ON CR TSTALLINE AND SLA TT CLEA VAGE. 237 
selves, men were not found to seek an explanation of slate- 
cleavage involving a less hardy assumption. 
The first step in an inquiry of this kind is to seek facts. 
This has been done, and the labors of Daniel Sharpe (the 
late president of the Geological Society, who, to the loss of 
science and the sorrow of all who knew him, has so sud- 
denly been taken away from us), Mr. Henry Clifton Sorby, 
and "others, have furnished us with a body of facts 
associated with slaty cleavage, and having a most important 
bearing upon the question. 
Fossil shells are found in these slate-rocks. I have 
here several specimens of such shells in the actual rock, 
and occupying various positions in regard to the cleavage 
planes. They are squeezed, distorted, and crushed; in 
all cases the distortion leads to the inference that the 
rock which contains these shells has been subjected to 
enormous pressure in a direction at right angles to the 
planes of cleavage. The shells are all flattened and spread 
out in these planes. Compare this fossil trilobite of normal 
proportions with these others which have suffered distor- 
tion. Some have lain across, some along, and some oblique 
to the cleavage of the slate in which they are found; but in 
all cases the distortion is such as required for its production 
a compressing force acting at right angles to the planes of 
cleavage. As the trilobites lay in the mud, the jaws of a 
gigantic vise appear to have closed upon them and squeezed 
them into the shapes you see. 
We sometimes find a thin layer of coarse gritty material, 
between two layers of finer rock, through which and across 
the gritty layer pass the planes of lamination. The coarse 
layer is found bent by the pressure into sinuosities like a 
contorted ribbon. Mr. Sorby has described a striking case 
of this kind. This crumpling can be experimentally imi- 
tated; the amount of compression might, moreover, be 
roughly estimated by supposing the contorted bed to be 
stretched out, its length measured and compared with 
the shorter distance into which it has been squeezed. We 
find in this way that the yielding of the mass has been 
considerable. 
Let me now direct your attention to another proof of 
pressure; you see the varying colors which indicate the 
bedding on this mass of slate. The dark portion is gritty, 
being composed of comparatively coarse particles, which. 
