THE IRON HILLS OF CORNWALL.
This singular deposit of ores is, we believe, peculiar to the locality. They are pro duced by the intrusion of a trap dike, which originates in the Azoic, though bursting through the overlying Mesozoic, near the South Mountains, and which enters the valley near Cornwall in Lebanon county, Pennsylvania. Here the volcanic or trap eruption terminates between the limestones of the valley and the primal slates, but in the great Eastern range of brown hematites which we are tracing.
The form which these igneous rocks assume at this point is suggestive. Generally they spread out on the surface and conceal the crater, if such existed; but here they form a crater, or separate so as to form a dish-shaped rim of trap nearly around the deposit of ore. These volcanic rocks contain but little iron, and present no evidence of having emitted the iron of this deposit from the bowels of the earth. We find here masses of iron almost in a metallic state ; but it is evident that these are the result of the great heat which operated on the brown oxides of this locality, existing before the intrusion of the trap. This volcanic rock proves itself of late origin, because we find it piercing the overlying Mesozoic.
The ores of Cornwall are not purely magnetic, but contain a small portion of brown oxide, sulphurets, and oxides of copper.
The formation of these ores is peculiar. They are evidently sedimentary, but owe their occurrence chiefly to the action of volcanic heat on the accompanying rocks, and perhaps the waters acting on the sublimated vapors, escaping from the fissures formed by the ejected trap, or through the influence of both combined.
Prof. Rogers, in his Geology of Pennsylvania, thus explains the formation of this peculiar deposit:— "At this locality the actions collecting the oxide of iron into its present conditions have been somewhat complicated. The ferruginous Primal slate has been meta morphosed, and its oxide of iron segregated and crystallized through the influence pro bably of highly-heated volcanic steam, and the same influence has produced a very general cleavage structure. During the same action, or subsequently, numerous injec tions of molten hot lava, resulting in dikes of trap rock, have invaded the stratum, and have still further changed the condition of the mass, infusing among it, probably by sublimation, some trappean mineral matter, and especially some sulphuret and carbonate of copper ; and since these subterranean influences, the atmosphere, through its rains, has exerted itself through countless ages to modify still further the chemical and phy sical condition of the shattered and fissured mass and its contained oxide of iron.
" This great iron-ore deposit, by far the most extensive and one of the most interest ing in the State, is situated at the outcrops of the Primal upper slates, where they rise from beneath the Auroral limestones, in Lebanon county, on the southeast border of the Kittatinny Valley.
"The ore-strata are embraced in three hills, having a nearly east-and-west range. These hills are flanked on the north by the Auroral limestones, and south by the over lapping unconformable Mesozoic red sandstone, which forms a high ridge, prolonging east and west and overlooking the valley.
" The eastern or Big Hill' is elevated 312 feet above the level of the creek at its base. The middle hill is 98 feet high, and the western hill 78 feet high.
" The bounding wall of the ore in the Big Hill is a heavy dike of trap, which varies in regard to texture and composition as the feldspathic or hornblendic element predomi nates. This massive dike, the thickness of which seems nowhere less than 40 feet, and probably greatly exceeds this, encircles the hill on three sides." It appears that this bounding wall of trap also exists on the fourth side, but is con cealed by debris. These walls of trap form a basin or receptacle for the ore, as shown in figure 172.
Iron ores occur in so many different farms, and under so many chemical combinations, that no one theory of formation can cover the coincidents and conditions with which and in which they are found. This deposit of ore differs in character and structure from the ores of the great limestone region which we have been tracing ; but the mate rial change must be ascribed to the trap dike which here invades the mineral range, and the volcanic heat which must have accompanied it ; and perhaps the solution given by Prof. Rogers is the best theory that can be offered. To account, however, for the formation of the isolated hematitic masses found throughout this great lime stone region, or the continuous range of hematitic ores which exist along the eastern slopes of the Blue Ridge, we must seek some other theory. But since there are so many theories we shall decline the task, merely presenting such facts in relation as may shed some light on the subject.