314 SAODDTES TiO he SIRO LANES: 
the waves. The only purpose of the comparison is to give at 
the outset some idea of the complexity of rock folds. 
Tangential thrust and gravity are assumed to be the causes 
of folds. No attempt will be made here to show this or to 
explain the cause of thrust, although in the last analysis it is 
probable that thrust is dependent upon gravity. At all times 
and in all positions rocks are subject to the force of gravity. 
Thrust and gravity act upon rocks of heterogeneous character. 
Rock heterogeneity, therefore, modifies the forms of folds. 
Folds are further modified by igneous rocks. In what follows, 
the effects of igneous rocks are at first excluded. 
We shall now attempt to analyze the rock waves or folds. 
For convenience, they will first be considered in two dimen- 
sions. 
SIMPLES ROEDS: 
Simple folds are classified by de Margerie and Heim” as fol- 
lows: A fold is upright or symmetrical when the axial plane is 
vertical, or nearly so, and the limbs have nearly equal dips in 
opposite directions at corresponding points (Fig. 1). 
A fold is inclined or symmetrical 
when the axial plane is inclined and 
the limbs have unequal dips in 
opposite directions at correspond- 
S ing points (Fig. 2). 
A fold is overturned or over- 
folded when the axial plane is 
inclined and the limbs have equal 
or unequal dips in the same direc- 
Fic. 1.—Simple upright fold. tion at corresponding points (Fig. 
3). An overturned fold is lying or recumbent when its axial 
plane is horizontal, or nearly so (Fig. 4). The different parts of 
an overturned fold are the arch limb, reversed limb, and trough 
Inbamlos (4, GG, IEG, 21). 
As to closeness of compression, folds are described by de 
* Les dislocations de 1’écorce terrestre, par EMM. DE MARGERIE et ALBERT HEIM, 
Ziirich, 1888, pp. 49-63. 
