ISâ im. THOMAS STERE Y HUNT ON THÉ 



metamorphisra. This hypothesis, as long since maintained by Mather for the rocks of 

 eastern North America, and later by Dana, asserts successive changes, — called by the latter 

 "grades in metamorphism," — from uncrystalline sediments through the Taconian and 

 other more massive crystalline schists to the granitoid gneisses. These various and 

 dissimilar groups of strata, as I maintained in 18*78, and as will to-day be admitted by 

 nearly all geologists, "are not the result of different and unlike changes vrhich one and 

 the same uncrystalline paleozoic series has suffered in different geographical areas, but, on 

 the contrary, belong to successive j)eriods in paleozoic and eozoic time. The great divisions 

 of the latter .... present in ascending order a progressive change in mineral character, 

 the nature of which has been shown ; . ... thus constituting a veritable passage in 

 time from the granitoid Ottawa gneiss at the base of the Laurentian, through the inter- 

 mediate Hixronian and Montalban divisions to the less markedly crystalline schists of the 

 Taconian." " Such a succession, I have since endeavored to shew, is the necessary result 

 of the secular process by which, from an undifferentiated primeval chaos, the various 

 groups of Primitive and Transition crystalline rocks haA^e been generated, as set forth in 

 the crenitic hypothesis^- already noticed in § 19-4 of the present essay. 



§ 202. The Taconian crystalline rocks were deposited over a large part of eastern 

 North America upon the eroded surfaces of more ancient eozoic groups, and in their 

 turn suffered greatly from movements of the earth's crust, and from erosion, preA'ious to 

 the beginning of Cambrian time. Over the more depressed portions of the worn surfaces, 

 the uncrystalline sediments of Cambrian, Ordoviciau, Silurian, and later periods, were 

 next successively laid down, alike on the Taconian and the more ancient crystalline 

 groups, not however without intervening movements of the earth's crust, which along 

 the eastern portion of the great paleozoic basin caused stratigraphical breaks, foldings, 

 and partial erosions of these later groups of sediments. Beyond the limits of this basin, to 

 the south and east, the sparse distribution of areas of paleozoic sediments, and their absence 

 from the higher levels among the crystalline rocks of the Atlantic belt, permit us to 

 suppose that the paleozoic seas did not iuA'ade these higher regions ; while the deposits 

 made by some of them at lower levels among these same crystalline rocks, have been in 

 great part removed by subsequent agencies. As a final resvilt of this process, we find, 

 within the great basiu, the Taconian rocks resting on various older crystalline groups, 

 and themselves overlaid directly by Cambrian, by OrdoA'iciau and by Silurian, while out- 

 side of the limits of the basin, areas of the same Taconian rocks are in parts overlaid by 

 mesozoic and by tertiary strata. 



§ 203. As regards the existence in other lands of a similar series of rocks to the Taco- 

 nian of North America, we have seen that Lieber, whose independent and careful studies 

 of this series in South Carolina we have resumed in Chapter IV (§ 69-*79), supposed them 

 to be the stratigraphical equivalent of the Itacolumite or diamond-bearing series of Brazil, 



" Hunt, Azoic Kocks, 1878, p. 253 ; see also ibid., p. 210. 



■"- " All physical theories properly so-called are hypotheses, whose eventual recognition as truths depends upon 

 their consistency with themselves, upon their agreement with the canons of logic, upon their congruence with the 

 facts which they serve to connect and explain, upon their conformity with the ascertained order of Nature, upon 

 the extent to which they api)rove themselves as trustworthy anticipations or previsions of facts verified by subse- 

 quent observation or experiment, and finally, upon their simplicity, or rather their reducing iiower." Stallo in 

 Concepts and Theories of Modern Physics, p. 85. 



