J. D. Dana—Age of the Green Mountains. 193 
logical and orological system constitutes nearly one-half of the 
whole range. The part of the Western belt which is not in- 
cluded in the above, is (exclusive of the Archzean area) the 
southern, or Westchester County, whose connection with the 
system has not yet been clearly made out. 
I. Western Section or THE Green Mountain Region. 
2. Rocks ; Stratification.—In mentioning the kinds of rocks 
we pass from north to south, and from west to east; the former 
direction follows the strike of the rocks and of the ridges, 
and the latter, transverse lines. 
Tt is important to note, in reading the following, that hydro- 
mica schist and mica schist are essentially the same in compo- 
sition, the former differing chemically from the latter only in 
the presence of a few per cent of water (not usually over 5). 
Physically the difference is wider; the hydromica schist being a 
fine-grained glossy slate (shading often into a variety that is 
called argillyte from its clay-slate aspect, and which is used as a 
roofing slate), and feeling more or Jess unctuous or talc-like.* 
This roofing slate from Fair Haven, near Castleton, Vermont, 
One of its most noted localities, is only a more finely grained 
hydromica schist ; for in recent determinations of the alkalies 
made for the writer by Prof. O. D. Allen of the Sheffield Sci- 
entific School of Yale College, and by Mr. O. E. Atwater of 
tioned ; but it becomes, to the southward, a belt of well char- 
acterized hydromica schist, yet with small beds of quartzyte 
MM some places, 
Next east of the main band of crystalline limestone (east of the 
meridian of lutland), exists a second band of hydromica schist, 
which, in the southern half of the State, is formed largely of 
quartzyte; thick strata of quartzyte and hydromica schist alter- 
hate, and the former often craduates vertically and in the direc- 
tion of the bedding into the latter by insensible shadings. Owing 
*It is the so-called taleose schist and taleoid schist of the Vermont Report, 
Which is shown in that Report (pp. 503-508, 522) to be non-magnesian. In this 
Paper the terms of the Report are changed in all cases to hydromica schist. 
