364 A.J. Bull—A Hypothesis of Mountain Building. 
being practically stationary in one and progressing rapidly in another. 
(Cf. I1-7/8 with II-13.) On the other hand, in different lines of 
descent, homologous structures may progress at different rates. 
(cf. the ambulacral end of each of the four graphs). 
Such considerations as these indicate that whilst a genus, species, 
or variety is characterized by a combination of structural elements 
at certain stages of development, a lineage, or line of descent, is 
characterized by a certain cluster of directions and rates of change. 
(To be continued. ) 
A Hypothesis of Mountain Building 
By A.J. Buri, Mise. Eilnstie EGS: 
HE following paper is an attempt to contribute to the solution 
of the problem of mountain building. It is here suggested 
that the folding of mountain ranges may be produced by the frictional 
drag of moving portions of the asthenosphere, and that these move- 
ments may be convective and result from its unequal heating by 
radio-active elements. 
The earth is gaining heat energy from the breaking up of radio- 
active molecules and the liberation of some of the energy of these 
heavy elements in the form of heat. It is at the same time losing 
heat by conduction through the crust and subsequent radiation. 
Now, it has not been possible to equate these two quantities, the 
average quantity of radium found in rocks by Strutt being many 
times greater than that necessary to make good the loss by radiation 
into space. This disagreement, which is doubtless due to causes not 
yet ascertained, does not in any way invalidate the established 
facts that the earth is both gaining and losing heat. 
The author is aware that many geologists have attempted to 
explain mountain building in terms of a cooling and contracting 
earth, but were such the case the contraction must have been con- 
tinuous and progressive with no periods of complete rest, with the 
result that the whole crust would be in compression, whereas the 
most characteristic phenomena of the throwing up of mountain 
chains are that they are local in place and occasional im time. More 
recently this hypothesis has been modified by supposing that the 
contraction only takes place at long intervals, and is due possibly 
to molecular rearrangement in the interior. 
In addition to this “ curiously local distribution of . . . com- 
pression ’’, as Oldham has called it, it must be remembered that the 
most intense compression and folding occurs in the centre of a 
mountain range, and that the folding becomes less on the outskirts. 
It has several times happened that the upheaval has affected freshly 
deposited material and has followed a long period of sinking, e.g. 
Malvernian, Armorican, and Alpine foldings. In contrast to the 
folded districts large portions of the crust have been but little 
