104 Mr, T. Sterry Hunt 



given line would, by destroying the equilibrium of pressure, cause 

 the somewhat flexible crust to subside ; the lower strata becoming 

 altered by the ascending heat of the nucleus would crystallize and 

 contract, and plications would thus be determined parallel to the 

 line of deposition. These foldings, not less than the softening of 

 the bottom strata, establish lines of weakness or of least resistance 

 in the earth's crust, and thus determine the contraction which 

 results from the cooling of the globe to exhibit itself in those regions 

 and along those lines where the ocean's bed is subsiding beneath 

 the accumulating sediments. Hence we conceive that the subsi- 

 dence invoked by Mr. Hall, although not the sole nor even the 

 principal cause of the corrugations of the strata, is the one which 

 determines their position and direction, by making the effects pro- 

 duced by the contraction not only of sediments, but of the earth's 

 nucleus itself, to be exerted along the lines of greatest accumulation. 



It will readily be seen that the lateral pressure which is brought 

 to bear upon the strata of an elongated basin by the contraction 

 of the globe, would cause the folds on either side to incline 

 to the margin of the basin, and hence we find along the 

 Appalachians, which occupy the western side of such a great 

 synclinal, the steeper slopes, the overturn dips or folded flexures, 

 and the overlaps from dislocation are to the westward, so that the 

 general dip of the strata is to the centre of the basin, on the other 

 side of which we might expect to find the reverse order of dips pre- 

 vailing. The apparent exceptions to this order of upthrows to 

 the south-east in the Appalachians appear to be due to small 

 downthrows to the south-east, which are parallel to and immedi- 

 ately to the north-west of great upheavals in the same direction. 



Mr. Hall adopts the theory of metamorphism which we have 

 expounded in the paper just quoted above, Canadian Naturalist^ 

 Dec. 1859, (see also Am. Jour. Sci. (2) xxv. 28*7, 435, xxx. 135,) 

 which has received a strong confirmation from the late researches 

 of Daubree. According to this view, which is essentially that put 

 forward by Herschel and Babbage, these changes have been effected 

 in deeply buried sediments by chemical reactions, which we have 

 endeavored to explain, so that metamorphism, like folding, takes 

 place along the lines of great accumulation. The appearance at 

 the surface of the altered strata is the evidence of a considerable 

 denudation. It is probable that the gneissic rocks of Lower Silu- 

 rian age in North America were at the time of their crystalliza- 

 tion overlaid by the whole of the palaeozoic strata, while the 



