312 FKAGMENTS OF SCIENCE 



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 important geological problem. 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 was 

 made for me by Mr. Hambly in the laboratory of Dr. 

 Percy at the School of Mines with the following 

 result : 



