Rocl-s! nrar Heathcote. 337 



converted into a feiTuginuus carbonato rock h\ decomposition of 

 the original silicate by carbonated water. Crystallization of the 

 carbonates proceeded from isolated centres, and eventually 

 became ei influent. Later silica-bearing solutions have replaced a 

 good deal of the carbonates by chalcedonic silica, but the 

 original boundaries of the carbonate areas are still traceable in 

 the cherty areas by fine dusty inclusions. 



The exact nature of the carbonates cannot be determined 

 under the microscope, but they probably consist of a mixture 

 of lime, magnesia and ferrous carbonates, the latter by subse- 

 quent partial oxidation have imparted a red-brown culour to 

 the uuiss. 



On the W. bank of the Mclvor Creek, about 200 yards N.W. 

 of this point, there is an exposure of the bedded chert series. 

 The relations of this to the calcareo-siliceous diabase rock can- 

 not be clearly made out. 



(h) Corundum in the Diabase. — Close to the jasperoid, 300 

 yards south of the selwynite outcrop, is a small gully, and in it 

 I found a mass of a heavy, purple and green coloured rock, 

 which was found to consist of a mixture of corundum and a 

 green micaceous mineral. The specimen was not in situ, but 

 could not have rolled far, as the gully terminated about 100 

 yards north of where it was found. 



On examining the workings at the selwynite outcrop, Mr. 

 Summers found a second lump of the same rock associated with 

 the seh\ynite. A section from the first specimen (No. 550) 

 (Pi. X VT., Fig. 1) shows prismatic, pink, highly refractory needles 

 and irregular masses of coiundum. They are noticeably pleochroic, 

 and have polarization colours about the same as those of quartz. 

 Stouter prisms, pi'obably of a pale orthochrombic pyroxene, 

 showing good prismatic cleavage, generally straight extinction 

 and bright polarization colours of the second order, also occur. 

 The backgi'ound is composed of a very pale green to colourless 

 micaceous mineral, probably chromiferous. Red bro\\-B to 

 opaque graina of chromite are also sparseh- scattered through the 

 rook slice. 



On examining sections cut from the siliceous diabase which 

 passes into jasperoid at the outcrop, 300 yards south of the 

 selwynite, evidence was obtained that the corundum occurred in 

 situ in the diabase. Under tlie microscope (No. 551) (PI. XVI., 



23 



