INTRODUCTION. 
71 
limestones, there were already foldings and plications, the consequence 
of a subsidence along the line of accumulation. Subsequently to the 
deposition of the latter formations, or at intervals during their accu¬ 
mulation, there have been other periods of subsidence, and consequently 
of folding and plication; so that these are not synchronous, nor are 
they conformable with each other. 
This successive accumulation, and the consequent depression of the 
crust along this line, serves only to make more conspicuous the feature 
which appears to be the great characteristic, that the range of mountains 
is the great synclinal axis, and the anticlinals within it are due to the 
same cause which produced the synclinal; and as a consequence, these 
smaller anticlinals, and their correspondent synclinals, gradually decline 
towards the margin of the great synclinal axis, or towards the margin of 
the zone of depression which corresponds to the zone of greatest accu¬ 
mulation*. 
This affords a partial explanation of the fact already observed, 
that the mountain elevations in the disturbed regions bear in their 
altitude a much smaller proportion to the actual thickness of the forma¬ 
tions, than do the hills in undisturbed regions. Furthermore it so hap¬ 
pens that so soon as disturbance takes place and anticlinals are formed, 
the beds are weakened at the arching, and become more liable to 
upwards or wrinkle. This is an illustration after a different manner of the old elementary process of 
producing foldings in sheets of paper, as illustrative of folded strata by lateral presure. Now, as 
a set of strata one or two hundred miles in width cannot slide over each other, as sheets of paper 
do if left to themselves during the process of depression, the beds on the lower side must either be¬ 
come extremely broken, or the higher portions become folded and plicated. That some fractures will 
take place below there can be no doubt, and these are probably such as we see filled with trappean 
matter. But the greater movement would undoubtedly take place in the higher beds, which necessa¬ 
rily assume positions and relations such as have been pointed out. This condition and manner of 
movement offers, moreover, an explanation of the form of trap-dykes, which are often narrower 
above in the synclinals and on synclinal slopes, the matter filling a fracture opened from below ; 
while in the case of such matter penetrating an anticlinal, it would necessarily widen above from 
the reversed conditions attending the fracture. 
# This mode of depression, which is the result of accumulation, and the production of numerous 
synclinal and anticlinal axes offers a satisfactory explanation, as it appears to me, of the difference 
of slope on the two sides of the anticlinals which have been so often pointed out as occurring in the 
Appalachian range, where the dips on one side are uniformly steeper than on the other. 
