65 



Scapolite is usually present in considerable amount. 

 It polarizes in brilliant colours, is uniaxial and negative, 

 and shows the other microscopical characters of this 

 mineral. 



The feldspars vary greatly in amount. In places they 

 form a considerable part of the rock, while no scapolite is 

 present. In other places the scapolite seems to take their 

 place and they are reduced to the rank of accessory con- 

 stituents. All three varieties of feldspar mentioned often 

 occur in the same specimen, their relative abundance 

 varying from slide to slide. The polysynthetically twinned 

 plagioclase in some cases equals the potash feldspar in 

 amount, but usually the potash feldspars seem to be 

 rather more abundant. 



Quartz is found only in a few of the thin sections and 

 is then present only in very small amount. 



When calcite survives, it can be seen that the original 

 rock had the character of a coarsely crystalline limestone 

 or marble. Under the action of the metamorphic processes 

 the silicates have grown into it in the form of rounded 

 grains which, increasing gradually in size, have finally left 

 the calcite merely as a filling of the surviving interstitial 

 spaces. The grains are about the same size as those of the 

 other minerals. 



An examination of thin sections of a suite of specimens 

 of this amphibolite — some of them still containing little 

 surviving bands of calcite and others of the harder and 

 more altered varieties — shows that pyroxene and scapolite 

 accompany the hornblende and feldspais in the former, 

 while as the alteration becomes more pronounced these 

 former minerals become less abundant and eventually 

 disappear, giving rise to a rock composed of hornblende 

 and feldspar, associated with which a little biotite is seen 

 in some specimens and with certain accessory minerals 

 common to both rocks. Although, as above mentioned, 

 no actual passage of pyroxene into hornblende could be 

 definitely observed, the hornblende individuals often have 

 a minutely serrated edge where they come against the 

 pyroxene, as if they were gradually enlarging themselves 

 at the expense of the latter mineral and thus replacing it. 



The amphibolite, representing the final product of the 

 alteration, while possessing a more or less distinct foliation, 

 has the "pflaster", "pavement", or mosaic structure 

 characteristic of rocks which have resulted from recrys- 



35064—5 



