6 F. L. Stilliuell: 



quartz. A vesicle tilled with quartz is rimmed with a network of green 

 prismatic crystals showing high polarisation colours and oblique extinc- 

 tion. Pleochroism, if any, is extremely faint, and the mineral looks 

 like epidote, but has not got the extinction angle of ©pidote. 



These characters, combined wdth its high basicity and chemical 

 composition, and its mode of occurrence as a dyke, are sufficient to 

 place the rock among the monchiquites. The felspar is too insigniti- 

 cant in quantity to displace it from this monchiquite group. 



Dr. Howitt (3) has described a dyke rock apparently similar to the 

 above, from Lansell's 180 Mine, on the Kew Chum line, and named it 

 limburgite. Miigge (3) in a review of Dr. Howitt's paper, suggested that 

 the name monchiquite should be used to agree with the mode of 

 occurrence, and the name limburgite- should be reserved for very basic 

 lava flo'ws. This is the proper interpretation of the terms, and there- 

 fore we may call our type rock monchiquite. 



An examination of a number of sections of lavas from other mines 

 reveals the same general chariicteristics of the monchiquite group, with 

 minor variations. A complete comparison is not possible, because the 

 lavas ai'e so frequently decomposed. Tliis decomposition is in no way 

 proportional to the depth below the surface, as show^n by the Central 

 Red, White and Blue rock. Further, the decomposition, once started, is 

 surprisingly rapid. A lava, exposed only for six months at 2600 feet 

 in the Koch's Pioneer was found to be crumbling in part, though quite 

 dry. In the Pearl Mine a lava exposed some thirty years along the 

 roof of a disused drive at 130 feet, has rotted sufficiently to form a 

 linear heap of mullock along the floor of the drive. 



In spite of this, fresh rocks, suitable for microscbiDical examination, 

 were obtained from a number of places, and the following notes may be 

 made: — Specimen No. 14, Lansell's 180 Mine, No. 2. The sample was 

 obtained from the 400-foot level, about 30 feet south of the shaft. This 

 mine is situated on the Sheepshead line, and adjoins the Central Red, 

 White and Blue Mine. Three l)ranches of the lava are present in this 

 mine, while there are only two in the other. In this rock olivine has 

 been more al)unda.nt, ))ut is now completely serpentinised. Augite is 

 present in two generations. The large augites are nearly colourless 

 and sometimes zoned, sometimes with houi-gla^s structure. The small 

 augites form the san)c kind of network as in the Ci'iitral Hed. Wliltc 

 and Blue rock. Tlie hornblende is present in al)0ut the same ]n-opor- 

 tinn, but the ihnenite is relatively much less abundant. The colourless 

 ground mass is in small piopoi-liou, l)ut a])])ears in the same residuum 

 as patches, crowded with hornblende and ihnenite microlites. Vesicles 

 are more developed, and this is always noticeable with the further 

 <kH'Oui|H)siti(in of tlic rork as recorded by ])r. Tlowitt. Many of these 

 liave an innei' cow of green anior[>hous siliia, rimmed with calcite, and 

 the alteration is s(jmetimes repeated. 



