Boult. — Occurrence of Gold at Harbour Cone. 437 



Bostonite. (Plate XII, fig. 8.) 



This rock, which, as mentioned in " General Geology," con- 

 stitutes almost the whole of the mountain and neighbourhood, 

 is, where not discoloured by iron-oxide, quite white. Two 

 varieties exist. The first is a finely crystalline rock in which no 

 crystals of feldspar can be seen with the unaided eye ; the 

 second a coarse variety in which large crystals of feldspar are 

 seen, often | in. in length. This latter exists along the foot of 

 the cliffs fronting the harbour, and it is doubtless to the 

 deep burying and consequent slow cooling that this coarsely 

 crystalline character is due. The remainder of the mountain is 

 composed of the finer variety. Under the microscope the rock 

 is seen to be composed of a number of polysynthetically twinned 

 lathes of feldspar showing an evident flow structure. The rock 

 is completely crystalline, the only other mineral present being 

 magnetite in small granules and rounded masses. The rock is 

 rather decomposed, the resulting product being white kaolin. 



Ulrich in describing the district calls the rock a trachyte, 

 to which family it undoubtedly belongs. Though the rock is a 

 lava, its characteristic feldspathic mineralogical characters ally 

 it to the hypabyssal bostonites. Harker (" Petrology for Stu- 

 dents ") defines these rocks as consisting " of feldspar, quartz 

 never being abundant, and ferro-magnesium minerals being 

 typically absent. Phenocrysts may or may not be developed, 

 the bulk of the rock being a groundmass of little feldspar rods 

 often with partial flow disposition." There evidently is a very 

 close resemblance between this rock and the bostonites, except 

 in their occurrence. The name has been applied to the rock 

 even though it is a lava. 



A determination of the alkalis gave soda (Na 2 0) = 5 - 7 per cent., 

 and potash (KoO) = 4'2 per cent. Since the rock consists almost 

 entirely of feldspar, this shows that the feldspar is anorthoclase, 

 rather rich in soda, thus allying the rock to the ceratophyres. 



A variety of the rock occurring in a band surrounding the 

 syenite is seen to be much harder than the mass of the bostonite. 

 It is densely impregnated with iron-pyrites. Under the micro- 

 scope its structure is that of the last variety, but it has large 

 masses and small grains of pyrites in it, together with patches 

 of red iron-oxide. This band is certainly due to the altering 

 effect of the intrusive mass of syenite. 



Bostonite Dyke. 

 This is a rock which occurs in a large dyke 8 ft. across, as 

 indicated in the map. Its course can be clearly traced for 5 or 

 6 chains, as the solid rock shows above the surrounding loam and 

 soil. 



