254 PROCEEDINGS OP SECTION C. 



Mr. W. Skey has given two partial analyses of the Thames 

 rocks, when decomposed to the pale grey state. 



Silica ... 



Alumnia 



Iron oxide — chiefly protoxide. 



Lime ... 



Magnesia 



Alkalies 



Water 



Specific gravity 2.510 2.296 



I. — From Waintahi Creek. 

 II. — Bed rock of principal claims at the Thames. 



The greater proportion of silica in II than in I is due to the 

 chlorite and calcite having been leached out, and the secondary 

 quartz left behind, as will be explained presently. 



In these rocks the ground-mass is always abundant, and more 

 or less devitrified ; and, except in the last stages of decomposition, 

 it always contains chloritic infilti'ations. In one of the Te A roha 

 specimens felspar microlites are abundant, and shew a well-marked 

 fluxion structure. 



The felspars, when recognisable, are always plagioclase, zoned 

 with inclusions. In one specimen I noticed twinning on the 

 pericline type, as well as on tlie usual albite type ; the same 

 specimen shewed a crystal which gave extinctions of 30° and 35° 

 on either side of the twinning plane, and is probably, therefore, 

 bytownite or anorthite. Generally the felspars are decomposed 

 to a colourless aggregate which is largely composed of calcite, but 

 may also in part be kaolin, a mineral which is not easy to recognise 

 in minute particles, especially if it be intimately associated with 

 an anistropic minei^al like calcite. The abundance of calcite in 

 the decomposed felspars confirms the idea that they were bytownite 

 or anorthite. Binary twins are rare. 



Chlorite, as pseudomorphs after augite, and as infiltrations in 

 the ground-mass and occasionally in the felspars. The chlorite in 

 the felspars and ground-mass is pale green and usually isotropic. 

 That in the augites is yellowish, or brownish-green, and is either 

 isotropic, or of a deep and grey colour between crossed nicols. It 

 is often seen to be in prisms either square or octagonal. This 

 chlorite has again been altered, in part or entirely, into a colourless 

 aggregate with brilliant polarization colours, and the cyrstals have 

 then generally narrow black borders. The colourless aggregate 

 may be talc. 



* With carbonic acid. 



