During volcanic eruptions lavas carry to the surface rock fragments-xenoliths. These may be both fragments from nearsurface (overwhelming majority) or deepseated rocks of the lower crust and upper mantle. The second group of xenoliths is characterized by a series of mineral parageneses that could emerge only at P-T conditions corresponding to depths from 20 to 130 km (MACGREGOR, 1968; SOBOLEV, 1973; LUTS, 1973; PERCHUK, 1973, etc.). About 1500 samples of deep-seated xenoliths collected in Kamchatka were examined which allowed us to get an idea of the bulk composition, density and state of rocks of the deep lithosphere beneath Kamchatka (Table 1). The composition of gas-liquid inclusions in the basic-hyper-basic xenoliths of basaltold lavas depends on the depth from which xenoliths are supplied (the depth is determined using the methods of geological thermobarometers) (PERCHUK, 1973; BAKUMENKO et al., 1975). The most deept-seated rocks are represented by high pressure associations composed of diopside, garnet (pyrope) and forsterite. These associations are followed by rocks of the group of spinel peridotite and by pyroxenites which at the Moho level are replaced by amphibole associations (KUTYEV, 1976). The lower crust is represented by essentially gabbroid associations (Table 2). Of great interest are the so-called carcass or porous xenoliths (KUTYEV, 1976; KUTYEV and KUTYEVA,