434 Pii % sson and Washington — Geology of Red Hill, JV. H. 



a, pale brownish yellow and carries inclusions of zircon sur- 

 rounded by pleochroic halos. It forms occasional tablets l mro 

 long by 0*5 broad which might rank as phenociwsts but the 

 greater part of it is scattered through the ground mass in 

 minute shreds, flakes and tablets varying from O05-O10 mm in 

 diameter. 



The aegirite-augite is in shapeless pieces which now and 

 then tend to elongated sections ; it has the following pleo- 

 chroism: a and b, clear grass-green, c yellow. The angle of c on 

 n is variable, about 10° and less toward the border in some 

 cases ; this shows it to be between pure aegirite and aegirite- 

 augite. 



Rosenhuschite. — Attached to aegirite there was observed a 

 bunch of radially divergent, colorless needles, whose refractive 

 index is about l - 6, birefringence rather high. They have a 

 negative optical extension in all cases. Are in nephelite. A 

 few other needles of the same mineral were observed with 

 these characters. They are not sillimanite, wollastonite or 

 pectolite and the optical and other characters combined with 

 the place of occurrence make it practically certain that the 

 mineral is rosenbuschite or an allied one. 



The feldspar phenocrysts whose size and shape has been 

 mentioned are of microperthite, the amount of albite and 

 orthoclase being about equal ; the albite lamellae commonly 

 show albite twinning. 



The groundmass is made of laths of alkalic feldspars which 

 average about 0*20 by O03 mm . Sometimes these are of albite 

 as shown by the twinning, sometimes they are of orthoclase. 

 In the angular interspaces between them is the nephelite and 

 sodalite, the last products of crystallization. The sodalite was 

 in somewhat larger areas than the nephelite and at times a 

 little cancrinite is with it. 



Mode. — The actual quantitative mineral composition was 

 determined by the Rosiwal method. With a moderately high 

 power the minerals were well individualized and could be 

 measured fairly accurately. There was of course some uncer- 

 tainty at times in discriminating between the feldspathoid 

 components and their borders, but these errors, provided a 

 sufficiently large number of measurements be made, tend to 

 balance one another. The average size of grain was 0*08 mm 

 and 150 sections of the grains were measured in a total dis- 

 tance of 12*00 mm with the results given in the table. 



The amount of zircon is probably too high ; a single large 

 grain was encountered in the traverse ; there is too little both 

 of it and of magnetite to determine it accurately, but the 

 amount is too small to be of practical importance. 



Chemical Composition. — The approximate chemical com- 



