MODE II. CLAY SLATE. 253 



often graduate into the softer. 3 Nev. Nord. 

 Beytr. 169. Or border upon the muriatic. genus, 

 and pass into schistose chlorite, or schistose talc, 

 or gneiss, or schistose mica. It often contains 

 quartz, both in veins and betwixt its lamina. 

 Voigt Prack. 4L More rarely felspar, schorl, 

 garnets or hornblende, and granular lime-stone. 

 Berg. KaL 205, 206. The softer sorts are re- 

 markably metalliferous. Berg. Kal. Voigt Prack. 

 40. The famous mountains of Potosi consist of 

 it chiefly. 1 Berg. Jour. 1792. 545. In Saxony 

 it is found in primitive lime-stone. 2 Berg. Jour. 

 1792, 134; and often mixed with it, as in Leske, 

 G. 328. It is so much the more siliciferous as it 

 approaches more to granitic mountains. Lasius, 

 121. It passes into rubble stone. 2 Berg. Jour. 

 1788. 493. In the argillites of the Pyrenees no 

 organic remains are to be found. Descrip. Py- 

 ren. 27- Saussure found it in the snowy regions 

 of Mont Blanc. 7 Sauss. 256."* 



Of the secondary argillite, or clay slate, Mr. 

 Kirwan gives the following description : 



" There can be no doubt but argillite is fre- Secondary, 

 quently of secondary origin; Ferber acknow- 

 ledges it to be partly primeval, and partly se- 

 condary. 4 N. Act. Petropol. 289. Gruner 



* Geol. Es3. 183. 



