[en] An experimental apparatus was designed and built to allow water-saturated permeabilities as low as 10^-18 m² to be measured on cores of diameter 5 cm and length 10 cm under steady-state flow conditions. This same apparatus can also be utilized in a transient (pressure-decay) mode in order to measure permeabilities several orders of magnitude lower than the steady-state limit. Tests were conducted on samples of pumice, fractured welded tuff, and welded tuff, representing a permeability range of seven orders of magnitude. Based on present measurements and calculations, the following results were obtained: Liquid-saturated permeability of the pumice core from Mount St. Helens was 2.76 x 10^-12 m²; the corresponding Ergun constant was 4.43 x 10¹¹ kg/m⁴. The ultimate compressive strength of this material was found to be greater than 1.8 MPa, but less than 3.6 MPa; liquid-saturated permeability of the unfractured welded-tuff core was 5.6 x 10^-19 m²; liquid-saturated permeability for the fractured welded-tuff core was found to decay to 2 x 10^-18 m² after long-time-scale exposures to continuous-flow and applied-load conditions, independent of the initial fracture state (open vs closed); with an initially closed (naturally existing) fracture, core permeability decreased by a factor of about 2 over a 200-h test period; with an initially open fracture, core permeability decreased by a factor of about 4 under the influence of a comparable load-time history to that experienced in the natural-fracture test; final core permeability was found to be reduced by an order of magnitude from its initial level during a total 700-h test period; and the final effective hydraulic fracture aperture was calculated to be 10^-6 m for both tests on the fractured welded-tuff core; the final effective fracture permeability was calculated to be 10^-13 m², five orders of magnitude greater than the matrix-material prmeability. 28 references, 10 figures