a aes 
a 
GEOGNOSY OF THE APPALACHIANS. 459 
Rogers that these rocks so much resemble the underlying hypozoic 
gneiss, as to be readily confounded with them ; and when compared 
with the latter, as displayed in the southern district, it is difficult 
to believe that we have in this so-called azoic or metamorphic 
series of the Montgomery and Chester valleys, anything else than 
a repetition of these same crystalline schists which have been de- 
scribed along their southern boundary, representing the Green 
Mountain and the White Mountain series. We thus avoid the dif- 
ficulty of supposing that we have in this region two sets of ser- 
pentinic rocks, and two of mica-schists, lithologically similar, but 
of widely differeit ages,—a conclusion highly improbable. It 
should be said that Rogers, in accordance with the notions then 
generally received, looked upon serpentine as an eruptive rock, 
which had altered the adjacent strata, converting the mica-schists 
into steatitic and chloritic rocks. ` 
This so-called azoic series, according to Rogers, underlies the au- 
roral limestone of Pennsylvania, thus apparently occupying the 
horizon of the primal paleozoic division or Potsdam series. We 
find, however, in his report on the geology of the state, no satis- 
factory evidence of the identity of the two series. On the con- 
trary, a very different conclusion would seem to follow from certain 
facts there detailed. The azoic or so-called metamorphic primal 
strata are said to have a very uniform nearly vertical dip, or with 
high angles to the southward, while the micaceous and gneissic 
strata of the northern subdivision of the southern district of so- 
called hypozoic rocks, limiting these last to the south, present 
either minute local contortions or wide gentle undulations, with 
comparatively moderate dips, for the most part to the northward. * 
From this, I think we may infer that the nearly vertical strata must 
be, in truth, older underlying rocks belonging, not to the paleozoic 
systeth, but to our second series of crystalline schists. We con- 
clude, then, that while the gneisses to the northwest, and probably 
those along the southeast rim of the Mesozoic basin of Pennsyl- 
vania are Laurentian, the great valley southward to the Delaware 
is occupied by the rocks of the Green Mountain and White Moun- 
tain series. The same two types of rocks, extending to the north- 
east, are developed about New York city, in the mica-schists of 
Manhattan and the serpentines of Staten Island and Hoboken ; 
* Rogers, Geology of Pennsylvania, I, pp. 69-74, and 154-158, 
