312 FRAGMENTS OF SCIENCE. 



Gradually, from bottom to top the coarseness dimin- 

 ishes, and near the upper surface we have a layer of 

 exceedingly fine grain. It is the fine mud thus con- 

 solidated 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 de- 

 posited 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 sand- 

 stone. 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. I$ow these spots are composed of the finer 

 mud, and they could not, on account of their fineness, 

 lite the pencil like the surrounding gritty portions of 

 the slate. Here is a beautiful example of these spots: 

 you observe fhlily 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 instruc- 

 tive 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 experi- 

 ence of our boyhood into evidence of the highest sig- 

 nificance as regards a most important geological prob- 

 lem. From the magnetic deportment of these slates, 

 I was led to infer that these spots contain a less amount 

 of iron than the surrounding dark slate. An analysis 



