476 DR. A. JOWETT ON THE [Oct. 1913, 



waves better than the volcanic rock in which it occurs. It breaks 

 most easily along the bedding-planes, the newly-broken surface 

 being covered with mica- flakes. Extremely fine-grained layers 

 alternate with relatively coarser sediment. In the fissures and 

 cavities, the bedding is less horizontal and regular than in the 

 sheets of sediment. 



High powers of the microscope are necessary for the determination 

 of the minerals even in the coarser layers, the finest material 

 consisting of fragments too minute to be dealt with in an ordinary 

 rock-section. 



The following minerals have been distinguished : quartz, felspar 

 (orthoclase, microcline, and plagioclase twinned on the albite-plan), 

 pale and dark micas, magnetite and red oxides of iron, augite, and 

 chlorite. 



In addition, a few small fragments of glassy lava containing 

 minute felspar-laths, have been found in almost every section 

 examined. These fragments are always of the same order in size 

 as the mineral-fragments among which they occur. The fragments 

 are generally angular, but the larger grains tend to be subangular. 

 The relative proportions of the constituents vary much in different 

 specimens, the coarser varieties being richer in quartz, felspar, and 

 larger flakes of mica, and the finer-grained types containing more 

 mica (in minute flakes) and chlorite. The pale-green sediments 

 with abundant chlorite seem to be generally associated with amygda- 

 loidal lavas containing felspar-phenocrysts ; but, on the whole, the 

 sediments are very much alike, irrespective of their immediate 

 surroundings. The cementing material is usually calcite, which is 

 frequently in optical continuity over small areas ; sometimes it 

 consists of chalcedonic silica. 



The material which forms the matrix of the well-rounded boulders 

 of the true conglomerates interbedded with the lavas is very 

 different from that described above, though the difference is perhaps 

 more in degree than in kind. It is reddish or purplish-brown 

 usually, and is much coarser-grained and softer. The fragments 

 are approximately equal in size, some being well rounded, others 

 subangular and angular. The great majority of the fragments 

 ■consist of felspathic lavas with considerable interstitial glass. The 

 felspars are sometimes quite fresh, showing twinning of the Carlsbad, 

 albite, and pericline types, and giving the extinction of labradorite. 

 The glass is usually crowded with magnetite dust, and is amygda- 

 loidal ; but, with the exception of a few pseudomorphs after olivine, 

 no other original minerals could be recognized in it. If we may 

 judge from the imperfect evidence available, the fragments are mainly 

 of basaltic or andesitic lavas. A few rounded grains of quartz and 

 flakes of muscovite also occur. Green chloritic material, in radial 

 aggregates, forms the matrix. Thin beds of sediment similar to 

 this, but without boulders, have been found among and at the 

 surface of the conglomerates, and even among the lavas in a position 

 similar to that of the horizontal sheets of sediment of the more 

 usual fine-grained type. 



