THE TECHNOLOGIST. [June 1, 1885. 



■610 MINERAL SUBSTANCES FOR WRITING ON. 



of the question ; for had not the iron been of a very superior quality, 

 it could never have stood the stretching, for it is no less than 68| times 

 thinner than the ordinary IC tin-plate. Of course, in a mercantile 

 view, it would never answer to make such thin sheets ; in fact, it is 

 hardly worth while making even taggers ; and most makers of tin- 

 plates would rather be without such orders,. unless in large quantities. 

 For the present, Messrs. William Hallam and Co., of Swansea, stand at 

 the top of the tree. 



The slate is one of the most indispensable auxiliaries in our schools, 

 nor can it be thoroughly superseded in them by any other writing- 

 material. Spite of its common appearance, it deserves our notice and 

 attention, especially as its production forms a not unimportant branch 

 of trade. 



Opinions are divided as to whether granite or slate compose the 

 primitive rocks of our globe. The slate, being originally a glowing, 

 molten mass, enveloped in a dense gaseous atmosphere, underwent a 

 diminution of its heat, at least on the surface, by the continuous 

 radiation of heat into infinite space. Gradually chemical compounds 

 so difficult of fusion began to disengage themselves in the shape of 

 finely-laminated crystals, and, the cooling process still continuing, to 

 attach themselves to the surface of the earthy globe, thus forming a 

 thin covering, a slight crust over the glowing globe, separating the 

 same from its vaporous atmosphere. Such was the origin of the crust 

 of our globe, which could now more rapidly increase in strength, as the 

 immediate influence of the internal heat was obstructed, and the 

 vapoury combinations present could begin to deposit themselves, at 

 least partially in a fluid form, upon the crust of our earth. 



This lowest formation is termed primary rocks, and upon them the 

 subsequent strata, known as secondary rocks, were deposited. It is to 

 this period of the Neptunian formations, and more particularly to its 

 earlier portion, that slate belongs. Thus slate is always and everywhere 

 the lowest and oldest rock, and would consequently be found in every 

 part of the world, were it not so often overlaid by other enormous 

 superincumbent strata. It also forms the principal element in numerous 

 mountain ranges. Veins of other substances frequently intersect the 

 slate, especially greenstone, porphyry and granite causing the slate 

 strata very often to be much dislocated, curved in the most varied 

 manner, and, as a rule, giving them a considerable inclination. Me- 

 tallic veins are likewise frequently found in these strata. 



The three chief elements in this group are : clay slate, mica- slate, 

 and gneiss. 



Clay-slate, which is the subject matter of these remarks, is a con- 

 fused mixture of very fine particles of mica, some quartz, felspar, and 

 talc, sometimes mixed with coaly particles, hornblend or chlorite, 

 generally of an uniform appearance. In its purest form it is used as 

 slate for rooting, tablets, and pencils, the latter being a curious and 



