198 Prof. C. Lapworth—The Secret of the Highlands. 
IX.—Essential Principles of Mountain Structure. 
(a.) General Principles. 
1. All sedimentary strata were originally deposited in an approxi- 
mately horizontal position, and they owe their present inclination, 
undulation, or contortion to the effects of lateral compression, or 
tangential thrust and counter-thrust of the exterior parts of the earth’s 
crust, and the results of this lateral pressure in distorting the strata 
are most typically displayed in mountain regions. 
2. Where the local force of compression is comparatively slight 
or ineffective, the originally horizontal strata are bent into a series 
of gentle undulations (normal flexures or amphiplexes) composed of 
alternate arches and troughs (anticlinal and synclinal folds), whose 
axes are normally vertical, and whose beds dip in opposite directions 
(orthoclinic or amphiclinic strata). ‘These folds are of two kinds— 
major and minor, each major fold being generally made up of a 
series of minor tndulations (Fig. 6, Plate V.). 
3. Where the lateral pressure is of excessive intensity, the anti- 
clinal and synclinal waves become crushed more closely together 
into a series of much narrower folds, and the entire rock-mass loses 
greatly in horizontal extension, but gains proportionately in height, 
giving origin to what is known as a mountain range, the major fold 
forming the crest, and the harmonic minor folds constituting the 
flanks of the range (Fig. 7). 
4, At the foot of a mountain range the inward thrust and the 
outward counter-thrust are approximately equal in amount, and 
opposite in direction, and the resulting folds are normal and regular 
(normal or amphiplexal folds). But as we proceed towards the centre 
of the range, while the thrust inward remains approximately the 
same, the counter-thrust outward is aided by the effects of the gravity 
of the mass above, and these two unequal forces are applied to the 
stratum obliquely with respect to each other. As a natural con- 
sequence, the axes of the rock-folds no longer remain vertical, but 
slope obliquely outwards—i.e. in that special direction in which the 
folding and ascending strata encountered the least resistance to their 
extension (Fig. 7). 
5. In a simple and equal-sided mountain-range these phenomena 
being correspondingly developed upon the two opposite sides of the 
range, give origin to the well-known /fan-structure (Fig. 7), seen in 
greatly denuded mountain forms, the younger beds upon the flanks 
of the range being reflexed and inverted in position, apparently dip- 
ping inwards in both directions below the older strata of the ridge 
above. 
6. In a typical complicated mountain-system of vast antiquity 
(which may be regarded theoretically as a series of simple mountain 
ranges pressed more closely together), this special fan-like structure 
must be again and again repeated (Fig. 8), and after denudation has 
taken its full effect, its newer strata will of necessity be found in the 
apparent anticlinals, and its older strata in the apparent synclinal 
forms (Fig. 9). 
