336 Washington — Charnockite Series of Igneous Rocks. 



some of the districts rocks composed almost wholly of soda- 

 lime feldspar form a large part of the complex. It is worthy 

 of note that a peculiarity of the soda- lime feldspars in the 

 hypersthenic rocks of these districts seems to be the frequent 

 absence of twinning lamellae, so that at tirst glance some of this 

 feldspar might be taken for orthoclase. This peculiarity is 

 noted independently by Kolderup, Holland, Bugge, Adams, 

 and Rogers, is hinted at by Lacroix, and has been noted in 

 descriptions in the preceding pages. Microcline, or rather 

 anorthoclase with the usual microcline twinning, is a constant 

 component of the more silicic members. Orthoclase seems to 

 be constantly absent. 



Ilmenite, or a titaniferous magnetite, is constantly present, 

 even in the most silicic members, and would seem to be the 

 only essentially titanium-bearing mineral in these rocks. 



In their chemical characters, also, the hypersthene rocks of 

 the several districts are very similar, as may be seen from a 

 study of the analyses cited in the preceding tables.* In all the 

 districts there is great variation in silicity, SiO„ varying from 

 well over 70 per cent to about 50 or even considerably below. 

 The iron oxides are consistently higher than magnesia, f often 

 very considerably so, except in the most femic members, in 

 which the magnesia is largely in excess. A notable amount of 

 lime is always present, even in the most silicic rocks, but in 

 general (except in the most femic members), its amount is not 

 more than enough to satisfy the excess of alumina over alkalies 

 to form anorthite. That is, the lime is wholly salic, whence 

 the prevalence of hypersthene, except in the most femic rocks 

 where it is partly femic, forming the diopside molecule and 

 entering modal hornblende and augite to a much less extent. 

 On the other hand there is never, even in the most silicic 

 rocks, an excess of alumina, when satisfactory analyses are con- 

 sidered. With this is to be correlated the paucity in biotite. 

 The alkalies are rather high and soda dominates largely over 

 potash, except in the most silicic rocks, where the potash 

 increases until its amount is about equal to that of the soda. 

 In the great majority of types titanium is consistently low. 

 Phosphorus is always very low. 



All the regions named show these general characters in 

 common, but each possesses certain individual peculiarities in 

 the extension of the series. These peculiarities may be attrib- 

 uted either to initial differences in the composition of the 

 general magma at each ; to differences in the extent to which 

 differentiation has progressed ; or, as seems probable in some 

 cases, to lack of adequate knowledge of the region. 



* Only a few of those representing the other regions are given here, 

 f The molecular, not the percentage, relations are referred to here and 

 subsequently. 



