LOW-ANGLE FAULTING 29 
and in a position to carry thrusts from the west. Because of the 
erosion of the crest of the anticline, support from the east limb had 
been to a considerable extent removed, and frontal resistance to a 
thrust from the west greatly reduced. With resistance in front 
lessened and resistance beneath unchanged, or very much less 
diminished, lateral thrusts developed shearing stresses which caused 
the overthrust. This type would, therefore, be an overthrust due 
to rotational strain fostered by the special attitude of the strata 
and especially by the lessening of the resistance to a forward move- 
ment of the upper layers because of preceding erosion. ‘The shear- 
ing then took place along a bedding plane as a line of weakness. 
The pretty structural explanation of the southern Appalachian 
overthrusts offered by Hayes was entirely dependent for its work- 
ing qualities upon appropriate stratigraphic formations of widely 
different competency. Similarly, though to perhaps lesser degree, 
the erosion-thrust of Willis is dependent upon appropriate stratig- 
raphy and antecedent history. Admitting that each of these 
explanations fits the particular case, or type of cases, for which 
it was devised (which was probably all that the authors intended), 
it is clear that an explanation on either of these lines cannot fit 
the type of overthrust which is so wonderfully displayed in the 
Scottish Highlands. In these remarkable dislocations the low- 
angle overthrusting did not occur until after the continuity of 
bedding over the overthrust area had been completely interrupted 
and displaced by repeated slice faults at the ordinary angle of 40° 
to 45°. The Scottish overthrusts did not follow any one weak 
formation, as did the overthrusts in the southern Appalachians, 
but cut straight through the various rocks of many previously 
faulted blocks. It is clear that a more general raison d’étre for 
low-angle faulting must be sought. 
2. Rotational Strain in Homogeneous Material 
Piling up of material a possible factor.—One of the most char- 
acteristic features of the Caledonian diastrophism which produced 
the faulted structure of the Scottish Highlands was the development 
of a remarkable imbricate structure prior to breaking along the great 
thrust planes. It seems to be well established, both from the field 
