Lamplugh : On Movements in Rocks. 183 
Not in the depths alone, but also close under the surface, 
are the subtle activities in progress. The alteration of rocks 
when exposed to the air, which we call weathering, equally 
implies re-adjustment among the particles ; with every change 
of condition, however slight, some of the ingredients shift 
their places and enter into new concatenations. Every quarry- 
man knows that his freshly-cut stone differs somewhat in 
hardness and other qualities from stone of the same kind that 
has been seasoned awhile. The precise cause of the difference 
is often not easy to discover, but it must mean some change 
in the relation of particle to particle. 
In the aggregate, these individually minute re-adjustments 
of the rock-particles are immensely potent in the construction 
of the earth, though their effect is not immediately impressive 
to our senses. There are other movements, however, which 
have produced results that everyone recognises. These are 
the movements that have affected the strata as a whole, 
pressing them into folds, stretching and breaking them asunder 
by faults, and crushing them into a mass of fragments by 
shearing. 
Take, for example, the case of folding. We know that the 
sediments were originally spread out in nearly horizontal 
sheets; yet the rocks into which they have been converted 
are rarely horizontal. The strata have been tilted in one 
direction or another, and sometimes so steeply that they now 
stand nearly or quite on end. Frequently they have had to 
pack into a narrower space, and in order that this might be 
achieved the more rigid rocks have been bent into folds, more 
and more tightly, until at last they have been doubled sharply 
back upon themselves again and again. It is very remarkable 
that hard and brittle rocks should have maintained their 
integrity though squeezed into most complicated forms, as if 
they were plastic. The distortion, however, has been wrought 
by forces that were not only intense but also very gradual 
and very prolonged, so that the individual particles of every 
rock-band had time to re-arrange themselves, each seeking as 
much relief as possible from the growing pressure. We shall 
frequently find convincing evidence of this if we look care- 
fully at the arches of the folds, for it will be seen that the beds 
are generally thicker there than in the two limbs, because of 
this transference of particles ; indeed, it sometimes happens, 
when the pressure has been very intense, that the limbs are 
squeezed out entirely, and all the material has crept into the 
arches. Even in a very gentle fold, the creep is generally 
apparent: for example, on measuring a slight fold in the 
Carboniferous Limestones of the Isle of Man I found that the 
beds which were seven feet thick on the sides of the arch were 
swollen to eight feet on its crest. (See Plate XIL., fig. d). 
1gtt May t. 
