44 REPORT OF MEETINGS FOR 1901 



those of upheaval. The two may be in close contiguity, or 

 they may be far apart ; but, however they are situated with 

 regard to each other, they are clearly correlative. As this 

 fact is one that has an important bearing upon the arguments 

 that follow, it may be as well to make its nature quite clear 

 by means of one or two illustrations like the following : — If 

 we take two or three leathern straps and lay them length- 

 ways one upon another, and press the opposite ends towards 

 each other, it will usually happen that the straps will be 

 folded into a series of bends, curving alternately upward and 

 downward, the closeness of the upfolds to the downfolds will 

 be, of course, dependent upon the length of the straps and 

 the distance of the ends from each other. The number of 

 the folds, or, what comes to the same thing, their magnitude, 

 will depend upon the manner in which the pressure is applied. 

 Furthermore, if we make a chalk mark across the edges of 

 the straps, before they are folded, and keep our eyes upon 

 these marks as the folding goes on, we shall see that there 

 is a small sliding movement of one strap past the other, the 

 existence of which is rendered still more manifest if the pile 

 of straps consists of half a dozen or more. Besides this 

 differential movement there is an evident tendency for the 

 straps to separate a little from each other, so that small 

 spaces are opened at certain points. These facts are of 

 some importance in connection with what is to follow. 



We may realise the correlation of the downfolds with the 

 upfolds also in another way. Perhaps a water-bed, such as 

 is sometimes used for invalids, will serve for this illustration. 

 When a patient is laid upon the water-bed the part beneath 

 him sinks with his weight, and a corresponding rise of the 

 surface of the bed around ensues as a consequence of the 

 displacement of the water. Downfold and upf'old again are 

 correlated. As yet another illustration we may take note of 

 what happens to a pan-full of dough when it is undergoing 

 kneading — just as much of the plastic mass rises above 

 the general level as is forced out by the kneading action of 

 the operator's hands. These illustrations are all very common- 

 place, but, nevertheless, they serve well to enable us to realise 

 what takes place on the large scale where the outer portions 

 of the Earth's crust are undergoing flexure. 



