T. Mellard Meade — Mountain-Building. 353 



effects may follow, such as the folding and formation of foot-hills by 

 the gliding of the upper beds down the sloping flanks of the older 

 beds; but it is unnecessary for me to dwell upon them here, as my 

 object is to enable those to grasp who have not yet done so, the idea 

 of successive cumulative expansions, as I conceive them to have acted 

 in the building up of mountain chains. 



But these expansions, caused by a general but fluctuating rise of 

 temperature, diminish, and finally cease, by the dying out of the cause 

 producing them. In the absence of compression no more folds are 

 initiated, nor is the amplitude of the old folds increased. Meantime, 

 the ever active elements in the form of air and water are busy at 

 work, reducing and carving out of these folds domes and ridges, the 

 mountain forms and scenery we are familiar with. Thus the cycle 

 of change is completed, and the broken-up rocks are returned as 

 detritus to the sea. 



A general but fluctuating fall of temperature now sets in, and the 

 rocks composing what I have called the strata-plate contract. This 

 contraction can only be met in one of two ways — either by stretching 

 or Assuring. In their nature rocks are incapable of stretching by 

 tension, excepting it be in a very minor degree, and compressive 

 extension could only partially compensate for the profound changes 

 of volume which take place. No doubt the strata-plate will be 

 eaten into, and underlain to a considerable extent, by semi-molten 

 matter in a plastic condition ; and the shrinkage of this, combined 

 with the irregular shrinkage of the non-homogeneous material of the 

 sedimentary strata, must inevitably initiate fractures. These fractures, 

 we know, take the form of two series of normal faults, each series of 

 which has a more or less definite direction and parallelism, and are 

 classified as strike or dip faults, accordingly as they roughly follow 

 the strike or dip of the strata. The voluminal contraction of the 

 strata-plate is met by the sinking of wedge-like blocks of strata along 

 and between these lines, or, rather, shear planes called faults, and 

 the earth's crust thus remains solid by keying up. In adapting them- 

 selves to the voids these blocks are continuously or intermittently 

 sinking, and certain secondary folding along a large fault often occurs. 

 This is fully explained by the fact that the strata nearest to the 

 earth's surface shrink least, so that the wedge, in adapting itself to 

 the void below by sinking, is often in compression in the upper 

 layers, which is met by the turning up of the edges of the strata 

 against the fault-plane. 



That this succession of events takes place in Nature, can be readily 

 settled by appeal to any typical section of a mountain range, or to 

 any of the numerous sections taken through the folded regions of 

 Britain published by the Geological Survey. 



This latter, it is almost needless to say, constitutes evidence of the 

 best kind, as the authors were simply recording facts, having no thought 

 in their minds of the theoretical relations here expounded. The 

 posteriority of normal faulting to folding has been remarked upon by 

 even so early an observer as Playfair. l Every section I have seen 

 1 " Illustrations of the Huttouian Theory," pp. 62, 63. 



DECADE IV. VOL. III. NO. VIII. 23 



