher levels suggests that the Layer 2/3 boundary is a metamorphic front within the sheeted dikes controlled by the disappearance of chlorite, or a porosity transition marking the disappearance of interpillow voids and large-scale fractures, depending on whether the boundary is defined in terms of velocities or velocity gradients.

Date of initial receipt: 19 August 1994
Date of acceptance: 24 February 1995

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