The hilly aspect of several parts of the central plateau is not due to foldings of the strata, which for the most part appear to be horizontal, but chiefly to the excavating action of the rivers, whose valleys are deeply eroded in the plateau, especially on its borders.
In Carinella they are generally deficient and the intestine straight; in young specimens of this species, however, they occur, though less regular and more in the form of incipient foldings by which the digestive surface is, increased.
These valleys are not synclinal foldings of rocks; they seem to be erosion-valleys.
Cohnheim's hypothesis of " embryonic residues " provides that early in the development of the embryo some of the cells, or groups of cells, are separated from their organic continuity during the various foldings that take place in the actively growing embryo.
The rocks underlying it have been subjected to successive foldings and crumplings by forces acting chiefly from the direction of the Atlantic Ocean, with alternating prolonged periods of waste and denudation.
Near the centre of the range there are broader foldings, carved into castle and cathedral shapes.
New masses of granites protruded next from underneath, and the Bothnian deposits underwent foldings in their turn, while denudation was again at work on a grand scale.
A new series of Jatulian deposits was formed and a new system of foldings followed; but these were the last in this part of the globe.
Such foldings, however, can often be distinctly seen, either on some cliff or coast-line, or in the traverse of a piece of hilly or mountainous ground.
It is only in the outer foldings of the highlands that Palaeozoic fossiliferous deposits are found - Silurian, Devonian, Carboniferous and Permo-Carboniferous.
Such foldings of strata must always die out unless they are abruptly terminated by dislocations.