DEFORMATION OF ROCKS 209 
But the rock beds as they occur in nature differ from the 
bunch of paper in that they are of varying thickness and 
strength. The major readjustments of the rock beds occur 
between the thick and strong strata, and within the weak and 
soft strata. In these latter, therefore, the rearrangement of the 
particles is far more profound than would be the case if such 
beds were folded alone. In closely folded districts among other 
evidence of readjustments the polishing effects of accommoda- 
tion between the beds may always be seen in the slickensided 
surfaces of the major and stronger beds, and in cases of compli- 
cated folding the same phenomena are observable between the 
thinnest lamine. A most striking instance of this polishing is 
seen in the Jura Mountains, where at many places both sides of 
the strong, thick layers of Jurassic limestone are polished as 
smoothly as if glaciated. They reflect the sun like an imperfect 
mirror. At some places where the folds are steep, layers have 
fallen down along these movement planes, exposing great sur- 
faces of beautifully polished rock. In the folded Cambrian 
quartzites of Doe River, Tennessee, the polishing of the layers 
by accommodation is scarcely less strikingly illustrated. 
If a given bed in the center of a rock formation be plicated 
and the layers above and below be folded in a strictly parallel 
manner, in passing away from the central bed in either direc- 
tion those on either side are less closely folded, and finally 
the crenulations become slight. If the folds are close in the 
center they die out with great rapidity. The above follows 
directly from the laws of deformation of solid masses, and is 
illustrated by Fig. 4. It should be observed that the more 
crenulated lines are longer than the less crenulated ones. In so 
far as this is imitated in nature this implies that there is differ- 
ential movement between the layers, for originally all of the beds 
must be supposed to have been of the same length. Such dit- 
ferential motion doubtless does occur in strata in which the folds 
differ in character, for the more closely folded beds must be 
subjected to severer thrusts or have been originally weaker, so 
that the thrust is more effective, or have been in a position in 
