J. D. Dana—Helderberg rocks in the Connecticut Valley. 348 
but generally not distinct. Following the outcrops of the 
upper stratum of quartzite (No. 2) to the southward, the strike 
changes to N. 65°-70° E., making the dip S. 20°-25° KE. in 
direction, with the amount unchan 
dip 55° to the eastward; and higher up West Mountain, half 
a mile to the south, the strike is N. 50° E., and dip 80° to the 
eastward; the slate varying much in strike. Thus, while the 
Helderberg beds have a small dip everywhere, and the smallest 
as they approach the clay-slate toward the top of the ridge, the 
slate is very high in dip. Moreover, this difference charac- 
terizes the formations in other parts of the town of Bernard- 
ston; and hence, although no section of the two together was 
found exhibiting to the eye their relative position, the facts 
left little doubt of their unconformability. ; 
Again, the Helderberg slate is a widely different rock from 
the older clay-slate, it being strictly a mica slate, Unaera 
n mica, usually lying on their sides or ae to the 
plane of bedding, which show their foliation and luster on a 
cross fracture of the slate, or by an oblique view of the surface 
of a slab. These prisms are from a line to half a line in 
aga and length. The prismatic edges are rounded, so that 
€ pri i 
: a 
the aspect of phyllite (chloritoid) in some clay slates ; but the 
folia are easily separable and flexible. There are also minute 
T . 
texture; what unevennesses it has are not due either to mica 
th garnets. The Helderberg slate carries this feature through 
the region, and far beyond it; and the clay-slate 1s as persistent 
i its different characteristics. a ee 
The Helderberg garnetiferous mica slate is in all cases lam1- 
* Hitchcock, in the Re ‘ation, makes both slates 
aia 1 rt of the American Association, : 
peed in character and spclacuanic, In the Vermont Report, the section repre- 
ts the lower slate as conformable, but the text leaves it in doubt. wy 
He examination of the slate for water been made by oberg of ne 
Wes, assistant in the Sheffield Scientific pence Pueetie it g onan Fogo 
cent. In a blowpi i f the little prisms of mi 
of water were ‘3 = : owpipe trial tng be Figo p rdly sufficient to watrant 
‘ und. a ms 
calling the slate a hydromica slate: and, moreover, it does not feel at all greasy. 
& 
ee 
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