DISCUSSION OF ANALYSES 185 



from Loon lake except for its freedom from nietamorphism. Hyper- 

 sthene is lacking, and there is rather a predominance of hornblende over 

 augite. which latter is colorless instead of green. Chemically it is closer 

 to the rock from Diana than to that from Loon lake, but is lower in 

 lime and magnesia than either. The agreement is, however, very close. 



Likewise the syenite from Essex county, ^lassachusetts, just described 

 anew by Mr. H. S. Washington, shows great similarity with the preceding.* 

 It lacks hypersthene, and the augite is like. that in the Mount Ascutney 

 rock. The specimen analyzed is more acid than the Adirondack rock, 

 with lower alumina, higher iron, and slightly lower alkalies, with the 

 potash somewhat in excess of the soda. These differences are all slight, 

 and of the essential identity of the rocks there can be no question. As 

 stated b}^ Washington for the Massachusetts rock, all belong to the variety 

 of augite-syenite called " akerite " b}'- Brogger and the '' akeritetype " by 

 Rosenbusch — in other words, are quartzose augite-syenites. The analysis 

 quoted by Washington of an akerite from Norwa}'' (column V) is ap- 

 pended, though, as stated by him, it is an acid representative of the 

 group. Furthermore, it is unusually low in lime, much more so even 

 than the Mount Ascutney rock. The analyses bring out clearly the con- 

 siderable variation to which these rocks are subject. Not improbably 

 they could all be duplicated in each locality. 



All the rocks represented in the first ^ve analyses are quartz-syenites 

 and quite acid representatives of the syenite group, approaching the 

 acidity of granites. Column VI gives an analysis of a more normal 

 syenite, introduced merely to empha«?ize the departure of the others 

 from the ordinar}'' type. 



Banatite is the name given by Bnigger to rocks of the monzonite 

 group (orthoclase-plagioclase rocks) which range between 62 and 67 per 

 cent of silica. The analysis (column VII) will indicate the differences 

 between them and these akerite-syenites, namely, the higher amount of 

 lime and magnesia and lower alkalies. Rocks of the monzonite group 

 are widely exposed in western Norway, a petrographical province which 

 has many features in common with tbe Adirondacks, in close associa- 

 tion with rocks of the gabbro group, anorthosites, norites, and so on. 

 They have recently been exhaustively investigated by Kolderup,t and 

 will be reverted to later. 



Geologic Age 



Quite fortunately the exposures in the railroad cuts near Loon lake 

 furnish data for a rough determination of the age of the syenite. For 



* Jour. Geology, vol. vi, pp. Y96~798. 



t Bergens Museums Aarbog, 18')C, no. V. 



XXViri—BuLL. Geol. Soc. Am., Vol. 10, 1898 



