Geological Results of the Earth’s Contraction. 185 
Il. Reasons why this action should not produce perfectly regu- 
lar and uniform folds.—Irregularities would proceed— 
1. F'rom a variation in the thickness of the bed, in consequence 
of which there would be a difference in the gravity of the mass 
in different parts. 
2. From a want of uniformity in the material or its state of 
induration, causing the cohesion to vary, and hence also the 
be equal along a given line. b 
_4 From any irregularity which there might be in the contrac- 
tion going on (for there should be some such contraction) beneath 
e area which is subjected to the lateral pressure. 
A fifth reason might be added, but it is of a general nature and 
will form the subject of another‘;communication. The four spe- 
cified are sufficient to set aside any objections to the view urged 
on the score of the irregularities which exist. 
Ill. Effects of gravity on the inclined strata*—When the beds 
become very much inclined, or dip at a large angle, the more 
sandy layers if not too much indurated, would settle bedily down- 
ward; the clayey layers would also settle, but owing to their co- 
hesion when moist, they would become: flexed or crimpled. 
Thus plications would be produced, from gravity alone ; a fact 
abundantly illustrated in the metamorphic rocks of New England 
and other countries; and it might happen that small plications 
should in the same manner be produced between non-plicated beds. 
V. Intruded igneous rocks occurring with plicated beds.—The 
occurrence of dikes or intruded masses of igneous rock in a pli- 
cated region, is no certain evidence that the intrusion was the 
cause of the plication, as the. two. effects, on the principles ex- 
plained, might be concomitant results of the same general operation. 
Sxeconp Serres, Vol. ILI, No. 8.—March, 1547. 24 
