OF THE MORE DISTURBED ZONES OF THE EARTH’S CRUST. 445 
out a great portion of its length, this lofty and rugged zone of mountains consists 
é of two approximately parallel chief crests. The great feature in the geological struc- 
ture of the whole zone is the presence of belts of closely plicated Mesozoic and Ter- 
tiary strata on both flanks of each of these great constituent ranges. But the most 
striking, and, at first view, perfectly enigmatical feature, is the inward plunge of 
the newer strata beneath the older, in the sides and at the base of both chains. 
When, however, the plicated strata are structurally arranged and traced, we find 
that this phenomenon assumes the character of a symmetrical folding of the 
rocks in two opposite directions from each high central axis. The individual 
foldings, with scarce an exception, lean outwards from the central tracts of 
the mountains, or from the quarters of igneous disturbance, rupture, and maxi- 
mum metamorphism of the crust. In other words, the axis planes of the pli- 
cated strata of the flanks of the Alps dip inwards towards the centre of the chain ; 
those nearest to it at a low angle, and those more remote at angles steeper and 
__ steeper as the waves recede, expanding to the outer base of the range. High on 
the flanks of the Alps, or, what is the same thing, deep in towards the roots of the 
j mountain, where only the synclinal bends, of the flexed strata, have been protected 
_ from denudation by inward folding, these closely compressed troughs lie pinched 
___ in between the older strata in oblique inward inclination. The transverse sec- 
| tions expose these bendings, which are called Vs by some of the Swiss geologists. 
| Here then we behold an exact counterpart in the stratification or structure of a 
r 4 single flank of the Alps, of that folding with inversion which characterizes the 
| Appalachian chain, or that of the Ardennes, a single side of the Alps being the 
equivalent of the whole of either of those zones ; it consists, that is to say, of a belt 
undulated in one direction. Crossing the Alps, or rather one of its component great 
chains, we find another simzlar belt of the same strata, plicated in the same way, 
| with their axis planes dipping also under the crest or orographic axis of the 
__ mountain, but of course, to the opposite quarter of the compass as compared with 
the plicated zone of the other flank. This is, I conceive, a true picture of that 
feature which, hitherto imperfectly analysed, has been called by some of the geo- 
 logists of Switzerland, expressively enough, 
The Fan-like Structure of the Alps. 
Viewed as a single chain, this mountain system consists, then, of two belts 
‘undulating in opposite directions; but, as already stated, it is for the most part of 
its length a double chain; and J think each range, especially, where these are 
_ widest apart has a plicated belt of strata upon each of its slopes, so that, for some 
- districts at least, the fan-like structure is twice repeated; in other words, there 
are four belts of closely folded waves, each having its axis planes dipping to- 
wards the base of its own high mountain system. 
