Fence Wall Geology- — Foerste. 369 
mingled, so that in such cases the determination of the bound- 
aries becomes more conjectural. 
Studies based upon preceding principles having led to a 
rough delineation of the area formerly occupied by any class 
of rocks it becomes necessary to correllate this area as deter- 
mined by boulders more closely with the area exposed by the 
bed-rocks during erosion. For this purpose recourse is had 
chiefly to topographical features. 
The most common of these are differences of elevation be- 
tween two adjacent areas geologically distinct, due to the fre- 
quency with which rocks of different geological ages show 
different degrees of resistance to the action of erosion. This 
is likely to result in the formation of single hills Vvhen the 
original area was a boss of some igneous rock ; long, narrow 
broken ridges or valleys when the original area was long and 
narrow, whether sedimentary or igneous in origin ; flattened 
or much diversified areas of greater extent but of marked differ- 
ence in general elevation when the original areas were of con- 
siderable extent. In such cases the boundaries between neigh- 
boring formations are apt to be found nearer the base of the 
hills or the top of the sides of the valleys expressive of the 
variable resistance offered to erosion by the different geolog- 
ical formations. 
Any abrupt change in the character of a rock, from a sand- 
stone to a conglomerate, a shaly series, an igneous formation, 
and the like is a potential line of weakness. Owing to a vari- 
able degree of hardness and tenacity the rocks along such 
planes are apt to become separated during folding and a mod- 
erate amount of sliding or faulting may take place on these 
planes, and give rise to additional fractures along the plane of 
separation. — The difference of velocity with which rocks im- 
mediately on either side of such planes transmit earthquake 
shocks is also likely to find its expression in a general loosen- 
ing of the strata along those planes, accompanied often by 
some sliding. — Ordinary faults in addition to a general plane 
of separation are often accompanied by minor fractures in the 
vicinity owing to friction during faulting. All of such frac- 
tures in whatever way caused are liable to intensify the differ- 
ences of elevation at the boundaries of adjacent geological 
areas, by offering regions favorable for the cutting action of 
erosion, whether by ice or water; and such fractures usually 
