THE 
GEOLOGICAL MAGAZINE. 
NEV SERIES, DECADE (il VOL. x. 
No. VIII. AUGUST, 1883. 
(@)aevi CEN AG, oy 41S) AM @ibwasS. 
wen 
].—Tue Sroret or tHE HicHLanps. 
By Prof. Cuartes Lapwortu, F.G.S. 
(PLATE VIII.) 
(Continued from p. 199.) 
(b). Special Principles of Mountain Structure, etc. 
Heim’s theory of inclined folds (overfolds). 
gee theory of the gradual formation of overfolds and overfaults 
in a stratified system is thus developed by Professor Heim : '— 
Wherever a stratified rock series is subjected to the lateral hori- 
zontal thrust of the earth’s crust, a fold is originated. This fold 
consists of two opposed and symmetrical members, with more or 
less remote axes, viz. (Plate VIII. Fig. 1) 
(1) An upward bend, or arch; and 
(2) A similar downward bend, or trough, parallel to the former. 
Connecting this arch and this trough lies a zone forming a common 
or middle limb. 
(a) As the crushing continues, the arch rises, the trough sinks, 
and the middle limb revolves as a relatively neutral intermediate 
portion, and gradually attains a perpendicular attitude. As the 
pressure still increases, the middle limb is even forced somewhat 
beyond the vertical, and becomes inverted in position.’ In this way 
the trough-curve is brought below the arch-curve, and there originates 
first an overhanging, and finally an inclined fold (Plate VIII. Fig. 2). 
During this process the lateral pressure has squeezed thinner those 
strata of different materials which are inclosed together in the 
central parts of the fold, while those in the main curves have been 
rendered somewhat thicker; but all three limbs of the fold have 
experienced the effects of the pressure in a similar measure. 
If the compressing force has been neutralized at this stage, we 
obtain in this way an inclined fold, whose arch, middle limb, and 
trough limb are formed of about equal strength. 
(b) In the case (a) described above, if the middle limb is very 
short, so that its double breadth has become wholly insufficient to 
withstand the amount of the compressing force—that is to say, if 
the pressure is more than sufficient to overturn the middle limb, 
the mechanical arrangement becomes considerably altered. 
1 Heim, Mechanismus der Gebirgsbildung, Band I, pp. 220-223. 
DECADE II.—YOL, X.—WNO. VIII. 22 
