238 FRAGMENTS OF SCIENCE. 
owing to their size, shape and gravity, sink first and con- 
stitute the bottom of each layer. Gradually, from bottom 
to top the coarseness diminishes, and near the upper surface 
we have a layer of exceedingly fine grain. It is the fine 
mud thus consolidated from which are derived the German 
razor-stones, so much prized for the sharpening of surgical 
instruments. When a bed is thin, the fine-grain slate is 
permitted to rest upon a slab of the coarse slate in contact 
with it; when the fine bed is thick, it is cut into slices 
which are cemented to pieces of ordinary slate, and thus 
rendered stronger. The mud thus deposited is, as might 
be expected, often rolled up into nodular masses, carried 
forward, and deposited among coarser material by the rivers 
from which the slate-mud has subsided. Here are such 
nodules enclosed in sandstone. Everybody, moreover, who 
has ciphered upon a school-slate must remember the 
whitish-green spots which sometimes dotted the surface of 
the slate, and over which the pencil usually slid as if the 
spots were greasy. Now these spots are composed of the 
finer mud, and they could not, on account of their fineness, 
bite the pencil like the surrounding gritty portions of the 
slate. Here is a beautiful example of these spots: you 
observe them, on the cleavage surface, in broad round 
patches. But turn the slate edgeways and the section of 
each nodule is seen to be a sharp oval with its longer axis 
parallel to the cleavage. This instructive fact has been 
adduced by Mr. Sorby. I have made excursions to the 
quarries of Wales and Cumberland, and to many of the 
slate yards of London, and found the fact general. Thus 
we elevate a common experience of our boyhood into 
evidence of the highest significance as regards a most im- 
portant geological problem. From the magnetic deport- 
ment of these slates, I was led to infer that these spots con- 
tain a less amount of iron than the surrounding dark slate. 
An analysis was made for me by Mr. Hambly in the 
laboratory of Dr. Percy at the School of Mines with the 
following result: 
ANALYSIS OP SLATE. 
Dark Slate, two analyses. 
1. Percentage ot iron 5.85 
2. " " 6.13 
Mean. 5.99 
