REPORT OP THE DIRECTOR AND STATE GEOLOGIST 1900 r75 



The pyroxenes and hornblende which these rocks contain are 

 precisely like those in the anorthosites, strongly suggesting com- 

 munity of origin. The feldspars are alkali feldspars with closely 

 corresponding soda and potash content. In the general rock 

 garnet is a much less conspicuous feature than in the anortho- 

 sites, and is often wholly absent. This is but natural, since the 

 garnet is not primary but has resulted from the interaction of 

 feldspar and magnetite. It is a lime iron alumina garnet, and 

 the necessarj^ lime for its formation is lacking in the alkali feld- 

 spar of the syenite. A further distinction between the two rocks 

 lies in the abundance of zircon in the syenite. It 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 calcula- 

 tion 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 properly 

 belonging wuth 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 percentage, 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 

 wonld 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 suffi- 

 ciently numerous to furnish a very fair representation of the gen- 

 eral 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 lofo of silica and prob- 

 ably higher, and that, since their feldspar is universally micro- 



