( 407 ) 

 From which follows the following- norm: 



quartz 25.38 Q = 25.38 



orthoclase 6.67 i Sal : = 86.58 



albite 35.63 ( F = 61.20 



anorthite 18.90 ) 



diopside 1.54 / p _ 6.03 



bronzite 4.49 ) 



ilmenite 0.91 j m 3 qq 



magnetite 2.09 \ 



Few — 9.03 



and the rock is placed in Class I (Persalane) ; Subclass 1 'Persa- 

 lone); Order 4 (Brittanare); Rank 3 (Coloradase) ; Subrank 4 (Yellow- 

 stonose). 



If we take into consideration that the diopside-molecnle is cry- 

 stallized as amphibole, there is a satisfactory agreement to be observed 

 between norm and mode. 



If Fe s O, is reduced to FeO, the molecular proportions, calculated 

 on a sum of 100 (Ti0 2 added to SiO,) give the following values: 



SiO, A1 S 8 FeO MgO CaO Na,0 K,0 

 74.14 9.97 2.83 2.63 5.05 4.58 0.81 



so that the formula according to Osann is : 



sACFacfnmk Series 



I 74.14 5.39 4.58 5.93 6.78 5.76 7.46 8.5 9.2 1.56 av 



II (10.17 8.6411.19) 



In I a-f c + f = 20j in II a + c -f f = 30. 



Both dacites, therefore, show a great chemical similarity, which 

 is also expressed by the graphic notation, fig. 1. The somewhat 

 greater basicity of the second rock, which contains more CaO and 

 MgO, a little less SiO s , and half the quantity of K a O, manifests 

 under the microscope by the character of the feldspars and the greater 

 quantity of amphibole. 



In order to compare them some analyses have been brought 

 together, which show a great similarity with the two discus ed 

 above : 



27 



Proceedings Royal Acad. Amsterdam. Vol. XI. 



