HuTTON. — On the Lyell District. 389 



crossed by a dyke of hornblende dolerite. About three miles 

 from Lyell we come upon unaltered Maitai slates, followed 

 at five miles by pink and white granites. 



When a junction between the granite and slates can be 

 well seen it is always sharply defined, and the granite is 

 generally altered for from lin. to 6in. in depth into a finer- 

 grained rock, with little visible mica. The altered slate is 

 highly micaceous, and is sometimes reddened for a foot or so 

 from the granite. The evidence is therefore conclusive that 

 the granite is eruptive. 



The granite is a rather coarse-grained rock, wuth white 

 felspar and both muscovite and biotite in about equal quan- 

 tities, and abundant. Its specific gravity is about 2-64. Under 

 the microscope, with polarized light, the quartz shows as a 

 rather fine granular mosaic (" granulitic" of Michel-Levy), with 

 larger pieces scattered about. Gas- and liquid-cavities are 

 present as usual, and the larger pieces have delicate pale-green 

 hairs of an undetermined mineral running through them 

 rather abundantly, as well as small prisms of apatite. The 

 felspar appears to be all orthoclase, often showing cross- 

 hatching due to the presence of microcline. The muscovite 

 and biotite show their usual characters in convergent polarized 

 light. Iron-oxides are very scarce. The granite boulders in 

 Lyell Eiver differ from that in the road-cutting in containing 

 plagioclase and in having less muscovite, but the quartz is the 

 same in both. 



The altered granite, near the contact with slate, is fine- 

 grained and brownish-white, with the mica not conspicuous. 

 Specific gravity, 2-67 to 2-78. Polarized light shows the 

 quartz in much finer grains than in the unaltered granite, but 

 it still exhibits the peculiarity of two sizes. Muscovite is 

 present in small crystals, and is rather abundant, but biotite 

 is absent or rare. Felspar is not recognizable as crystals, 

 but forms a kind of ground-mass of a greyish colour, with 

 bright points of quartz scattered through it, thus approaching 

 an elvanite. 



Higher up the Buller the main mass of granite agrees with 

 the boulders in the Lyell Eiver, so that it would seem that 

 the main mass is a plagioclase - biotite granite, but that 

 towards the western margin the potash minerals, muscovite 

 and orthoclase, are developed at the expense of the soda-lime 

 and magnesia minerals, plagioclase and biotite. 



The hornhlende dolerite is a dyke in the granite about two 

 miles from Lyell on the Nelson Eoad. It is very tough and 

 difficult to break so as to get unweathered specimens ; com- 

 pact, black, showing a ground-mass with irregular crystals of 

 augite and altered olivine. Specific gravity, 3-04. The micro- 

 scope shows an almost holocrystalline ground-mass, some- 



