ON THE THEORIES OF ELEVATION AND EARTHQUAKES. G9 
large that the vertices of any of them might be cut off by denudation, while 
the lower portion should penetrate to a sufficient depth to conceal the in¬ 
verted curvature, the strata might present the character of inverted and not 
that of folded strata. Instead, however, of such a mass as PS (fig. 6) 
being compressed into folded strata, and retaining the general jjositiou re¬ 
presented in the diagram, the whole might, as already intimated, lie com¬ 
pletely turned under the lateral contiguous muss, and tlius present an 
inutance of inverted stratification on a much larger scale than that which 
would probably result from tire folds in tlie manner just nientioneil. 
§ Thickness of tJie FmciuiT<l Portions of the Earth's Crust. 
29. 'fhe preceding investigations may enable us to form some conjecture 
respecting the thickness of those portions of the earth's crust which have 
been elevated and ilislocated in the manner represented! in figs. +, 5, 6. In 
coinideriug the two masses PS and P'S' (fig. 5), an obvious difficulty pre¬ 
sents itself in conceiving how there can be sufficient space to a<lmit of 
their titmiug into the positions there represented. In fact, this angular 
movement necessarily requires either that the distance AA' should be con¬ 
siderably increased by the upheaval, or tliat the masses should become so 
Compressed that the sum of their diagonals MN, MN' should be reduced to 
little more than the original lengths of the sides MP, IMP'. Similar remarks 
will apply to the case of fig. 6. If PM should Im considerably greater 
than tlie depth MS, tlic angular movement otfers no liiffictdty either in a 
geometrical or mechanical sense; for theii the sides IMP, MP'would ap¬ 
proximate mote nearly to equality with the two diagonals, and the support¬ 
ing power would become less able to counteract the tendency of the force 
of gravity to communicate the angular movement in question. If, on the 
contrary, the depth should be considerably greater than the original lengths 
of the sides MP, MP', the two masses could nt'vor, I conceive, be brought 
into the relative positions represented in fig. 5. Now cases arc probably 
not unfrequent in which anticlinal and synclinal lines occur with alterna¬ 
tions of opposite and rapid dips, at intervals not exceeding a few miles. In 
such cases it appears difficult to understand how the elevated crust also can 
exceed a very few miles in thickness. 
§ Contraction of the Earth’s Crust. 
SO. It appears to have been the opinion of some geologists that many of 
the phanoniena of elevation, especially tliose which have apparently re¬ 
tired the action of an intense horizontal force to produce them, might be 
accounted for on the hy|H)the»is of a contraction of the earth s crust 
due to the loss of heat by the central nncleits contained within it This 
View of the subject however has never assumed that determinate form which 
wn demand for it here more than a passing notice. I have shown that the 
hyi^lbesis on which it rests is not essential to account for the most intense 
pressure; and I may also remark, that so far from a contraction of 
fhe general elevated area being necessary to account for phsciioincna of ele¬ 
vation, one of the principal difficulties of the subject consists in conceiving 
great angular disjilacements, such as have been above described, without 
supposing the transversal dimensions of the elevated area to have been in¬ 
creased instead of diminished, 
§ Contemporaneity of Elevation. 
31. The contemjHiraneity of elevation may be considered with reference 
to horizontal space or to vertical height. The most general view which has 
