144 ^^'^ American Geologist. Manii, lyoi. 
sequence of renewed contraction, the flatteninj^ may occur 
elsewhere. 
This hypothesis of the alternation of periods of deformation 
with periods of spheroidal recovery is geologically useful, as 
it suggests an explanation of a certain periodicity in geolog- 
ical phenomena. For instance, the later half of Palaeozoic time 
may have been a time of slow tetrahedral collapse, culminat- 
ing in an instability which led to the great mountain move- 
ments which closed the Palaeozoic ; then followed a quiet period 
of slow restoration of the spheroidal form, causing the series 
of marine "transgressions" which are the dominant feature of 
the geological history of the Mesozoic era. 
\'ertical Range of Defor.matiox. 
Reluctance to admit the possibility of such changes is re- 
duced when we recollect how insignificant are the dififerences 
in level, when compared with the size of the earth. The use 
of exaggerated diagrams leads to unconscious magnification of 
the extent of the polar flattening, and of the difference between 
the continental summits and the oceanic depths. The study of 
large-scale maps has been authoritatively recommended. The 
examination of true scale curves and outlines may help us to 
realize the actual conditions. The accompanying figure''' shows 
a section of the earth's crust from Stromboli to Vesuvius. The 
thick black band represents the section cross the Mediter- 
ranean ; the line ah marks the depth of the Atlantic ; the upper 
curve shows where the surface would be if there were no polar 
flattening. The lowest line marks the depth of one-hundredth 
of the earth's radius. The thickness of this zone in compar- 
ison with the size of the earth is shown on Fig. 7, h, which is a 
sector of a circle, with the zone of a shown, reduced to its true 
relative size. The polar flattening is barely recognizable, and 
the difference between sea-bottom and mountain summit is 
marked only by variations in the thickness of a line. 
The diagram illustrates the insignificance of the deforma- 
tions required ; and that crustal disturbance occurs much 
deeper than the layer with which the tetrahedral theory is con- 
cerned is shown by the fact that the estimated centre of origin 
of the Lisbon earthquake lies far below. 
• Based on Lingg's 'Erdpofil.' 
