338 



NEW YORK STATE MUSEUM 



by no means rises to the dignity of an essential constituent but 

 is much more abundant and attains a larger size than in the 

 usual igneous rock. The syenites contain quartz almost without 

 exception, and the amount increases toward the acid end of the 

 series, the calculation of the analysis of column 14 showing 14$ 

 of that mineral. 



Analysis 10 is of a green gneiss which occurs associated with 

 granite and granitic gneiss near Piercefield. Its field relation- 

 ships to the syenite are not plain, and the doubt about its prop- 

 erly belonging with them is not cleared away by the analysis, 

 which falls slightly out of the series in its magnesia-lime ratio 

 and in its total magnesia. The rocks nearest it in silica per- 

 centage, 8 and 9, have this ratio, 1:3 and 1:3.5 respectively, as 

 against 1:1.7 in 10. Its ratio is nearest to that of 13. On the 

 other hand, it can be argued that its general great similarity in 

 composition would seem to ally it closely with the syenites, and 

 that these show a great variation in the magnesia-lime ratio, 

 even though it approaches so near to equality in no other. 



General characters of the Adirondack eruptives. The analyses in 

 the preceding table are thought to be sufficiently numerous to 

 furnish a very fair representation of the general characters of 

 the Adirondack eruptives, except for the lack of analyses of the 

 granites. The latter vary greatly, ending with very acid rocks 

 composed almost wholly of quartz and feldspar. It is quite safe 

 to say that they will reach 75^ of silica and probably higher, and 

 that, since their feldspar is universally microperthite, the ratio 

 of soda to potash will remain substantially as it is in the 

 syenites. 



The gabbros and anorthosites are quite normal representatives 

 of these groups. But in the transition rocks between these and 

 the syenites we find low magnesia, low ratio of lime and mag- 

 nesia to alkalis, and approximately equal amounts of soda and 

 potash, and these characters continue to the end of the series. 

 The soda-potash ratio is a slowlv rhnn^inor one, the potash beins: 

 at first below, but eventually overhauling and passing the soda 

 in the more acid rocks. In these respects the syenites, and prob- 

 ably the granites, depart somewhat from the corresponding 

 rocks of the Ekersund-Soggendal area in Norway, which also ac- 



