3894. SJ. D. Dana—Taconic Rocks and Stratigraphy. 
eastward extension of the Pittsfield limestone and the quartzyte 
south of it, and (4) in the Bear Mountain region with Monu- 
ment Mountain west of it; the localities of Archzean being 
recognized by the presence of hornblendic rocks, sometimes 
zircon-bearing and chondroditic limestone, from eastern Tyring- 
ham northward and marked on the map by the letter A. The 
colored portion of the.map is that covered by the limestone. 
The next subject is: 
2. The Limestone and the conformably overlying strata. 
1. Kinds of rocks and their distribution.—The kinds of rocks 
observed in central and northern Berkshire, with the adjoining 
border of New York, are the same as in the southern portion, 
already considered, except that the mica schists north of the 
town of Sheffield have not yet been found to be staurolitic. 
The limestone is mostly dolomitic, except to some extent (as 
made known by analyses in the Geological Report of Professor 
KH. Hitchcock (1841, p. 80), in the more western towns of 
Berkshire. The same analyses show that the beds are rarely 
true dolomite, but partly calcite, by having a deficiency of 
magnesia, and that consecutive beds vary much in constitution. 
The subject needs investigation. 
The rock varies in grade of crystallization, being coarsest (a) 
to the southward, and also (0) to the eastward. It varies in 
color, being usually white to the eastward, often gray in the 
more western belts of Berkshire, and generally gray west of 
Berkshire. It varies in impurity, being often very micaceous 
in the vicinity of the schist ridges, so as sometimes to be decep- 
tive, and very quartzose near the quartzyte. Tremolite and 
white pyroxene, abundant minerals in the Sheffield limestone 
and its continuation southward, (to New York Island, if all 
is the same limestone, as is probable,) occur also in belts of 
Monterey, eastern Great Barrington, Tyringham and Lee; and 
minute tourmalines are occasionally present. 
The schists are hydromica schists to the westward and mica 
schists to the eastward, along with indefinite intershadings 
through chloritic hydromica schists. Hydromica and chlorite 
are hydrous species, so that the difference in the metamorphic 
schists mentioned is largely a difference as to the presence of 
water; a hydromica containing 4 or 5 per cent (but like mica 
in other constituents), and chlorite containing 12 to 14 per cent. 
In the Greylock range the chloritic hydromica schist passes 
locally into a hydromica gneiss, which I designated in 1877 a 
protogine variety of gneiss.* 
The more western schists west of Canaan, N. Y., are much 
like a glossy roofing slate, showing in the color no evidence of 
the presence of chlorite, though it may contain much of it. 
* This Journal, II, xiv, 258, 260. 
