352 T. Mellarcl Reade— Mountain- Building. 



Without, however, considering the causes of changes of 

 temperature in the earth's crust, which I have treated of in other 

 communications, it is sufficient for our present purpose to accept the 

 fact that profound changes do occur. Beginning with Hutton and 

 Play fair, I can think of no geologist who has written upon the sub- 

 ject of metamorphisra and geological dynamics who fails to call to 

 his aid, in some form or other, the effect of heating upon the 

 sediments which compose the rocky covering of our globe. 



If it be assumed, then, that a great sheet built up of various strata 

 of sedimentary origin, with, perhaps, intercalations of ashy and 

 igneous beds, combinedly reaching in places thicknesses measured by 

 miles, is by slow degrees subjected to a fluctuating rise of temperature, 

 it is evident from the illustrative experiments referred to that not 

 only a linear vertical expansion will ensue, but that the horizontal 

 expansion, as much greater in proportion as the areal extent is 

 greater than the thickness, will produce, by small increments and 

 minor alternations, a creep, ending in an anticlinal fold in a position 

 determined by several conditions. 



It is thus seen that an actual movement or displacement of material 

 proportionate to the amount of expansion has taken place. 



The excess of the material of this compound sheet, or what may be 

 called the strata-plate, over the space it originally occupied, is com- 

 pensated for b}' a heaping up by folding along a line of maximum 

 pressure or least rigidity. In this way a permanent feature in the 

 form of a fold has been built up upon the earth's crust, which may be 

 increased in amplitude by future expansions, but which will remain 

 unaffected by any succeeding contractions of the strata-plate. 



Let us now consider how a contraction of the strata-plate can be 

 compensated for. I have shown in niy original work that it may be 

 met, in the case of small contractions, by what I have called com- 

 pressive extension, which is a lengthening of the strata by the 

 compression of the overlying strata going on pari fjassu with the 

 contraction due to a fall of temperature, so that, instead of separating 

 by Assuring, the strata are made to continually occupy the same 

 superficial horizontal space while, at the same time, becoming 

 thinner by compression. As the greatest expansion takes place at 

 the base of the deposits, or in the underlying crust, there is, in most 

 cases, a load sufficient to act upon and mould the contracting bed, and 

 in this way convert horizontal into vertical contraction, the rigidity 

 of the strata and power of conveying lateral thrust being at the same 

 time preserved. Therefore, in the case of a general rise of tempera- 

 ture of the strata-plate, but with minor fluctuations and falls of 

 temperature, the effect of every rise, however small, will — whether 

 the compensation be by compressive extension or by minor faulting 

 and keying up — tend to still further lengthen the strata and develop 

 the anticlinal fold, or to add to it other parallel anticlinals until a 

 complete folded range is finally formed ; or, as in the more extreme 

 cases, such as the Alps, a central core or a series of ellipsoidal domes 

 of gneissic rocks is forced up from below, throwing back the folds in 

 fan-like form, and further compressing them. Certain secondary 



