Notes on the Area of Intrusive Rocks at Bar go. 133 



now recognisable. The ground-mass of the rock is composed 

 of innumerable minute interlocking grains of quartz, which 

 are so arranged that a number of them are so nearly 

 optically the same that they become obscure almost at the 

 same time when rotated between crossed nicols. These 

 areas have in places a rounded or elliptical shape, suggesting 

 that each may represent an original crystallisation after- 

 wards broken into fragments, or perhaps more correctly, 

 separated by irregular flaws. The slice thus appears com- 

 posed of adjoining dark and light areas, which alternate on 

 rotation. In this quartzose mass are innumerable flakes and 

 rounded plates of two micas, rounded grains and crystals of 

 magnetite, which are all aggregated more in some places 

 than in others. 



Subjoined is a quantitative analysis of this rock : — 



I 



^0. 2. — HOENFELS. 





I'A 







•15 



Ti.O, 







•17 



Si.O, 







62-28 



Al.,03 .. 







2016 



Fe.,03 .. 







•53 



Fe.O 







3-84 



Ca.O 







•82 



Mg.O 







2-54 



K.,0 







6-40 



Na.,0 .. 







1-29 



H,0 







1-86 





10004 



Moisture 



•72 



Sp. gr. 



... 



... 



2-744 



This sample was selected as apparently representing 

 one of the least originally quartzose sediments. If this is 

 the case, it shows in a marked manner the increase of silica, 

 which I have before referred to, and one may also observe 

 that, as is the case generally in these metamorphic rocks, the 

 amount of combined water is lessened in the process of 

 recrystallisation, together with a total removal of any 

 carbonaceous material. 



I found that still greater changes had been brought about 

 in the sediments immediately adjoining the contact. A good 



