510 W. ST. BENSON. 



plates, apparently replacing talc or serpentine. In one 

 instance, quartz, talc, and serpentine occur together with 

 much finely divided carbonate filling the original cracks. 

 In another rock quartz and carbonates alone represent the 

 olivine, the carbonates being in the cracks or arranged in 

 bands sometimes bent in a parallel series of lines, the 

 quartz being arranged in lines parallel with these. The 

 whole seems to be pseudomorphous after talc or serpentine, 

 at other times the carbonates may be more irregularly 

 distributed, the quartz in larger plates or in small irregu- 

 larly placed plates. Some areas again appear to be strictly 

 pseudomorphous quartz with very small amounts of car- 

 bonate or silicate. (See Plate 34, fig. 1.) Carbonates may 

 predominate in other areas, and a false appearance of 

 plagioclase twinning is given by the presence of clear 

 roughly parallel veinlets of quartz in the carbonates. In 

 one rock opal occurs in the central portion of a serpentine 

 mesh. Finally we have the complete replacement of the 

 olivine by carbonates without any trace of structure pre- 

 served. The alteration of olivine to chlorite also appears 

 to occur ; in one instance the birefringence of the green 

 material being too low for biotite, and the pleochroism 

 too weak. The chlorite is in simple subradiating groups. 



Ilmenite occurs as grains with an irregular rounded out- 

 line, sometimes slightly idiomorphic. It is generally decom- 

 posing into leucoxene, either peripherally or in a regular 

 tri-linear network. In these rocks the ilmenite is invari- 

 ably surrounded by, or associated with, a mass of chlorite 

 of a very pale apple-green colour, and which is composed of 

 fibres grouped in subradiate areas or tufts of low birefring- 

 ence. In this are irregular grains, or sometimes vermicu- 

 larly shaped patches of a carbonate, and rarely small flakes 

 of biotite. 



Pyrites occurs in certain of the rocks in idiomorphic 

 crystals or irregularly shaped grains. It is, 



