67 



No. I represents the first stage of alteration, and was 

 made from a specimen which shows an alteration of narrow, 

 lighter and darker coloured bands. The specimen was 

 broken across the strike of the rock and thus included 

 several of each of these bands, giving in this way an ap- 

 proximate average of the composition of the rock as a 

 whole. Under the microscope the lighter coloured bands 

 are seen to consist of calcite, pyroxene and a little horn- 

 blende. In the darker bands the calcite is largely replaced 

 by the silicates, the constituent minerals of these bands 

 being scapolite, pyroxene, some hornblende, some calcite, 

 and a little microcline. A very small amount of sphene 

 is also present in the rock. 



The analysis as given under No. I (a) represents the 

 composition of the specimen as collected ; that given under 

 No. i (b) represents the composition of the rock as it 

 appears when the calcite present (determined by calcu- 

 lation from the amount of C0 2 present and also by direct 

 experiment) is deducted and the amount of the remaining 

 constituents is recalculated on the basis of ioo. No. I (b) 

 therefore represents the percentage composition of the 

 silicated portion of the specimen, or, to put it in another 

 way, it represents, except in the case of the lime, the ad- 

 ditions made to the limestone by the granite magma in 

 this first stage of alteration. The specimen contains 34-50 

 per cent, of calcite, leaving 65-50 per cent, of silicates. 

 This silicated portion of the rock, as will be seen by com- 

 paring analysis No. 1 (b) with Nos. 2 and 3, bears a general 

 resemblance in composition to the two latter rocks, which 

 represent the subsequent stages of alteration, the percent- 

 age of silica being practically identical in all cases. 



No. 2 is the analysis of a typical specimen of the 

 amphibolite which alternates with thin bands of the 

 limestone at Maxwell's Crossing. It represents a second 

 stage in the alteration, this particular specimen being 

 practically free from calcite. Under the microscope it is 

 seen to be composed of hornblende and pyroxene, more or 

 less completely replacing each other in the alternate bands, 

 together with a considerable amount of scapolite, plagio- 

 clase and untwinned feldspar. The rock also contains 

 many minute rounded grains of sphene scattered every- 

 where through it, but holds no iron ore and no biotite. 



No. 3 is the analysis of a harder variety, a typical 

 amphibolite representing the last stage of the change. It 



35064— 5* 



