288 PENTTI ESKOLA 



We have found that the clinopyroxene gneiss has, in the majority 

 of cases, a composition that is nearly the same as that of the ordinary 

 Becket gneiss of the surrounding area, excepting that it has a higher 

 amount of lime. A bulk composition may therefore be computed 

 from the analysis of the gneiss quoted above (p. 268), by allotting 

 CaO enough to form anorthite with all the excess alumina (actually 

 present in the micas), to form diopside with all the (Mg,Fe)0 

 present, and titanite with all the TiOz present. Such a calculation 

 indicates practically no other change in the composition than an 

 increase in the percentage of lime from 1.30 to 2.97 per cent. 

 The rock would contain 25.2 wt. per cent quartz, while the actual 

 Becket gneiss has 27.7 wt. per cent. 



The actual clinopyroxene gneiss in many cases differs from 

 this imaginary rock in being poorer in quartz, and having soda in 

 excess over potash or potash over soda. The aplitic gneiss around 

 the skarn inclusions near Benson Pond is an example of a highly 

 potassic type low in soda and silica, being composed practically 

 of potash feldspar only. The analyzed rock from south Peru, on 

 the other hand, is an extremely sodic variety, low in potash and 

 silica. In both cases practically all the excess silica is gone, and 

 the feldspars and the pyroxene only have been left. 



Where a magnesic clinopyroxene occurs in an acid gneiss it 

 proves that the assimilation of limestone had happened nearly 

 in place without further development, while the occurrence of 

 pyroxene high in the iron compound, as was found in some cases, 

 indicates that some differentiation had taken place after assimila- 

 tion. Both cases occur in the banded Becket gneiss (cf. p. 277). 



The skarn exhibits a special case. It is supposed to be a 

 pneumatolytic contact-product formed in limestone in such a way 

 that adjacent magmas have brought in silica and metal compounds 

 which have replaced the carbonate. 



It is pretty clear, in the case of the clinopyroxene gneiss in 

 question, that it has been formed in part by direct assimilation of 

 limestone by the gneiss magma and in part by the assirnilation of 

 skarn previously formed by contact action. 



The main indication of the banded structure of the gneiss 

 seems to be that its intrusion and crystallization has happened 



