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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 7, 



cause, sediment and brick- earth, would be left behind. The author 

 also sees an agency competent to deposit an extensive spread of 

 brick-earth in the unlading of icebergs, — the coarser gravel sinking 

 more quickly, and the finer particles and angular splinters less 

 readily, and being liable to be stirred up and raised higher by every 

 fresh shower of gravel falling upon them. 



The author, having shown that the contour of the existing surface 

 of our softer strata and other observed phenomena seem due to the 

 sudden uprising of the land from beneath the sea, next inquires 

 whether there have been more than one such movement, and whether 

 these have been combined with periods of depression. Without en- 

 tering into any detail on this part of the subject, he remarked that, 

 the present contour being in the main the same as it was at the 

 period of the great-mammalian fauna (only less elevated), there 

 must have been a sudden elevation preceding that period, and that, 

 during subsequent depressions, the valley deposits were formed in 

 which the mammalian bones lie entombed, the valleys having been 

 again partially re-excavated by the repetition of a sudden but less 

 lofty upheaval. 



Lastly, it was pointed out that sudden vertical movements of the 

 surface on a grand scale are of as probable occurrence as those lesser 

 movements with which we are historically acquainted ; for the earth's 

 crust, to the depth of at least many miles, is rigid to such a degree 

 that great changes of position in its parts cannot occur without 

 actual disruption of the strata, as all faults testify. Its movements 

 are not those of a flexible or semifluid envelope. Now, the pres- 

 sure requisite to rupture nearly rigid strata will accumulate enor- 

 mously before they yield to it. causing probably a slow and gradual 

 movement from the want of absolute rigidity ; but when once they 

 are raptured, they will be thrown up or down with a sudden move- 

 ment. This will be true even on old lines of fault, for it will reqtiire 

 a great and new accumulation of force to overcome the friction on 

 the line of fault ; but when once the rocks are set in motion, the 

 resistance caused by the friction will be much less, according to a 

 well-known principle in mechanics. Again, if lateral pressure, 

 arising from failure of support, be the nature of the force which has 

 elevated tracts of the earth's surface (which is the author's persua- 

 sion), such a pressure might accumulate very greatly without pro- 

 ducing any vertical movement ; but as soon as any local circum- 

 stance determined the course of an anticlinal or fault, the edges of 

 the strata on one or both sides of that line would begin to be raised 

 with reference to the neighbouring parts, and the pressure which, as 

 in a bridge, had long existed in the plane of the crust without pro- 

 ducing motion, would now act at an angle to the beds with momen- 

 tarily increasing advantage, as in a bridge beginning to fall, tilting 

 them into inclined positions, probably crushing them, and producing 

 minor dislocations at the same time. 



Thus, by mechanical considerations, the author is led to believe 

 that the ordinary nature of the greater movements of the earth's 

 crust must be sudden. 



