Notes on the Area of Intrusive Rocks at Dargo. 141 



assumes that the measurement of 3 degs. was taken in the 

 plane OP (001); a less angle would still be in the Andesine 



^roup, 



unless it were less than 1 deo\* It also assumes 



that only one kind of plagioclase occurs in this rock, which 

 is not at all certain. 



(d) A very small amount of quartz, in homogeneous 

 portions, filling spaces between the minerals, otherwise 

 resembling that of the granitic rocks. 



The subjoined quantitative analysis is of this rock. Ko 

 mica was observable in these slices: — 



No. 4. — Quartz Diorite. 



Si.O, 





5203 



Al.,03 ... 





20-57 



Fe...03 ... 





1-60 



Fe.b 





6.97 



Ca.O 





7-80 



Mg.O ... 





5-39 



K.O 





134 



Na.^O 





2-37 



H.O 





1-27 





99-34 



Moisture 



•26 



Sp.gr.... 



.. 



2-855 



In order to be able to calculate with some degree of pre= 

 cision the percentages of the rock-forming minerals from 

 this analysis, it would be necessary to have a knowledge of 

 the composition of the amphibole and of the felspars. An 

 estimate of the probable composition of the rock can, how- 

 ever, be made by calculating the felspars out on the basis of 

 the optical measurements, and also the alteration products 

 in the assumption that the H^O indicates kaolin and 

 chlorite, which, according to the microscopic examination, 

 would probably be in the proportion of one of the former to 



Oligoclase ... 

 Andesine i **' 

 Labrador 



Formula. 



Ab2anl ... 

 . Ab 3 an 2 ... 

 . Ab4an3 ... 



Ablanl ... 



Obscuration angl 

 on O.P. (001). 



3° 35' 



2° 12' 



... 2° 58' 

 5° 10' 



Bosenbusch — Physiographie, &c., 2nd E., p. 535. 



