438 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



oldest rock known to geologists.* There are also found azoic 

 rocks, which appear to have been formed from the breaking up 

 and wearing down of the primitive rocks, being deposited bj 

 water as a sediment, and forming stratified rocks, among which 

 are the slate rocks. These slates are almost infinite in their 

 gradations. Even in the same quarry, admirable roofing slates 

 may run into a mica slate of no value. In the Lake Superior 

 region, where they lie at a very uniform dip, they have been 

 found to measure no less than fifteen miles in thickness. I have 

 measured one series in Massachusetts, amagnesian slate, which I 

 found to be five miles in thickness ; and Sir Wm. Logan supposes 

 all the series then to reach a thickness of thirty miles. Commenc- 

 ing at the Adirondack mountains and passing to the Mohawk 

 river, in the direction of the Catskill mountains, we find first the 

 unstratified rocks; and upon these, layers of stratified rocks. 

 Upon the latter we find the paleozoic rocks, those containing the 

 earliest vestiges of life, sandstones, limestones and shales. The 

 azoic rocks contain iron, always an oxide, in the slate, never of 

 a carbonate of iron. One of the oldest forms of iron ore is the 

 magnetic. In this system of rocks, the magnetic iron is found 

 in mountain masses. The whole region of Marquette county, in 

 Michigan, may be said to be an iron formation The magnetic 

 iron ore is diagnostic of the primitive formation. Magnetic 

 iron is composed of the protoxide of iron 31, and of the peroxide 

 of iron 68 per cent. Hematite, or the red oxide of iron, is com- 

 posed of iron two parts, and of oxygen three. Tho protoxide of 

 iron is so easily oxidized upon exposure to the atmosphere, that 

 it turns into the red hematite, which is the diagnostic sign when 

 you are exploring for iron. In New Jersey, iron is found com- 

 bined with manganese and zinc, and called Franklinite' iron. 

 This contains sixty-six to sixty-nine per cent, of iron, fifteen to 

 eighteen per cent, of manganese, and ten to seventeen per cent, 

 of zinc. It is usually found associated with limestone. 



The Chairman. — Tliere is a vein sixty feet wide, which begijjii 

 with sixty-six per cent, of zinc upon one side, and gradually 

 changes until upon the other side it is sixty-six per cent of iron, 

 the per centage of manganese remaining the same through the 

 whole. 



* Tlicre is true granite in the coal fields of Virginia, traversing the co.il measty^ 

 jn dykes. This granite, therefore, is of later date than the carboniferous rocks. 

 —J. R. 



