Br. C. Callaway — On Plagioclinal Mountains. 217 



Most mountain chains are produced by the puckering of the 

 earth's crust into parallel flexures, and Ihe greater systems consist of 

 a combination of these earth -waves. Each wave usually rises higher, 

 and the crests approximate more closely, towards the axis of the 

 chain. The Alleghanies of North America well illustrate this 

 structure. Towards the east, the anticlinals are squeezed up to such 

 a height by lateral thrust that they sometimes fall over on one side, 

 like an ocean wave rolling in upon a shallow shore. But, towards 

 the Mississippi Valley, the waves gradually flatten out into low broad 

 undulations. Of course, denudation has acted powerfully upon the 

 flexures, by marine action during their emergence from the ocean, and 

 by sub-aerial forces since their elevation. No trace of the original 

 curved outline of the earth-waves appears in the shape of the 

 ground. The tops of the anticlinals were planed off by the waves 

 of the sea, and rivers have since scored and fretted the surface into 

 a seemingly lawless irregularity. 



But amidst this apparent disorder, one law is clearly seen. This 

 law is that primary mountain ridges run parallel with the geological 

 strike. A range sometimes consists of a denuded anticlinal ; some- 

 times of a synclinal converted into a prominence by the erosion of 

 the anticlinals on each side ; sometimes of combinations of both, or 

 parts of either. But in all these cases, the strike of the mountain 

 chain corresponds with the strike of the beds of which it is com- 

 posed. Amongst examples of this law which will at once occur to 

 geologists, are the Pennine chain, the Cotteswolds, the North and 

 South Downs, the Swiss Alps, and the Andes. The normal structure 

 of mountains may be called orthoclinal. 



The reason of this correspondence between geographical and 

 geological strike is not far to seek. Take the case of an anticlinal 

 mountain. Here the ridge is simply the upper half of a mutilated 

 earth-wave. During the emergence of the land, the anticlinals were 

 first exposed to the action of the sea. Notwithstanding the erosion 

 of the ocean-waves, the anticlinal axes succeeded in raising them- 

 selves, though in a very mutilated state, into dry land. Subsequent 

 sub-aerial action would score and carve the slopes of the saddle into 

 ravines and valleys, but it would not interfere with its strike. 

 Synclinal ridges are formed in a similar manner ; but with one 

 difference. During emergence, the waves plane off the top of the 

 anticlinal, and expose softer strata which lie beneath. The anti- 

 clinals thus become lines of weakness, along which denudation 

 works with great rapidity till it reaches a lower level than the 

 synclinals, which stand up as ridges. The alternative between 

 anticlinal and synclinal ridge is, then, mainly determined by the 

 relative hardness of the strata. In both cases, the strike of the 

 chain will be parallel to the original earth-waves, that is, to the 

 geological strike. 



In reconstructing portions of the geology of Shropshire, the writer 

 has observed some remarkable exceptions to the above rule. The 

 southern half of the county is grooved by a series of parallel valleys 

 trending to the S. W. The intervening elevations, of course, strike in 



