GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Geologiska Sallskapet i Finland. 25
the ends of the box and the whole block reduced to about l/, of its
original thickness. The pressure needed was a few kg/cm2. The clay
was then lifted out of the box. The sides or a section were then photographed.
Plates 1 and II show the middle portion of some experiments.
When the paraffine layer remained in the centre it was crumpled
up into folds that are identical with natural ptygmatic folds. Generally
a slightly greater amount of clay flowed out of one of the apertures
and the paraffine layer was then bent into a looped shape (Plate
II, fig. 1). This case represents the mechanism of folding with a flat
lying anticlinal axial plane. The result is similar to that of nature.
Fig. 6. Set-up of experimenta on ptygmatic folding.
Experimental ptygmatic folding.
We find flat and even drawn out limbs (D) combined with a crumpled
anticlinal apex (compare fig. 1). Inspection of Plate 1, fig. 2, A
shows that the wavelength of the folds increases with the thickness
of $he sheet of paraffine. By changing the consistency of this layer
the wavelength can also easily be altered.
In fig. 2, Plate II the dark lines in C are the outcrop of originally
plane surfaces parallel to the hard layer painted black. But the tendency
of the clay to slip along horizontal shearing planes hae given
. a stepped outline to the sections. In A and B the thick line shows the
curve that would have ensued in perfectly plastic movement. The
dying out of the contortions away from the controlling vein is well
demonstrated.
Another inference can be drawn from the experiments. For simplicity
1 allowed only two opposing directions of escape for the clay.
If the vein is placed in the plane containing the directions both of
compression and elongation it is not folded. This is sirnply because
it stretches together with the clay to compensate the compression. A
more brittle layer, however, was not internally deformed but torn apart
and the loose pieces were folded. Spurr thought the flow during gneissic