340 NEW YORK STATE MUSEUM 



Adirondack rocks closely approach the monzonite type. They 

 also closely approach Brogger's akerite type (quartz augite 

 syenite) from near Christiania, and seem to occupy a position 

 intermediate between the two. Smyth's rock from Natural 

 Bridge, column 1 of the above table, would certainly fall within 

 the monzonite group, notwithstanding its high lime percentage; 

 and the rock from north of Tupper lake, column 9 of the origi- 

 nal table, belongs also in that group lying on the border land 

 between monzonite and banatite.^ Because lof this, it is per- 

 haps more logical to refer all the Adirondack syenite to that 

 group, though as a somewhat aberrant type. 



It would therefore appear that in each district a very similar 

 magma has given rise to a very similar rock series, and, it is 

 likely, thriough a similar differentiation process. The order of 

 succession of the different types can not be compared, since the 

 Adirondack succession is uncertain in one respect. The syenite 

 followed the anorthosite, and then came the granite, but the 

 position of the gabbro is uncertain. It is certainly later than 

 the anorthosite, and certain gabbroic dikes which have been 

 found cutting the syenite lead to the impression that it is later 

 than that, but there is some question as to the correctness of 

 their reference to the main gabbroi of the region. There may 

 have been two periods of gabbro outflow, one earlier and the 

 other later than the syenite. 



A few dikes, and a few small eruptive masses;, of three or four 

 different types, have been noted which are not referable to any 

 of the great masses apparently. But as yet their relationships 

 are obscure. All are younger than the anorthosite, and all are 

 metamorphosed. 



General metamorphosed condition of the intrusives. All these 

 igneous rocks have undergone severe metamorphism, as shown 

 by the partial or complete granulation of the original minerals, 

 the large amount of recrystaJlization, and the production of 



^The monzonite group was established by Brogger to contain rocks inter- 

 mediate between the granite syenite group (ortboclase rocks) on the one 

 hand, and the diorite gabbro group (plagioclase roclis) on the other ; hence 

 characterized by both orthoclase and plagioclase. Monzonite has a silica 

 percentage between 50% and 60%, banatite between 60% and 66%, and 

 adamellite over 66%. 



