GEOLOGICAL EXPLORATIONS AND LITERATURE— 1851. 33 



of the eruptive masses were occasionally subjected to denudation, and the ferruginous 

 particles were, under the action of violent currents, spread out in thin beds, or swept 

 into some depression of the surftice, forming a lenticular mass, upon which the strata 

 were afterwards accumulated. 



When the siliceous materials had become iuipregnated with metallic matter, 

 which may have been scattered more or less uniformly through it, a rearrangement 

 of the siliceous and ferruginous particles in some instances took place, under the 

 action of segregating forces, by which the whole mass assumed a banded structure. 



Subsequently, the whole series of beds, slaty, quartzose, ferruginous, and trap- 

 pean, were elevated and in all probability folded, perhaps at the epoch of the eleva- 

 tion of the granite ranges un the north and south of the ferriferous belt of the Azoic 

 system. 



From this quotation we see that while the authors reganh'il tlie ore 

 material as igneous, they nevertheless, in order to explain the handed nature 

 of some of the jasper ores, found it necessary to admit the action of segre- 

 gating- forces, and in order to accoimt for the lenticidar forms of some of 

 the deposits they made use of the ideas of denudation and deposition. The 

 reason for refusing to accept the theory of a sedimeutar^' origin for the 

 banded ores and jaspers in general is apparent from the following quotation 

 (pp. 67-68): 



At first glance this banded structure might be regarded by some as the result 

 of aqueous deposition, by which alternate seams of quartzose and feiTuginous matter 

 were si5read over each other, and the whole subsequently solidified and welded 

 together by heat; but if we examine the circumstances more closely it will be found 

 more difficult to account for all of the facts under this hypothesis than might at first 

 appear. The extreme tenuity of these bauds, which are often no thicker than a 

 sheet of paper, renders the supposition of their analogy to strata highly improbable. 

 In fact, this banded structure iu many of the Lake Superior ores — for example, at 

 the Cleveland iron knob — will be hardly apparent to the eye on fresh fracture of a 

 specimen, the weathered surface of which may present a beautiful series of intricate 

 convolutions of alternate bands of bright- red and steel-grey. Besides, on examining 

 this mountain mass we find every portion exhibiting equally fine and equally con- 

 torted series of convolutions. If these were really the result of aqueous deposition 

 we should expect from analogy with other deposits of a similar character that some 

 of the layers would be of more considerable thickness than others, and that, suppos- 

 ing the contortions to have been caused by lateral pressure of the plastic mass, in 

 some cases at least the foldings would exhibit a considerable radius of curvature, 

 which is not the case here. 



MON XXVIII 3 



