Igneous Pefthles. 2 *.»•'> 



extinction angle of 7 degrees from the 001 cleavage. It is dis- 

 tinctly biaxial, having a fairly wide axial angle; is negative; and 

 is referable to clinochlore. A little disseminated pyrites occurs 

 throughout the section, and is associated with brown iron oxide. 

 True spherulites are absent from the rock, but j)lumuse aggregates 

 of hornblende and felspar are common. 



Section W5 is practically identical with W6, except that the 

 spherulitic structure is not so well developed. Veinlets of quartz 

 and chlorite, with a little brown oxide, traverse the rock. 



Section W3, of a pebble from same locality as the foregoing 

 specimens. Microscopically, the texture is aphanitic, with the ex- 

 ception of a few porphyritic crystals of felspar. These appear 

 to be entirely calcic felspar, giving maximum extinction angles of 

 430 from the twin planes. The basis of the rock is not easily 

 deciphered, but it appears to consist of plagioclase, grains of Ijlack 

 iron oxide, and ? augite. Vesicles infilled with chlorite are rarely 

 present. The rock is probably a basic volcanic, approaching a 

 basalt. 



Section W9, pebble in conglomerate, near steel bridge over 

 Thomson River, is a highly chloi'itised. sericitised and carbonated 

 rock, probably a diabase. Apparently both plagioclase and ortho- 

 clase are present in the section ; chlorite (pennine) replacing ? 

 femic mineral; quartz showing strain effects and a little biotite and 

 ilmenite. 



Summary. 



A series of conglomerates, grits, and limestones, containing 

 igneous pebbles and debris, occurs near the Thomson river, Wal- 

 halla. Certain previous explanations, of the origin of the con- 

 glomerate, and of the igneous material, appear to be unsatisfac- 

 tory. It is shown that the explanation, which maintains that tlje 

 conglomerate is derived by post Silurian faulting and brecciation 

 of an accompanying dyke, is at variance with the field relations; 

 and also that there is little evidence of vulcanicity contemporaneous 

 with the deposition of the sediments. The Avriter concludes that 

 the conglomerate is a normal marine shore line deposit, and that 

 the igneous debris present in the conglomerates and limestones 

 is derived from pre-existing igneous rocks. 



Petrologically, the pebbles show considerable similarity. All 

 intermediate stages between a hornblende diabase (epidiorite), and 

 the spherulitic type of the same rock are represented in the 



llA 



