Boult. — Occurrence, of Gold at Harbour Cone. 439 



the microscope the rock is holocrystalline and porphyritic. It 

 consists mainly of a gioundmass of a number of small crystals, 

 rounded and irregular grains of segerine, together with an im- 

 mense number of fine needles of this mineral. The segerine is 

 of a grass-green colour, pleochroic and very birefringent. Small 

 needles and lathes of feldspar occur in such small proportions as 

 to render any determination of their mineralogical properties im- 

 possible. Throughout this groundmass are numerous porphy- 

 ritic, hypidiomorphic, and idiomorphic crystals of hornblende, of 

 a brown colour, strongly pleochroic. In places these are larger, 

 with typical hornblendic characters. In parts of the section 

 they form nests, each nest being composed of long laths and 

 basal sections of hornblende surrounded by smaller needles of 

 hornblende and small masses of segerine. These are comparable 

 to the basic secretions found in platonic rocks. 



The order of consolidation was evidently — (1) hornblende 

 (crystals of this having been separated out, there was evidently 

 a tendency for others to form in its vicinity) ; (2) segerine ; 

 (3) feldspar. 



The stages of crystallization seem to have to a great extent 

 overlapped, the segerine having been in some cases contempor- 

 aneously crystallized with the hornblende in the nests, and the 

 needles of segerine and feldspar being consolidated together. 



Treatment with acid and evaporation discloses cubes of 

 sodium-chloride under the microscope showing the presence of 

 nepheline. The name " tinguaite " has therefore been given to 

 the rock. 



Trachytoid Phonolite. 



Two varieties of this rock occur as dykes, possessing distinct 

 characters. 



No. 1. — This occurs, as shown, near the mouth of the vallev 

 containing the mine. The course of the dyke is shown by the 

 outcrop of the rock in situ lrom place to place along the line 

 shown on the map. It appears to be about 7 ft. broad, and is 

 inclined at about 10° to the vertical. It is in a state of great 

 decomposition, so that the question as to whether it was ever 

 nepheline-bearing is not to be determined. Under the micro- 

 scope it is holocrystalline and porphyritic. The phenocrysts 

 consist of scattered crystals of hornblende of rather irregular 

 outline, though sometimes rounded or elongated. Most of these 

 are of a deep-brown colour, intensely pleochroic, while others 

 show a dark- and light-green pleochroism. An interesting feature 

 of the rock is the occurrence of crystals showing a gradual change 

 from a brown-coloured core to a gieen margin. The interior, 

 then, possesses a lower extinction angle than the exterior. The 

 groundmass consists in part of granular calcite with brilliant 



