MODE XII. SLATE. 



107 



Sometimes veins of granite are found to pass 

 through this substance, which must not be ac- 

 cepted as only appearing in the finer form used 

 for slates, but also in coarse and thick schisti, 

 and sometimes, though rarely, even massive. 



It is doubtful whether the yellow argillaceous 

 schistus, which composes the famous mountain 

 of Potosi, belongs to this description ; as the Potosi. 

 argillaceous schistus, or the clay-slate of many 

 other countries, so remarkably metalliferous, 

 cannot be classed under this division; which 

 further evinces the utility, if not necessity, of a 

 far greater abundance of definitive denominations 

 in this new science. But Helms seems to con- 

 sider the Andes as chiefly composed of what 

 Kirvvan calls primeval blue argillite ; and he de- 

 scribes the yellow slate of Potosi as being ex- 

 tremely hard. If they contain from 10 to 20 of 

 iron, they belong to this division ; and as iron 

 commonly accompanies the richest ores, it is 

 probable that its presence is here indicated. 

 But Humboldt regards that amazing chain of 

 mountains as chiefly composed of what is called 

 argillaceous porphyry ; while those of New Spain 

 are of argillaceous schistus: roofing-slate, and 

 its correlatives, being regarded as rare. 



There are valuable quarries of slate in Corn- Quarries, 

 wall, Wales, Westmoreland, and Scotland. A 



