eee aa ee se a De: Lila 
EG i 
GEOGNOSY OF THE APPALACHIANS. 455 
tween the rocks ofthe White Mountains and those of the Adiron- 
dacks; but the Messrs. Rogers in the same year, 1844, published 
an essay on the geological age of the White Mountains, in which, 
while endeavoring to show their Upper Silurian age, they speak of 
them as having been hitherto regarded as consisting exclusively 
of various modifications of granitic and gneissoid rocks, and as be- 
longing ‘‘to the so-called primary periods of geologic time.” * 
They however considered that these rocks had rather the aspect of 
altered paleozoic strata, and suggested that they might be, in part 
at least, of the age of the Clinton division of the New York 
system ; a view which was supported by the presence of what were 
at the time regarded by the Messrs. Rogers as organic remains. 
Subsequently, i in 1847,} they announced that they no longer consid- 
of organic origin, without however retracting 
their opinion as to the paleozoic age ofthe strata. Reserving to 
another place in my address the discussion of the geological age 
of the White Mountain rocks, I proceed to notice briefly the dis- 
sei characters of the three groups of crystalline strata just 
mentioned, which will be shown in the sequel to have an impor- 
ae in geology beyond the limits of the Appalachians. 
I. The Adirondack or Laurentide Series. The rocks of this, 
series, to which the name of the Laurentian system has been given, 
may be described as chiefly firm granitic gneisses, often very 
coarse-grained, and generally reddish or grayish in color. They 
are frequently hornblendic, but seldom or never contain much mica, 
and the mica-schists, (often accompanied with staurolite, garnet, 
andalusite and cyanite), so characteristic of the White Mountain 
series, are wanting among the Laurentian rocks. They are also 
destitute of argillites, which are found in the other two series. 
The quartzites, and the pyroxenic and hornblendic rocks, asso- 
ciated with great formations of crystalline limestone, with graph- 
ite, and immense beds of magnetic iron ore, give a peculiar 
character to portions of the Laurentian system. 
he Green Mountain Series. The quartzo-feldspathic rocks 
of this series are to a considerable extent represented by a fine- 
grained petrosilex or eurite, though they often assume the form of 
a true gneiss, which is ordinarily more micaceous than the typical 
* Amer. Jour. Sci., I, i, 411. ° 
t Ibid, I, v, 116. 
