46 
POPULAR SCIENCE REVIEW. 
Slate is a very interesting example of tlie state to whicli a 
rock may be brought by those changes that act in the interior 
of the earthj on a grand scale,, without of necessity^ destroying 
all traces of the original construction of the material acted on. 
In a slate quarry it would be easy to pick up specimens show- 
ing perfect cleavage^ or^ in other words^ splitting into films, as 
thin as paper, in a direction altogether different from that of 
the original strata. In the same hand- specimen, the original 
stratification might be recognised by fossil shells, and by lines 
very clearly marked, but in which there is no tendency to 
split. Still, in the same specimen, there may be a tendency 
to break up into definite mathematical solids, almost always 
exhibiting the same set of angles. In the intervals between 
these latter planes (technically called crystals of sihca 
and pyrites will be found. Little cubical crystals of pyrites 
will also be disseminated through the slate, quite independent 
of cleavage joints or bedding. When we consider that the 
original rock in this case was certainly clayey mud, that any 
impurities have been crystallized out, but that the chemical 
composition and mechanical arrangement are hardly altered, 
and that there is no real resemblance to crystallisation in the 
mere facility of splitting, it is clear that the metamorphism is 
of a high order. Pressure enormous and long continued, pro- 
bably combined with an equable temperature for a long time, 
is the vera causa to which we must revert to account for these 
phenomena. But pressure will not fill the crevices, nor 
separate out the iron and silica, nor deposit these minerals 
afterwards. Pressure with a certain amount of chemical action 
must here have combined in the recesses of the earth, and the 
change is one that must have required time as well as force. 
The fact that cleavage is producible by pressure alone, even 
without any exercise of chemical action, is undeniable. It was 
illustrated by Mr. Sorby, in 1856, in reference not only to 
slates, but limestones, consisting of fully 50 per cent, of parti- 
cles of shell and other organic substances, the rest of the rock 
being crystalline. In fissile rocks of this kind the microscope 
shows that the particles of organic origin, irregularly distri- 
buted where cleavage does not exist, are pulled out, as it were, 
and lie parallel to the direction of the cleavage planes where 
cleavage has supervened, and that in these cases cleavage is quite 
independent of stratification. Even the shape of the cells in 
encrinite joints is elongated by the cleavage, and the very 
crystals themselves are pulled out in a similar manner. It is 
well known that the shells in roofing slate, when they have 
been retained, are distorted in a similar way. Thus a kind of 
metamorphosis is produced by mechanical action only. 
It is not necessary to pause here to explain that the varieties 
