196 J. D. Dana—Age of the Green Mountains. 
to be gneissoid rock, meaning apparently gneissoid mica 
schist.* 
It appears then, that even the mountain belt of Vermont in 
its northern half consists of rocks much like those of the re- 
gion either side; and they are essentially the same for the rest 
of the belt, with this difference, that to the south the mica 
schist is replaced to a large extent by gneiss. 
Again, according to the sections in the Vermont Geological 
Report and the text describing them, the schists of the mountain 
belt are conformable with the hydromica schist on the east and west. 
an Massachusetts, the schists east of the limestone belt are 
mica schist (as in the Hoosac Mountain) and micaceous gneiss, 
and they are conformable, according to Hitchcock and my own 
imperfect observations, with the beds of the limestone belt. 
But an Archean area extends northward from Connecticut, 
Mountain belt of gneiss, and that a diminution in the amount 
of feldspar makes the difference. 
In Connecticut the rocks need more study before general 
conclusions can be positively stated, owing in part to the Arch- 
ean areas which give complexity to the subject in the northern 
half of the State.+ 
Connection between differences in the rocks and geographical 
distribution—As has been shown, marked differences exist be- 
tween the rocks of the north and south, and between those of 
the west and east; but the differences are so systematically dis- 
tributed along the range that they are testimony to its essen 
unity. The differences are in the main just those that would 
have come for the most part, from differences in the grade 0 
metamorphism. Intenser metamorphism should be naturally 
looked for along or about the axis of the range than to the 
west of it; and so, also (as shown by the facts connected wit 
the Appalachians), on the eastern side of the axis than on the 
western; and thus it is in reality. 
In view of such facts we may safely hold that if to the south, as 
in Westchester county, the rocks are mainly gneiss, this alone 
would not be sufficient evidence of difference of system or age 
hfield. : 
+ One fact may have much importance when the investigations are completed, 
namely: that on the west of New Haven, and therefore the west side of the 
Connecticut Valley depression, the rocks are, first, hydromica schist, which is 12 
di guishable from 
ous mica schist; next, coarse gneiss and mica schist ; and all are strictly conformable. 
The chloritic hydromica schist west of New Haven includes so ibe 
labradorite-dioryte, a rock I have not found in the Taconic region of Berkshire. 
