SKQT^EXOE OF ERI'PTTOXS IN THE ABIROXDArivR 19] 



liistory, 1)ut the similarity with the Norwegian rocks just mentioned is 

 so great that it demands notice. The anorthosites are common to both 

 regions. The norites and quartz-norites are also represented in the Adi- 

 rondacks, partly as peripheral phases of the anorthosites which were 

 undoubtedl}^ produced by differentiation in place, and partl}^ as some- 

 wdiat later eruptions which cut the anorthosites and also the older 

 gneisses, but wdiich have not been noted cutting the syenite-gneisses. 

 These norites grade into very basic ilmenite-norites and into quite pure 

 masses of ilmenite in both regions. 



The rocks of the monzonite group are represented in the Adirondacks 

 by the syenitic and granitic gneisses here discussed. These rocks have 

 certainly a range in silica percentage sufficient to include the banatites, 

 adamellites and granites of Ekersund-Soggendal, and as far as can be 

 judged from thin-sections the more basic monzonite end of the series is 

 represented as well. In these rocks we meet the first considerable 

 difference in the two districts. The Adirondack rocks, so far as chem- 

 ically studied, i^un too low in iron, magnesia and lime, and too high in 

 alkalies to be classed in the monzonite group (see analysis VII of the 

 table), though the corresponding niineralogic difference is mainly to be 

 seen in the character of the plagioclase, which is oligoclase in the latter 

 and albite in the former. This likely points to some slight difference 

 in the composition of the original magma, but the general resemblance 

 is so close as to be very striking. Finally, in both regions the eruptive 

 activity closed at a later period with the formation of diabase dikes 

 accompanied by more acid rocks, syenite-porphyry in the Adirondacks 

 and augite-granite in Norway. 



A word of comparison with the Essex county, Massachusetts, ])etro- 

 graphical province may not be amiss. The igneous rocks of the latter, 

 according to Sears, consist of granites, syenites, and quartz-syenites, 

 nepheline-syenites, essexites, diorites, and gabbros, all of which are cut 

 by numerous dikes.-^ Leaving out the nepheline rocks, these are the 

 same types as occur in the Adirondacks ; but when the relative pre- 

 ponderance of the different varieties in the two provinces is taken into 

 consideration it is clear that the original magma in the Adirondack 

 region must have ])een considerably the more basic of the tw^o, being 

 lower in silica and the alkalies and higher in lime and magnesia; 

 hence the prominence of gabbros in the one and of alkaline syenites in 

 the other. Notwithstanding this considerable difiference, some almost 

 identical rock types appear in each as a result of differentiation. It is 

 of interest to note that the two areas present almost precisely the same 



* J. JI. Seals m Bulletin Essex Institute, vol xxvii, 1S95. 



