THE GREAT AZOIC BELT.
In this great zone of rocks, which encircle the entire Appalachian basin, and, which have an immense spread in many localities, we find the proper region of the magnetic ores or those peculiar to the trappean formations. The gneissic rocks and associate slates are generally embraced under the nomenclature of metamorphic or Azoic, though the terms do not properly express the character of these formations in all portions of the Appalachian basin.
A simple fact seems to have escaped the notice of our geologists, or if noticed by them our reading has been too limited to observe it.
The Azoic rocks express the meaning clearly as those without the ancient life, or those which contain no fossil remains ; as the Palaeozoic rocks are those in which are entombed the relics of past races.
The Azoic rocks are crystalline, sedimentary strata, metamorphosed by heat, and, consequently, could not have been formed during a late period, or at a time when life could exist. But the rocks which may be Azoic, or destitute of the ancient life, in the East, may belong to the Palaeozoic strata in the West, since all the evidences show that the region of heat was in the East and that the temperature as well as the sedimentary strata decreased to the West and North.* This is not only evident as a natural sequence, but as a fact. It is rare to find fossils in the Potsdam sandstone of the Blue Ridge, or, if found, they are broken, and present evidence that they were not in situ, or that they were drifted from other localities or formations. But in the West and North not only the
Potsdam sandstone, but several of the underlying formations, are full of well-preserved fossils. It therefore follows that the horizon of the Azoic rocks is not uniform, but that they ascend or descend according to their locality and the conditions of their creation.
In the East this formation approaches very nearly the Auroral limestones of Rogers, or the Valley limestone. It surrounds, as before observed, the entire Appalachian basin, but is not developed beneath the Gulf States, though it undoubtedly crosses from the Appalachian Mountain chain to the Rocky Mountains at some indefinite point south, and encircles the great basin with a zone or belt of unbroken gneiss. In some places this belt is wide and undulating, with comparatively low angles of dip ; but in others it is piled in mountain-masses and in oft-repeated and folded strata of the sharpest angles of formation, as in Southwestern Virginia, North Carolina, and Tennessee. In other localities it is low and narrow, as in Eastern Pennsylvania and New Jersey, but everywhere in the East it contains more or less iron ore and numerous veins of copper, lead, zinc, &c. &c. This belt is pierced by thousands of fissures; and extinct volcanoes and trappean formations exist from one end to the other.