Notes on Monckiqu'de Dykes. 7 



Specimen No. 9, 3827-feet, Victoria Quartz Mine. The complete 

 alteration of the olivine in this rock is very noticeable, and the ground 

 mass is perfectly isotropic. This New Chum dyke was found to be 

 very decomposed down to 1300 feet iu the Pearl Mine, and very hard 

 ap;ain in the Catherine Mine, Eaglehawk. Here the rock is similar 

 to that in the Victoria Quartz Mine, but the vesicles are far more 

 abundant, and many of them contain rims of a coarse fibrous carbon- 

 ate, probably calcite. 



Specimen No. 34, 1868-feet, Johnson'.s Reef Mine, No. I. This is the 

 Garden Gully lava, and here it is very dense, and the colourless ground 

 mass is scarcely perceptible. Ihnenite is scarce, vesicles are numerous, 

 and many are curiously lined with pyrite. The same dyke at the 

 Clarence United Mine contains a.n average amount of ilmenite. 

 Natrolite is well developed in the vesicles of this rock. The natrolite is 

 prismatic, positive in sign, with straight extinction and low polarisation 

 colours At the Koch's Pioneer Mine, the Garden Gully lava is not so 

 fresh, but a section of a 2-foot dyke shows it to be relatively coarse. 

 Olivine is completely serpen tinised. Large augites are present, some 

 of which have a core of inclusions. Ilmenite and hornblende are 

 present in average proportions. Small biotites are noticeable. The 

 ground mass is in general not isotropic, and consists mainly of a low 

 polarising mineral. This is probably original felspar, but may be 

 secondary, and developed during the alteration of the rock. 



The rocks from the places above mentioned come within the mon- 

 chiquite group, from which the following deviate : — 



Specimen No. 7, Goldfield's Mine, No. I. The GoldtieM's No. 1. is an 

 abandoned shaft at the southern end of the Nell Gwynne line of reef. The 

 specimen was obtained from the dump, and was a piece of a thick lava 

 met in sinking the shaft at the depth of about eighty feet. The first 

 glance under the microscope shows the rock to be generally Siimilar to 

 the true nionchiquites. Olivine phenocrysts are partly fresh, and only 

 jnirtly serpentinised. There is only one generation of augite. The 

 ilmenite is very abundant and hornblende is present. Blotite is 

 present in i-elatively large crystals. The feature of the rock is that the 

 ground nuxss is perfectly clear and colourless, and not isotropic. A 

 large vesicle is present in the slide, and contains a. little biotite and a 

 great deal of calcite, and these are mixed with a mineral which is 

 apparently identical with the true ground mass of the rock. Here it 

 is found to possess cleavage, and an extinction of three or four degrees. 

 It is untwinned and the polarisation colours are never above gi'eyish- 

 white. It is biaxial and positive, and has a refractive index less than 

 that of oil of cloves (1.5333). Its appearance is that of a felspar, and 

 its refractive index is that of orthoclase and anorthoclase. In addition 

 it has in some places an appearance suggestive of \pry fine lamellar 



