10 THE PRESIDENTIAL ADDRESS 
to fit the major periods of mountain movement, as determined by 
him from the radioactivity of minerals contained in the rocks. On 
this evidence the Alpine movement should date back from 20 to 
60 millions of years ago, the Hercynian 200 to 250 millions, and the 
Caledonian from 350 to 375 million years. 
In a preliminary attempt to modify Joly’s hypothesis Holmes 
postulated the occurrence of similar, but longer cycles (Magmatic 
Cycles) in a denser, ultrabasic layer underlying the basic one, the 
rhythm of which would be nearer to 150 million years. ‘The shorter 
cycles due to the basic layer are held in part responsible for periods 
of minor disturbance, and also to account for the individual varia- 
tions in effect, duration, and intensity of the larger ones. Each of the 
later movements has also evidently been limited and conditioned by 
the results of foregoing ones, and especially by areas of fracture 
and weakness on the one hand, and by large stable masses composed 
of rocks intensely consolidated, or already closely packed, on the 
other. 
More recently Holmes has developed the possibility that the loss 
of heat is mainly due to convection in the liquid substrata, and that 
convection is the leading cause of the drifting and other movements 
of the crust, and the disturbances that have occurred in it. He 
says :— 
‘Although the hypothesis involving sub-crustal convection 
currents cannot be regarded as established, it is encouraging to 
find that it is consistent with a wide range of geological and geo- 
physical data. Moreover, it is by no means independent of the 
best features of the other hypotheses. It requires the local 
operation of thermal cycles within the crust, and it necessarily 
involves contraction in regions where crustal cooling takes place. 
It is sufficiently complex to match the astonishing complexities 
of geological history, and sufficiently startling to stimulate 
research in many directions.’ 
The phenomena are difficult to disentangle as the number of 
operating causes has been so great and many of them are not fully 
understood. But, underlying them all there is unquestionably the 
pulse within pulse which Suess saw and of which Joly pointed 
the way to explanation. 
The view at which we have arrived is neither strictly uniformi- 
tarian nor strictly catastrophic, but takes the best from each 
hypothesis. As Lyell showed, most of the phenomena of geology 
can be matched somewhere and sometime on the earth of to-day ; 
but it would appear that they have varied in place, intensity, phase, 
and time. And, as Lyell was driven to accept evolution to explain 
the history of life on the earth, so must we employ the same word to 
