Although it is generally easy to obtain small non-disturbed specimens of soft rocks of weakly welded ignimbrite tuff from exposures，it is virtually impossible to acquire complete borehole core samples of “Shirasu”，the Quatemary rhyolitic
ignimbrite tuff which is widely distributed in southern Kyushu，Japan．This difficulty is attributable to the quality of the rocks，in which volcanic glass and coarse-grained materials with large pumiceous fragments are weakly welded to each other．The rocks are therefore soft and tend to break under external forces orvibrations．
A new sampling technique，the Hybrid Method，which is based on air bubble drilling，has recently been developed．This enables us to obtain complete core samples of “Shirasu”．A continuous non-disturbed core sample 100 cm long was obtained from southern Kyushu using this method．Rock textures were observed in addition to measurements of penetrative hardness distribution in the core．Images obtained under the optical microscope and by SEM shows charactehstic piled textures of volcanic glasses of bubble wall type，and welded textures．Distribution of penetrative hardness values shows that this parameter varies from 0.1 kgf/cm to 35 kgf/cm in these rocks．