1012 
the past few thousand years were due to variations in 
the tides, especially in the submarine tides at the 
boundary of the Atlantic and Arctic Oceans, where a 
thin layer of cold arctic water overlies the warmer, 
more saline, Atlantic water. According to Pettersson, 
this “tide-generating” force reached maxima about 3500 
B.C., 1900 B.c., 250 B.c., and a.p. 1433. At tidal maxima 
the arctic icecap 1s more readily broken up than at 
tidal minima, and more ice drifts out mto the Atlantic. 
This floating ice, by lowering the surface temperature 
of the oceans and increasing the local contrasts, shifts 
the storm tracks southwards and causes an increase 
in the number and intensity of depressions, conse- 
quently tidal maxima should also be maxima of stormi1- 
ness and rainfall. There were in fact rainfall maxima 
about 2000 B.c. and 500 B.c. and a period of great 
storminess in the 12th to 14th centuries, but the agree- 
ment breaks down before 3000 B.c., during the Climatic 
Optimum. I think Pettersson’s ‘“‘tide-generating force” 
may have contributed to the climatic variations since 
3000 3B.c., but to be effective it requires a nice adjust- 
ment of ice conditions in the Arctic such as can have 
occurred very rarely and, geologically speaking, only 
for short periods. 
THEORIES OF CLIMATIC CHANGE DUE TO 
TERRESTRIAL CAUSES 
The Hypothesis of Continental Drift. Most theories 
of climatic change rest on the implicit assumption 
that geological deposits were laid down in the latitudes 
and longitudes in which they are now found. That 
assumption was seriously challenged by Wegener [19], 
who put forward the theory that in geological time 
the continents have drifted over the earth, moving 
relatively to each other and also, very widely, relative 
to the poles. He side-stepped the problem of climatic 
change completely; to him a decrease of temperature 
in any district simply meant that that district was 
moving into higher latitudes. Glacial deposits, wherever 
they are now, were all formed in high latitudes, the 
coal measures mark the Carboniferous equator, and 
so on. 
For some time this theory attracted wide support, 
but difficulties, both geological and meteorological, have 
multiplied. The statement that the great glacial de- 
posits of late Palaeozoic age in South America, Africa, 
India, and Australia were formed in a single primeval 
continent (Pangaea) through which the South Pole 
followed a wandering course, breaks down under de- 
tailed examination. The distribution of temperate floras 
of early Tertiary age in zones surrounding the present 
North Pole amounts to a proof that at those times 
the pole occupied its present position and not a point 
in the North Pacific as depicted by Wegener. His 
correlation of Quaternary glaciations is regarded by 
geologists as impossible. Finally, Wegener himself real- 
ised that continental drift cannot explain the succession 
of glacial and interglacial stages. The theory has no 
single definite fact to support it, for even the supposed 
westward drift of Greenland has not been proved; as 
a theory of geological climates it is now almost obsolete. 
CLIMATOLOGY 
The Geographic Control of Climate. The zonal dis- 
tribution of climates at present is by no means perfect; 
owing to warm and cold ocean currents and to the 
positions of the continents, any isotherm may cross 
many degrees of latitude at sea level. The isotherm of 
32F in January, for example, ranges from 35°N in 
eastern China to 70°N north of Norway. There is also 
a strong vertical zoning of climate, temperature de- 
creasing upwards at about 3F per 1000 ft. From this 
it appears that, other things being equal, a geological 
period with large high continents should be cold and 
one with wide oceans and low continents broken up 
into islands should be warm. It will be shown that this 
geographic factor is quantitatively sufficient. 
The Geographic Cycle. The geological record shows 
that the world has passed through a number of cycles 
of elevation and erosion. At the close of a long period 
of quiet, folding of the earth’s crust begins. The con- 
tinents rise and extend to the full limit of the con- 
tinental shelves, and the ocean floor sinks. Erosion 
forms great thicknesses of sedimentary deposits which 
cause further subsidence and folding into great moun- 
tain chains, accompanied by voleanic activity. Glaciers 
form on the mountains and in favourable conditions 
grow into ice sheets. This locking-up of water reduces 
the level of the sea still further. Eventually the period 
of disturbance ends, the mountain ranges reach their 
greatest elevation and are rapidly worn down by ero- 
sion, accentuated in many cases by the glaciers. The 
general level of the land falls and that of the oceans 
rises, and the continental borders are again flooded. 
This brings in a long period of low relief and small 
land masses which continues until another period of 
disturbance begins. 
Geographic cycles are not all of the same intensity. 
The major cycles, culminating in widespread glaciation, 
appear to run their course in about 250 million years, 
but these tend to be broken by minor cycles which 
lead to nothing more than local glaciation. The succes- 
sion of events is shown diagrammatically in Fig. 2. 
An important point is that in the Quaternary, and 
probably in the earlier periods of disturbance also, 
glaciation was not coincident with mountain building, 
but lagged some five to ten million years behind it. 
Various reasons have been assigned for this lag: 
1. At the end of a long period of warm climate the 
oceans are warm, and a long time is required to cool 
them. As soon as the polar seas froze, or extensive 
glaciers reached the sea, cooling of the great mass of 
the oceans would be fairly rapid, but until some ice 
existed, they would cool very slowly. 
2. A smooth dome is not a favourable basis on which 
glaciers can grow into ice sheets. The mountains must 
first be eroded into peaks and valleys, with further 
isostatic elevation. 
3. Mountain building alone is not enough for glacia- 
tion, which must wait until some other factor, such as 
a decrease of solar radiation, acts as a trigger. 
4. Recently Wagner [31] suggested that mountain 
building is accompanied by a great release of earth 
heat, and this raises the temperature of the ground 
