57 



distance from the Mount, showing that the dolomitization of 

 these beds had taken place before the period of volcanic ac- 

 tivity. Many of the blocks of limestone contain layers cf 

 black flints in situ, and in mass measure up to 6 and 9 ft. in 

 diameter. The flints, which became separated from their matrix 

 before ejected, are frequently bleached and flaked by the heat 

 to which they have been exposed. 



The presence of large blocks of lava in th© tufa is proof that 

 the periods of activity were intermittent, for such consolidated 

 lava must have been formed during periods of rest, when the 

 magma may have cooled sufficiently to form a solid plug to the 

 vent, only to be broken and ejected in angular masses when 

 the volcanic forces broke out afresh. Some very large masses 

 of limestone occur almost at the highest point of the cone, 

 showing that the forces of ejectment were intense even in its 

 later stages. 



The finer material, comprehended under the general term 

 of ash, is highly interesting. It may be said to consist of papilli, 

 ash, and fine earthy material, the latter consisting largely of 

 grains of sand. The presence of alluvial matter extensively 

 mixed with the ash points tO' the conclusion that the erup- 

 tions were a mixture of mud and scoriaceous material. That 

 this should be the case seems probable from the geological 

 features of the district, and the indications that the volcanic 

 foci were at shallow deptli. The sedimentary rocks that are 

 represented among the ejected material are almost entirely 

 limited to those of Eocene age, which form the superficial geo- 

 logical features of the neighborhood. One or two small frag- 

 ments of a siliceous quartzite were observed in the tufa that 

 may have come from an c*!der series below the Tertiary beds, 

 but the almost total absence of such older rocks from the 

 ejectamenta is suggestive. 



The Eocene limestone is the great water-bearing stratum 

 of the district. The depression of the Blue Lake and others, 

 including the Crater Lake, are below this subterranean water- 

 line ; hence they contain water, which in the case of the Blue 

 Lake has an average depth of 250 ft. The whole of this 

 south-eastern country, at the level indicated, is saturated to an 

 enormous extent. There is every reason to think that these 

 hydrographic conditions existed at the time of volcanic 

 activity, and may have acted as an important factor in creating 

 the igneous outbreak. It would also give character to the 

 eruption, and make the vents, more or less, mud volcanoes. 

 There are, indeed, some evidences that a lake existed in or 

 near the Mount Gambler crater before the original eruption, or 

 during a period of quiet. Laminated mud, in irregular blocks, 

 can occasionally be seen in the ejected material. One remark- 



