d2 TbAKSACTIOX^S of the AMEniCAN IXSTITUTE. 



without producing the phenomena which are observed in the Apa- 

 lacliiau and other mountain ranges. 



You can iiave some conception of this process of folding of the 

 inner beds, by taking a pile of sheets of paper, so coniining tlie edges 

 that the_y cannot slide one over the other, and then depressing the 

 centre ; as the lower sheets assume the curving direction, the upper 

 ones will curve upwards, or wrinkle, in consequence of the contrac- 

 tion of the space between the edges. An elementary illustration of 

 this kind was formerly used to show the effects of lateral pressure in 

 the folding of strata. Now, a set of strata of mud and sand, 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. 



I have shown, by the slow accumulation of tlie sediments and the 

 evidences of shallow water, that the subsidence was gradual ; we have, 

 moreover, proof that it Avas periodical ; for we find the lower 

 Ilelderberg group resting unconformably upon the upturned beds of 

 the Hudson River grouj), showing that previous to the deposition of 

 these limestones, there were foldings and plications of the lower 

 strata, along the line of accumulation. Subsequently to the deposition 

 of the later formation, there have been other periods of subsidence, 

 and consequently of folding and plication. These movements, there- 

 fore, are not synchronous, nor are they conformable with each other ; 

 and hence arises much of the difficulty of tracing the geological 

 structure in disturbed and mountainous regions of country. 



Now, according to the view I have taken, the zone of greatest 

 accumulation should ])e that of the greatest depression and ultimately 

 of greatest elevation, and consequently the foldings of the strata 

 would be more extreme along that line. This condition we find to 

 be true ; and as we approach the margin of this zone of depression, 

 and as tlie beds Ijecome gradually thinner, the foldings become less 

 extreme, and die out in gentle undulations on the west. 



It might, perhaps, be suggested that these foldings, coming from tlie 

 depression and necessary contraction of the interior of a great trough 

 or synclinal, would be removed on their subsequent elevation, and 

 the beds might assume in some degree, at least, their original position ; 

 but I have already explained that tills is not the mode of elevation. 

 The movement has been a gradual one ; for the restoration of the 

 equilibrium of pressure and temperature and the elevation has been 

 continental, and not local. 



Before leaving this part of the subject, I will endeavor to give 



