238 FRAGMENTS OF SCIENCE. 



owing to their size, shape and gravity, sink first and con- 

 stitute the bottom of each layer. Gradually, from botton. 

 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 OF SLATE. 

 Dark Slate, two analyses. 



1. Percentage ot iron 5.85 



2. " " 6.13 



Mean. 5.99 



