Dr. Harold Jeffreys — Causes of Mountain-Building. 215 



Treacher and White (Proc. Geol. Assoc, xix, p. 385, 1906) identified 

 the Uintacrinus sub-zone south-west of Kirby House (to the east of 

 our section), and again at Prosperous Farm (about the same distance 

 to the west). In both of these instances the exposures were, if 

 anything, nearer to the Tertiary boundary than the roadside section. 

 Thus it seems probable that the outcrop of qnadratus-c\\'nW. here is 

 of the nature of an outlier, precisely similar to the one that the 

 above-named authors record at Laj'lands Green, Kintbury. 



There are now three patches of this zone known to occur at the 

 western end of the London Basin, namely, Boxford, Kintburj'-, and 

 Inkpen. It can hai'dly be an accident that all three occur in an 

 almost perfectly straight line, which has a north-east to south-west 

 trend. It is true that the Boxford outlier is associated with peculiar 

 lithological conditions and great attenuation of the zones, but in the 

 case of the two more southerly outcrops there is no such peculiarity. 

 On the present evidence it seems reasonable to postulate the existence 

 of a pre-Tertiary syncline along this line, with, perhaps, a parallel 

 complementary anticline on the eastern side which is responsible for 

 the Hampstead Marshall inlier. 



Y. — The Causes of Mountain-Bcjilding. 

 By Harold Jeffreys, M.A., D.Sc. 



AN article by Mr. 11. M. Deeley in the Geological Magazine for 

 March, pp. 111-120, is mainly devoted to an attempt to find 

 a cause of mountain-building more potent than the compression due to 

 cooling, of which he says that "many physicists . . . are quite 

 satisfied that it is not capable of accounting for the amount of 

 compression required". The only physical argument he quotes in 

 support of this statement is that of Osmond Fisher, which he may 

 therefore be presumed to consider the most conclusive ; 3'et it is 

 certain that no physicist would now admit that Fisher's reasoning 

 has any validity. It rests entirely on Kelvin's theory of the cooling 

 of the earth, which has had to be completely revised on account of 

 the discovery of the extensive distribution of radio-active matter in' 

 the earth's crust. The time available has been found to be about 

 twenty times greater than on Kelvin's theory, and the cooling has 

 therefore had time to extend to a mucli greater depth and to produce 

 accordingly a very much greater compression. Our present knowledge 

 indicates that the compression has been enough to sliorten the 

 circumference of the earth by from 133 to 227 kilometres, according 

 to the precise distribution of radio-active matter assumed. The 

 level of no strain is at the same time found to be at a depth of about 

 80 kilometres, so tbat the crust-movements due to compression would 

 extend to a considerable depth. 



Mr. Deeley next states that the compression required to make the 

 Alps would be 1,050 kilometres, and deduces, apparently by a simple 

 division by tt, that the diameter must have decreased by about 

 334 kilometres, which he says is greater than can possibly be 



