president's address SECTION C. 



18T 



Petrological and Chemical Characters. 



The names given by Howitt to these rocks do not at once suggest 

 their derivation from an alkali magma. His descriptions of their 

 niineralogical contents and a glance at the analyses which he made 

 of them, however, show their relationships with the group of the 

 ceratophyres, and, indeed, Howitt has given that name to several of 

 the rock-sections which he had prepared from this series. 



Three analyses which he made are here recorded : — 



* SiOa determined indirectb'. 



1. Quartz-Mica-Porpbyrite (Quartz-Ceratopliyre) , Navigation Creek. 



2. Qviartz-Mica-Porphyrite (Quartz-Ceratophyre), Navigation Creek. 



3. Quartz- Porpliyrite (Quartz-Ceratophyre), Mount Elizabeth Creek. 



Quartz-mica-porphyrite (QvaHz-ceratoplvyre). — This rock shows 

 a microcrystalline granular ground mass of quartz and felspar with 

 minute microliths of chloi'ite replacing probably amphibole. The por- 

 phyritic constituents are as follows : — 



Oligoclase of an acid variety showing both albite and Carlsbad- 

 twinning. 

 Quartz in corroded and fractured crj'stals. 

 Chlorite pseudomorphs after a magnesia-iron-mica. 



Associated with this t}-pe are the rocks described by Howitt as quartz- 

 granophyrites. These in the field occur as dyke-like masses, but their 

 microscopic characters are similar to those shown usually by acid lava 

 flows. Fluidal structure is sometimes seen, the rocks appear to have 

 been originally glassy, and subsequent devitrification has produced 

 large spherulitic areas and a somewhat granophyric intergrowth of 

 quartz and felspar. In some of the less spherulitic varieties por- 

 phyritic felspars occur. Most of these are acid-oligoclase, but some 

 have the optical characters of anorthoclase, and the rock is clearly a 

 spherulitic variety of quartz-ceratophyre. 



Quartz-porphyrite (Quartz-ceratophyre). — These in the field con- 

 sist of dykes of a milk-white colour. Under the microscope it is seen 

 that quartz and felspar are the only constituents. The fine-grained 

 ground mass of quartz and felspar has a few porphyritic crystals of 

 the same minerals scattered through it. The felspars were too 

 kaolinised for precise determination, but the analysis shows that they 

 were soda rich, and that the rock is a quartz-ceratophyre. 



