22 HENRY S. WASHINGTON 



The complete absence of biotite and the great poverty or gener- 

 ally absolute lack of the more salic rocks in magnetite and ilmenite 

 are very characteristic. The rather common presence, though in 

 very small amounts, of olivine in these rocks is interesting. The 

 investigation of Soellner/ with analysis by Dittrich, shows that it is 

 an almost purely ferrous fayalite. As is well known, the olivine 

 found elsewhere in highly silicic rocks, as granite and rhyolite, is 

 always fayalite, not common olivine or the magnesian forsterite. 

 The entire absence of nephelite tephrites andbasanites is remarkable 

 in view of the highly sodic character of the general magma. 



Norm and mode. — The rocks show some interesting relations be- 

 tween the norm and mode. They are quite normative as regards the 

 quartz and feldspars, both soda-microcline and andesine-labradorite, 

 and only slightly abnormative as regards the augite of the trachytes 

 and basalts, and the aegirite of the aegirite pantellerites. The 

 departure of the mode from the norm is, however, very marked in 

 the presence of the sometimes abundant cossyrite and the presence 

 of small amounts of fayalite in the pantellerites. 



It is interesting to note that the presence of the soda-hornblende, 

 cossyrite. goes hand in hand with normative sodium metasihcate. 

 In some cases, as in the trachytes, a very little cossyrite is present 

 with neither acmite nor sodium metasihcate in the norm, but in 

 general the amount of this mineral is correlated directly with that 

 of sodium metasihcate in "the norm, the rocks richest in cossyrite, 

 especially as phenocrysts, showing most excess of soda over alumina. 

 On the other hand, the rocks, in which aegirite is largely dominant 

 over cossyrite, the aegirite pantellerites, and comendites, show very 

 little or no sodium metasihcate, but, in general, large amounts of 

 acmite in the norm. 



This relation between sodium metasihcate and cossyrite, the 

 norm of which shows 8 . 66 per cent of sodium metasihcate, furnishes 

 an instructive commentary on Harker's'' criticism that the norm 

 may contain compounds "which are foreign to igneous rocks and 

 some of which are not known in nature." Cross^ has briefly dis- 



' J. Soellner', Zeits. Kryst., XLIX (1911), 138. 



^ A. Harker, Natural History of Igneous Rocks, London, 1909, p. 365. 



3W. Cross, Q.J.G.S., LXVI (1910), 499. 



