474 ©. H. Hitcheock—Helderberg Rocks in New Hampshire. 
6. Clay slate-—This is the auriferous slate of Lyman and Bath. 
disappear beneath the Helderberg. The case is analogous to 
those i 
Seely silica. Silica 72°98, peroxide of iron 6°85, alumina, 
5-99, magnesia ‘86, potash 5°61, soda 9°92 =101-21. Thus 
this slate resembles the common schist of the Lyman. series, 
Possibly it was derived from the decomposition of the latter. 
The synclinal form of these slates confirms the general theory 
of structure in this neighborhood, already presented. For, if 
this area isa basin, it must lie upon older strata; and, if both 
sides of the slate are flanked by a similar succession of strata, 
the latter must be relatively older. In fact, this slate is flanked 
first by the Lyman schist, and that in turn by the Lisbon group. 
On the southeast, the latter joins on to the Cods series, and on 
the west it comes in contact with the same slate, in both cases 
newer rocks; but ultimately gneiss is found after one or more 
ew Hampshire report gives four sections across this group, 12 
Lisbon, Orford, Lyme and Hanover, with an average thickness 
of about 10,000 feet. Subsequent investigations modity all 
our published statements respecting this group, The limestone 
and gneiss of the sections must be eliminated; the proof 0 
this is very evident, The gneiss in Hanover underlies the 
. . . : an horn- 
line, and thus all the rocks are repetitions instead of one un- 
broken series, In Haverhill, the gneiss with limestone undet- 
* This Journal, III, vol, v, p, 90, 
