"Sees ee 
American Geology, by T. S. Hunt. 413 
the steeper slopes, the overturn dips or folded flexures, and the 
overlaps from dislocation are to the westward, so that the general 
ip 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 southeast in the Appalachians appear to be due to small 
downthrows to the southeast, parallel to and immediately to the 
northwest of great upheavals in the same direction 
adopts the theory of metamorphism which we have 
expounded in the pages of this Journal and in the paper just 
quoted above, (see also Am. Jour. Sci. [2,] xxv, 287, 485, xxx, 
135), which has received a strong confirmation from the late re- 
searches of Daubrée. According to this view, which is essen- 
tially that put forward by Herschel and Babbage, these: changes 
have been effected in deeply buried sediments by chemical reae- 
tions, which we have endeavored to explain, so that metamorph- 
ism, like folding, takes place along the lines of great accumula- 
tion. 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 Silurian age in North America were at 
the time of their crystallization overlaid by the whole of the 
palzozoic strata, while the metamorphism of carboniferous strata 
in eastern New England points to the former existence of great 
deposits of newer and overlying deposits, which were subse- 
quently swept away. : 
On the subject of igneous rocks and volcanic phenomena, Mr. 
Hall insists upon the principles which we were, so far as we 
know, the first to point out, namely their connection with great 
accumulations of sediment, and of active voleanos with the newer 
deposits. We have elsewhere said: “the volcanic phenomena 
of the present day appear, so far as we are aware, to be con- 
fined to regions of newer secondary and tertiary deposits, which 
Wwe may suppose the central heat to be still penetrating, (as shown 
by Mr. Babbage), a process which has long since ceased in the 
seozoic regions.” ‘To the accumulation of sediments then we 
referred both modern volcanos and ancient plutonic rocks; these 
latter, like lavas, we regard in all cases as but altered and dis- 
placed sediments, for which reason we have called them exotic 
rocks. (Am. Jour, Sci. [2,] xxx, 183). Mr. Hall reiterates these 
